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CHAPTER V OOGENESIS AND SPERMATOGENESIS. REDUCTION OF THE CHROMOSOMES

" Es kuromt also in der Generationenreihe der Keimzelle irgendwo zu einer Reduktion der ursprunglich vorhandenen Chromosomenzahl auf die Halfte, and diese ZaMtn-reduktion ist demnach nicht etwa nur ein theoretisches Postulat, sondern eine Thatsache."

Boveri. 1

Van Beneden's epoch-making discovery that the nuclei of the conjugating germ-cells contain each one-half the number of chromosomes characteristic of the body-cells has now been extended to so many plants and animals that it may probably be regarded as a universal law of development. The process by which the reduction in number is effected, forms the most essential part of the phenomena of maturation by which the germ-cells are prepared for their union. No phenomena of cell-life possess a higher theoretical interest than these. For, on the one hand, nowhere in the history of the cell do we find so unmistakable and striking an adaptation of means to ends or one of so marked a prophetic character, since maturation looks not to the present but to the future of the germ-cells. On the other hand, the chromatin-reduction suggests questions relating to the morphological constitution of nucleus and chromatin, which have an important bearing on all theories of the ultimate structure of living matter and now stand in the foreground of scientific discussion among the most debatable and interesting of biological problems.

Two fundamentally different views have been held of the manner in which the reduction is effected. The earlier and simpler view, which was suggested by Van Beneden and adopted in the earlier works of Weismann, Boveri, and others, assumed an actual degeneration or casting out of half of the chromosomes during the growth of the germ-cells — a simple and easily intelligible process. Later researches conclusively showed, however, that this view cannot be sustained, and that reduction is effected by a rearrangement and redistribution of the nuclear substance without loss of any of its essential constituents. It is true that a large amount of chromatin is lost during the growth of the egg. 2 It is nevertheless certain that this loss is not directly connected with the process of reduction ; for, as Hertwig

1 Zcllensiudien, III., p. 62. * Cf. Figs. 97, 116.

233


234 REDUCTION OF THE CHROMOSOMES

and others have shown, no such loss occurs during spermatogenesis, and even in the oogenesis the evidence is clear that an explanation must be sought in another direction. The attempts to find such an explanation have led to some of the most interesting researches of modern cytology ; and though only partially successful, they have raised many new questions which promise to give in the end a deeper insight into some of the fundamental questions of cell-morphology. For this reason they deserve careful consideration, despite the fact that taken as a whole the subject still remains an unsolved riddle in the face of which we can only return again and again to Boveri's remark that whatever be its theoretical interpretation the numerical reduction of the chromosomes is itself not a theory but a fact.



Pig. 114. — Formation of the polar bodies before entrance of the spermatozoon, as seen in the living ovarian egg of the sea-urchin, Toxopneustes (X 365).

A. Preliminary change of form in the germinal vesicle. B. The first polar body formed, the second forming. C. The ripe egg, ready for fertilization, after formation of the two polar bodies (p. b. 1, 2) ; e. the egg-nucleus. In this animal the first polar body fails to divide. For its division see Fig. 89.


A. General Outline

The general phenomena of maturation fall under two heads : viz. oogenesis, which includes the formation and maturation of the ovum, and spermatogenesis, comprising the corresponding phenomena in case of the spermatozoon. Recent research has shown that maturation conforms to the same type in both sexes, which show as close a parallel in this regard as in the later history of the germ-nuclei. Stated in the most general terms, this parallel is as follows : l In both sexes the final reduction in the number of chromosomes is effected in the course of the last two cell-divisions, or maturation-divisions, by which the definitive germ-cells arise, each of the four cells thus formed having but half the usual number of chromosomes. In the female but one

1 The parallel was first clearly pointed out by Platner in 1889, and was brilliantly demonstrated by Oscar Hertwig in the following year.


GENERAL OUTLINE


235


of the four cells forms the " ovum " proper, while the other three, known as the polar bodies, are minute, rudimentary, and incapable of development (Figs. 89, 97, 114). In the male, on the other hand, all four of the cells become functional spermatozoa. This difference between the two sexes is probably due to the physiological division of labour between the germ-cells, the spermatozoa being motile and very small, while the egg contains a large amount of protoplasm and yolk, out of which the main mass of the embryonic body is formed. In the male, therefore, all of the four cells may become functional ; in the female the functions of development have become restricted to but one


Primordial germ-cell.


Obgonia.


Primary oocyte or ovarian egg.

Secondary oocytes (egg and

first polar body).


Mature egg and three polar bodies



• Division-period (the number of divisions is much greater).


Growth-period.


Maturation-period.


Pig- 115. — Diagram showing the genesis of the egg. [After BOVERI.]

of the four, while the others have become rudimentary (cf. p. 124). The polar bodies are therefore not only rudimentary cells (Giard, '76), but may further be regarded as abortive eggs — a view first put forward by Mark in 1881, and ultimately adopted by nearly all investigators. 1 The evidence is steadily accumulating that reduction is accomplished by two maturation-divisions throughout the animal kingdom, even in the unicellular forms ; though in certain Infusoria an additional division occurs, while in some other Protozoa only one maturation-division has thus far been made out. Among plants, also, two maturation


1 A beautiful confirmation of this view is given by Francottes's ('97) observations on a turbellarian, Prostheceraus, The first polar body is here often abnormally large, all gradations having been observed from the normal size up to cells nearly as large as the egg itself. Such polar bodies are occasionally fertilized and develop into small gastrulas, first forming a single polar body like the second polar body of the egg. Here, therefore, two of the four cells are exceptionally capable of development. It may be added that Fol long ago observed the penetration of the small polar bodies by spermatozoa in the echinoderms; and this has been more recently observed by Kostanecki in mollusks.


236 REDUCTION OF THE CHROMOSOMES

divisions occur in all the higher forms (Muscineae, pteridophytes, and phanerogams), and in some, at least, of the lower ones. Here, however, the phenomena are complicated by the fact that the two divisions do not as a rule give rise directly to the four sexual germ-cells, but to four asexual spores which undergo additional divisions before the definitive germ-cells are produced. In the flowering plants there are only a few such divisions, which give rise to structures within the pollen-tube or embryo-sac. In the archegoniate cryptogams, on the other hand, each spore gives rise, by repeated divisions, to a "sexual generation " (prothallium, etc.) that intervenes between the process of reduction and that of fertilization. The following account deals primarily with reduction in animals, the plants being afterward considered.

I. Reduction in the Female. Formation of tfie Polar Bodies

As described in Chapter III., the egg arises by the division of cells descended from the primordial egg-cells of the maternal organism, and these may be differentiated from the somatic cells at a very early period, sometimes even in the cleavage-stages. As development proceeds, each primordial cell gives rise, by division of the usual mitotic type, to a number of descendants known as oogonia (Fig. 115), which are the immediate predecessors of the ovarian egg. At a certain period these cease to divide. Each of them then grows to form an ovarian egg, its nucleus enlarging to form the germinal vesicle, its cytoplasm becoming more or less laden with food-matters (yolk or deutoplasm), while egg-membranes may be formed around it. The ovum may now be termed the oocyte (Boveri) or ovarian egg.

In this condition the egg-cell remains until near the time of fertilization, when the process of maturation proper — i.e. the formation of the polar bodies — takes place. In some cases, e.g. in the sea-urchin, the polar bodies are formed before fertilization, while the Ggg is still in the ovary. More commonly, as in annelids, gasteropods, nematodes, they are not formed until after the spermatozoon has made its entrance ; while in a few cases one polar body may be formed before fertilization and one afterward, as in the lamprey-eel, the frog, and Amphioxus} In all these cases the essential phenomena are the same. Two minute cells are formed, one after the other, near the upper or animal pole of the ovum (Figs. 97, 1 16); and in many cases the first of these divides into two as the second is formed (Fig. 89).

A group of four cells thus arises, namely, the mature egg, which gives rise to the embryo, and three small cells or polar bodies which take no part in the further development, are discarded, and soon die

1 Cf. p. 189.


GENERAL OUTLINE


237


without further change. The egg-nucleus is now ready for union with the sperm-nucleus.




, ' r-: \



D



E H

Fig. xx6. — Diagrams showing the essential facts in the maturation of the egg. The somatic number of chromosomes is supposed to be four.

A. Initial phase; two tetrads have been formed in the germinal vesicle. B. The two tetrads have been drawn up about the spindle to form the equatorial plate of the first polar mitotic figure. C. The mitotic figure has rotated into position, leaving the remains of the germinal vesicle at g.v. D. Formation of the first polar body; each tetrad divides into two dyads. E. First polar body formed; two dyads in it and in the egg. F. Preparation for the second division. G. Second polar body forming and the first dividing; each dyad divides into two single chromosomes. H. Final result; three polar bodies and the egg-nucleus (9), each containing two single chromosomes (half the somatic number) ; c. the egg-centrosome which now degenerates and is lost


238 REDUCTION OF THE CHROMOSOMES

A study of the nucleus during these changes brings out the following facts. During the multiplication of the oogonia the number of chromosomes is the same as that occurring in the division of the somatic cells, and the same number enters into the formation of the chromatic reticulum of the germinal vesicle. During the formation of the polar bodies this number becomes reduced to one-half, the nucleus of each polar body and the egg-nucleus receiving the reduced number. In some manner, therefore, the formation of the polar bodies is connected with the process by which the reduction is effected. The precise nature of this process is, however, a matter which has been certainly determined in only a few cases.

We need not here consider the history of opinion on this subject further than to point out that the early observers, such as Purkinje, Von Baer, Bischoff, had no real understanding of the process and believed the germinal vesicle to disappear at the time of fertilization. To Butschli ('76), Hertwig, and Giard ('76, '77) we owe the discovery that the formation of the polar bodies is through mitotic division, the chromosomes of the equatorial plate being derived from the chromatin of the germinal vesicle. 1 In the formation of the first polar body the group of chromosomes splits into two daughter-groups, and this process is immediately repeated in the formation of the second without an intervening reticular resting stage. The egg-nucleus therefore receives, like each of the polar bodies, one-fourth of the mass of chromatin derived from the germinal vesicle.

But although the formation of the polar bodies was thus shown to be a process of true cell-division, the history of the chromosomes was found to differ in some very important particulars from that of the tissue-cells. The essential facts, which were first carefully studied in Ascaris by Van Beneden ('83, '87), and especially by Boveri ('87, 1 ), are in a typical case as follows (Figs. 116, 117): As the egg prepares for the formation of the first polar body, the chromatin of the germinal vesicle groups itself in a number of masses, each of which splits up into a group of four bodies united by linin-threads to form a " quadruple group" or tetrad (Vierergruppe). The number of tetrads is always one-half the usual number of chromosomes. Thus in Ascaris {mcgaloccphala, bivalcns) the germinal vesicle gives rise to two tetrads, the normal number of chromosomes in the earlier divisions being four ; in the mole-cricket there are six tetrads, the somatic number of chromosomes being twelve ; in Cyclops the respective numbers are twelve and twenty-four (one of the most frequent cases); while in Artemia there are eighty-four tetrads and one hundred and sixty 1 The early accounts asserting the disappearance of the germinal vesicle were based on the fact that in many cases only a small fraction of the chromatic network gives rise to chromosomes, the remainder disintegrating and being scattered through the yolk.


GENERAL OUTLINE


eight somatic chromosomes — the highest number thus far accurately counted. As the first polar body forms, each of the tetrads is halved to form two double groups, or dyads, one of which remains in the egg



Fig. 117 A. The egg with the spermaloioiin just e shaped tetrads (only one clearly shown), the been (bur. B. The tetrads seen in profile.


C. First polar body formed, contain) single chromosomes, completing the 1


dyads. H. I. The dyads rotatii lg. A'. Each dyad has divided ir er stages see Fig. 90.)


240


REDUCTION OF THE CHROMOSOMES


while the other passes into the polar body. Both the egg and the first polar body therefore receive each a number of dyads equal to one-half the usual number of chromosomes. The egg now proceeds at once to the formation of the second polar body without previous reconstruction of the nucleus. Each dyad is halved to form two single chromosomes, one of which, again, remains in the egg while its sister passes into the polar body. Both the egg and the second polar body accordingly receive two single chromosomes (one-half the usual number), each of which is one-fourth of an original tetrad group. From the two remaining in the egg a reticular nucleus, much smaller than the original germinal vesicle, is now formed. 1

Primordial germ-cell.


Spermatogonia.


Primary spermatocyte.

Secondary spermatocytes.

Spermatids. Spermatozoa.



Division-period (the number of divisions is much greater).


• Growth-period.


M aturation-period.


Fig. zz8. — Diagram showing the genesis of the spermatozoon. [After Boveri.]

Essentially similar facts have now been determined in a considerable number of animals, though, as we shall presently see, tetradformation is not of universal occurrence, nor is it always of the same type. For the moment we need only point out that the numerical reduction of chromatm-masses takes place before the polar bodies are actually formed, through processes which determine the number of tetrads within the germinal vesicle. The numerical reduction is therefore determined in the grandmother-cell of the egg. The actual divisions by which the polar bodies are formed merely distribute the elements of the tetrads.


1 It is nearly certain that the division of the first polar body (which, however, may be omitted) is analogous to that by which the second is formed, i.e. each of the dyads is similarly halved. Cf. Griffin, '99.


GENERAL OUTLINE 24 1

2. Reduction in the Male. Spermatogettesis

The researches of Platner ('89), Boveri, and especially of Oscar Hertwig ('90, 1) have demonstrated that reduction takes place in the male in a manner almost precisely parallel to that occurring in the female. Platner first suggested ('89) that the formation of the polar bodies is directly comparable to the last two divisions of the sperm mother-cells (spermatocytes). In the following year Boveri reached the same result in Ascaris, stating his conclusion that reduction in the male must take place in the " grandmother-cell of the spermatozoon, just as in the female it takes place in the grandmother-cell of the egg," and that the egg-formation and sperm-formation really agree down to the smallest detail ('90, p. 64). Later in the same year appeared Oscar Hcrtwig's splendid work on the spermatogenesis of Ascaris, which established this conclusion in the most striking manner. Like the ova, the spermatozoa are descended from primordial germ-cells which by mitotic division give rise to the spermatogonia from which the spermatozoa are ultimately formed (Fig. 118). Like the oogonia, the spermatogonia continue for a time to divide with the usual (somatic) number of chromosomes, i.e. four in Ascaris mcgalocephala bivalens. Ceasing for a time to divide, they now enlarge considerably to form spermatocytes, each of which is morphologically equivalent to an unripe ovarian ovum, or oocyte. Each spermatocyte finally divides twice in rapid succession, giving rise first to two daughter-spermatocytes and then to four spermatids, each of which is directly converted into a single spermatozoon. The history of the chromatin in these tzvo divisions is exactly parallel to that in the formation of the polar bodies (Figs. 119, 120). From the chromatin of the spermatocyte are formed a number of tetrads equal to one-half the usual number of chromosomes. Each tetrad is halved at the first division to form two dyads which pass into the respective daughter-spermatocytes. At the ensuing division, which occurs without the previous formation of a resting reticular nucleus, each dyad is halved to form two single chromosomes which enter the respective spermatids (ultimately spermatozoa). From each spermatocyte, therefore, arise four spermatozoa, and each sperm-nucleus receives half the usual number of single chromosomes. The parallel with the egg-reduction is complete.

These facts leave no doubt that the spermatocyte is the morphological equivalent of the oocyte or immature ovarian egg, and that the group of four spermatozoa to which it gives rise is equivalent to the ripe egg plus the three polar bodies. Hertwig was thus led to the following beautifully clear and simple conclusion : " The polar bodies are abortive eggs which are formed by a final process of


242


REDUCTION OF THE CHROMOSOMES


division from the egg-mother-cell (oocyte) in the same manner as the spermatozoa are formed from the sperm-mother-cell (spermatocyte). But while in the latter case the products of the division are all used as functional spermatozoa, in the former case one of the products



Pig. 119. — Diagrams showing the essential facts of reduction in the male. The somatic number of chromosomes is supposed to be four.

A. Ii. Division of one of the spermatogonia, showing the full number (four) of chromosomes. C. Primary spermatocyte preparing for division ; the chromatin forms two tetrads. D. E. F. First division to form two secondary spermatocytes each of which receives two dyads. G. H. Division of the two secondary spermatocytes to form four spermatids. Each of the latter receives two single chromosomes and a centrosome which passes into the middle-piece of the spermatozoon.


1

of the egg-mother-cell tecomes the egg, appropriating to itself the entire mass of the yolk at the cost of the others which persist in rudimentary form as the polar bodies." l

1 *9o> i> P* I2 & '


GENERAL OUTLINE 243

3. Weismanris Interpretation of Reduction

Up to this point the facts are clear and intelligible. Before coming to closer quarters with them it will be useful to make a digression in order to consider some of the theoretical aspects of reduction ; though the reader must be warned that this will lead us into very uncertain ground traversed by a labyrinth of conflicting hypotheses from which no exit has yet been discovered.

The process of reduction is very obviously a provision to hold constant the number of chromosomes characteristic of the species ; for if it did not occur, the number would be doubled in each succeeding generation through union of the germ-cells. 1 A number of writers have contented themselves with this simple interpretation, Oscar Hertwig, for example, regarding reduction as " merely a process to prevent a summation through fertilization of the nuclear mass and of the chromatic elements." 2 A moment's reflection reveals the entire inadequacy of such an explanation. As far as the chromatin-mass is concerned, it does not agree with the facts ; for in reduction with tetrad-formation the chromatin-mass is reduced not to one-half, but to one-fourth. That reduction must mean more than mere mass-reduction is moreover proved by the fact that the bulk of the nucleus may enormously increase or decrease at different periods in the same cell, irrespective of the number of chromosomes. The real problem is why the number of chromosomes should be held constant. The

1 Of the many earlier attempts to interpret the meaning of the polar bodies, we need only consider at this point the very interesting suggestion of Minot ('77), afterward adopted by Van Beneden ('83), that the ordinary cell is hermaphrodite, and that maturation is for the purpose of producing a unisexual germ-cell by dividing the mother-cell into its sexual constituents, or "genoblasts." Thus, the male element is removed from the egg in the polar bodies, leaving the mature egg a female. In like manner he believed the female element to be cast out during spermatogenesis (in the " Sertoli cells "), thus rendering the spermatozoa male. By the union of the germ-cells in fertilization, the male and female elements are brought together so that the fertilized egg or oosperm is again hermaphrodite or neuter. This ingenious view was independently advocated by Van Beneden in his great work on Ascaris ('83). A fatal objection to it, on which both Strasburger and Weismann have insisted, lies in the fact that male as well as female qualities are transmitted by the egg-cell, while the sperm-cell also transmits female qualities. The germ-cells are therefore non-sexual. The researches of many observers show, moreover, that all of the four spermatids derived from a spermatocyte become functional spermatozoa. Minot's hypothesis must, therefore, in my opinion, be abandoned.

Balfour doubtless approximated more nearly to the truth when he said, " In the formation of the polar cells part of the constituents of the germinal vesicle, which are requisite for its functions as a complete and independent nucleus, is removed to make room for the supply of the necessary parts to it again by the spermatic nucleus" ('80, p. 62). He fell, however, into the same error as Minot and Van Beneden in characterizing the germ-nuclei as " male " and "female"; and, as shown at pages 194, 353, it has been found that a single germnucleus is able to carry out development of an embryo without union with another.

2 '90, I, p. 1 12. Cf. Hartog, '91, p. 57.


„, REDUCTION OF THE CHROMOSOMES

o. k ,. :icaiiin^ of the phenomena was first seriously considered by »c-«a.i.i;i in his essays of 1885 and 1887 ; and, although his conclun-'o* *«•« 01 j. highly speculative character, they nevertheless gave so



ffg. i»o. — Reduction in the spermatogenesis of Aicaril mtgafocephiila.vai.6ii aitn

A-tl. Successive stages in the division of the primary sperm atocyte. The onxin.n: telicul umkrxocs a very early division of the chromatin -pa nulrs which then form a doubly split spim HuviUl, /'. This shortens ((.'),. ind breaks in two to form (he two tetrads (D in profile. £ viev endwise). F. G. H. First division to form two secondary spermatocytes, each rrcciring t«o dya /. Secondary spermatocyte. J, K. The same dividing. L. Two resulting spermatids, each « 


great a stimulus to the study of the entire problem that his views deserve special attention. Weismann's interpretation was based on a remarkable paper published by Wilhelm Roux in 1883,' in which are

1 For division of the spermatogonia see Fig. 55; for the corresponding phf univalent see Fig. 148.

  • Ubtr Jit BiJeutung Jer Kirnthtilungsfigurin.


GENERAL OUTLINE 245

developed certain ideas which afterward formed the foundation of Weismann's whole theory of inheritance and development. Roux argued that the facts of mitosis are only explicable under the assumption that chromatin is not a uniform and homogeneous substance, but differs qualitatively in differerit regions of the nucleus ; that the collection of the chromatin into a thread and its accurate division into two halves is meaningless unless the chromatin in different regions of the thread represents different qualities which are to be divided and distributed to the daughter-cells according to some definite law. He urged that if the chromatin were qualitatively the same throughout the nucleus, direct division would be as efficacious as indirect, and the complicated apparatus of mitosis would be superfluous. Roux and Weismann, each in his own way, subsequently elaborated this conception to a complete theory of inheritance and development, but at this point we may confine our attention to the views of Weismann. The starting-point of his theory is the hypothesis of De Vries that the chromatin is a congeries or colony of invisible self-propagating vital units or biophorcs somewhat like Darwin's "gemmules" (p. 12), each of which has the power of determining the development of a particular quality. Weismann conceives these units as aggregated to form units of a higher order known as " determinants," which in turn are grouped to form " ids," each of which, for reasons that need not here be specified, 1 is assumed to possess the complete architecture of the germ-plasm characteristic of the species. The "ids" finally, which are identified with the visible chromatin-granules, are arranged in linear series to form " idants " or chromosomes. It is assumed further that the " ids " differ slightly in a manner corresponding with the individual variations of the species, each chromosome therefore being a particular group of slightly different germ-plasms and differing qualitatively from all the others.

We come now to the essence of Weismann's interpretation. The end of fertilization is to produce new combinations of variations by the mixture of different ids. Since, however, their number, like that of the chromosomes which they form, is doubled by the union of two germ-nuclei, an infinite complexity of the chromatin would soon arise did not a periodic reduction occur. Assuming, then, that the " ancestral germ-plasms" (ids) are arranged in a linear series in the spiremethread or the chromosomes derived from it, Weismann ventured the prediction ('87) that two kinds of mitosis would be found to occur. The first of these is characterized by a longitudinal splitting of the thread, as in ordinary cell-division, " by means of which all the ancestral germ-plasms are equally distributed in each of the daughter-nuclei after having been divided into halves." This form of division, which

1 Cf. the Germ-plasm, p. 60.


246 REDUCTION OF THE CHROMOSOMES

he called equal division (Aequationstheilung), was then a known fact. The second form, at that time a purely theoretical postulate, he assumed to be of such a character that each daughter-nucleus should receive only half the number of ancestral germ-plasms possessed by the mother-nucleus. This he termed a reducing division (Reduktionstheilung), and suggested that this might be effected either by a transverse division of the chromosomes, or by the elimination of entire chromosomes without division. 1 By either method the number of " ids M would be reduced ; and Weismann argued that such reducing divisions must be involved in the formation of the polar bodies, and in the parallel phenomena of spermatogenesis.

The fulfilment of Weismann's prediction is one of the most interesting results of recent cytological research. It has been demonstrated, in a manner which seems to be incontrovertible, that the reducing divisions postulated by Weismann actually occur, though not precisely in the manner conceived by him. Unfortunately for the general theory, however, transverse divisions have been certainly determined in only a few types, while in others, of which Ascaris is the best-known example, the facts thus far known seem clearly opposed to the assumption. On the whole, the evidence of reducing divisions, i.e. such as involve a transverse and not a longitudinal division of the chromatin-thread, has steadily increased ; but it remains quite an open question whether they have the significance attributed to them by Weismann.

B. Origin of the Tetrads

I. General Sketch

In considering the origin of the tetrads or their equivalents, it should be borne in mind that true tetrad-formation, as described above, has only been certainly observed in a few groups (most clearly in the nematodes and arthropods). But even in cases where the chromatin does not condense into actual tetrads these bodies are represented by chromosomes in the form of rings, crosses, and the like, which are closely similar, and doubtless equivalent, to those from which actual tetrads arise, and present us with the same problems. With a few apparent exceptions, described hereafter, the tetrads of their equivalents always arise by a double division of a single primary chromatin-rod or mass. Nearly all observers agree further that the number of primary rods at their first appearance in the germinal vesicle or in the spermatocyte-nucleus is one-half the usual number of chromosomes, and that this numerical reduction is due to the fact that the spireme-thread segments into one-half the

1 Essay VI., p. 375.


ORIGIN OF THE TETRADS


247


/



5 6

7

a



usual number of pieces. Apparently, however, there are two radically different types of tetrad-formation as follows.

In the first type the tetrad arises by one longitudinal and one transverse division of each primary ckromatin-rod, the latter effecting the reduction demanded by _

Weismann , shypothesis(Fig. /\ £j Ls U

I2i,l). To give the usual graphic representation, let us, for the sake of discussion, assume the somatic number of chromosomes to be four, designating the spireme-thread as a b c d, each letter representing a chromosome, each of which we may in turn assume to consist of a series of four granulesor "ids "(Fig. 121). In ordinary mitosis the spireme would segment into a — b — c — d t which then would divide lengthwise to form pairs of identical sister

, abed chromosomes — •

abed To form the tetrad, on the other hand, the spireme first segments into two rods ab and cd, each of which, in view of its subsequent history, may be regarded as

bivalent, representing tWO Pig. 121. — Diagrams of tetrad-formation; I, with

chromosomes United end to one transverse an d one longitudinal division (copepod

, __ .. type); II. with two longitudinal divisions (Ascaris type).

end (VOm Rath, RUckert, A _ D succes sive stages; chromatin-granules num Hacker). Eagh of these beredfrom 1 to 8. The two types diverge at C. In D

divides once longitudinally, giving the identical pairs or

dyads — , and once transversely, giving the tetrads

ab cd a


U


8



ab ab


ab ab


the granules of each constituent of the tetrad fuse to form a homogeneous sphere.


a


b b


d m d


Inspection of Fig. 121, 1, shows that through the second or transverse division, each member of the tetrad receives only half the number of ids contained in the original segment. This number, four, is the same as that assumed for a single chromosome ; and, since each of the two tetrads contributes one chromosome to the germ-cell, the latter receives


248 REDUCTION OF THE CHROMOSOMES

but half the usual number both of chromosomes and of ids. This mode of tetrad-formation has been most clearly demonstrated in insects and copepods, and an equivalent process occurs also in mollusks, annelids, turbellarians, and some other animals, as described beyond. In the second type, illustrated especially by Ascaris, the tetfad is apparently formed by two longitudinal divisions of each primary chromatin-rod, and no reducing division occurs. If, therefore, we adopt the same terminology as before, we have first ab and cd, then

ab cd

ab cd


ab cd j r n ab — , and finally


cd

— , by two longitudinal divisions. In

cd


ab cd ab

this case, according to Brauer's careful studies, each chromatin-granule (" id") divides at each longitudinal division of the primary rod. The four chromosomes of the tetrad are therefore exactly equivalent, being derived from the same region of the spireme-thread, and containing the undiminished number of " ids " (Fig. 121, II).

The contradiction may be stated in a different way. In the first type of tetrad formation, the number both of granules and of chromosomes is first doubled (i.e. in the assumed case, through the formation of two tetrads, each consisting of four chromosomes, or eight in all), and then reduced to half that number by the two successive maturation-divisions. In the second type, on the other hand, the number of chromosomes is likewise doubled, but that of the granules is quadrupled, so that, although in both types the two maturation-divisions reduce the number of chromosomes to one-half, only in the first type do they reduce the number of granules or " ids," as Weismann's hypothesis demands. We must therefore distinguish sharply between the reduction of the chromosomes and that of the "ids." The former is primarily effected by the segmentation of the primary spiremethread, or the resolution of the nuclear reticulum, into one-half the usual number of segments (i.e. the "pseudo-reduction" of Riickert); and here the real secret of the reduction of the chromosomes lies. The reduction of the "ids," if they have any real existence, is a distinct, and as yet unsolved, question.

2. Detailed Evidence

We may now consider some of the phenomena in detail, though the limits of this work will only allow the consideration of a few typical cases.

(a) Tetradfonnation with one Longitudinal and one Transverse Division. — In many of the cases of this type the tetrads arise from ring-shaped bodies which are analogous to the ring-shaped chromosomes occurring in heterotypical mitosis (p. 86). First observed by HenkingCgi) in Pyrrhocoris, tetrad-origin of this type has since been found in other insects by Vom Rath, Toyama, Paulmier, and others,


ORIGIN OF THE TETRADS


249


in copepods by Riickert, Hacker, and Vom Rath, in pteridophytes by Calkins and Osterhout, in the onion, Allium, by Ishikawa, and in various other forms where their history has been less clearly made out. The genesis of the ring was first determined by Vom Rath in the mole cricket {Gryllotalpa, '92), and has been thoroughly elucidated by the later work of Riickert ('94), Hacker ('95, 1), and Paulmier ('99). All these observers have reached the same conclusion ; namely, that the ring arises by the longitudinal splitting of a primary chro matin-rod, the two halves remaining united by their ends, and opening out to form a ring. The ring-formation is, in fact, a form of



ontogenesis of the mole-cricket


Pig. 132. — Origin of the tetrads by ting- formation Gryllotalpa. [VOM RATH.]

somes united end to end anrl longitudinally split cxct-pl at Ihc free ends. B. C. Opening out of (he double rods to form rings. I), Concentration of the rings. E. The rings broken up into letrads. /'. First division -figure established.

heterotypical mitosis (p. 86). The breaking of the ring into four parts involves, first, the separation of these two halves (corresponding with the original longitudinal split), and second, the transverse division of each half, the latter being the reducing division of Weismann. The number of primary rods, from which the rings arise, is one-half the somatic number. Hence each of them is conceived by Vom Rath, Hacker, and Riickert as bivalent or double ; i.e. as representing two chromosomes united end to end. This appears with the greatest clearness in the spermatogenesis of Gryllotalpa (Fig. 122). Here


2 SO


REDUCTION OF THE CHROMOSOMES


the spireme-thread splits lengthwise before its segmentation into rods. It then divides transversely to form six double rods (half the usual number of chromosomes), which open out to form six closed rings. These become small and thick, break each into four parts, and thus



Fif. 113.— Formation of the tetrads and polar bodies in Cyclops, slightly si full number of tetrads is no! shown.) [Rt'c:KKKT.]

A. Germinal vesicle containing eight longitudinally split chromatin-rods (half the number). II. Shortening of the rods; transverse division (to furm the tetrads) in C. Position of the tetrads in the first polar spindle, the longitudinal split horizontal, phase: longitudinal divisions of the tetrads. E. The first polar liody formed; seco spindle with the eight dyads in position fur the ensuing division, which will be a trait


9. Ana


give rise to six typical tetrads. An essentially similar account of the ring-formation is given by Vom Rath in Euchceta and Calanus, and by Riickert in Hetcrocopc and Diaptomus.

That the foregoing interpretation of the rings is correct, is beautifully demonstrated by the observations of Hacker, and especially of


ORIGIN OF THE TETRADS


251


Ruckert, on a number of other copepods {Cyclops, Canthocamptus\ in which rings are not formed, since the splitting of the primary chromatin-rods is complete. The origin of the tetrads has here been traced with especial care in Cyclops strcnuus y by Ruckert C94), whose observations, confirmed by Hacker, are quite as convincing as those



a





Pig. 134. — Diagrams of various modes of tetrad-formation. [HACKER.]

a. Common starting-point, a double spireme-thread in the germinal vesicle ; d. common result, the typical tetrads ; b. c. intermediate stages : at the left the ring-formation (as in Diaptomus, Gryllotalpa, Heterocopt) ; middle series, complete splitting of the rods (as in Cyclops according to Ruckert, and in Canthocamptus) ; at the right by breaking of the V-shaped rods (as in Cyclops strenuus, according to Hacker.


of Brauer on Ascaris, though they led to a diametrically opposite result.

The normal number of chromosomes is here twenty-two. In the germinal vesicle arise eleven threads, which split lengthwise (Fig. 123), and finally shorten to form double rods, manifestly equivalent to the closed rings of Diaptomus. Each of these now segments transversely


252


REDUCTION OF THE CHROMOSOMES


to form a tetrad group, and the eleven tetrads then place themselves in the equator of the spindle for the first polar body (Fig. 123, C), in such a manner that the longitudinal split is transverse to the axis of the spindle. As the polar body is formed, the longitudinal halves of the tetrad separate, and the formation of the first polar body is thus demonstrated to be an "equal division" in Weismann's sense. The eleven dyads remaining in the eggs now rotate (as in Ascaris),



B. Later stage of the same, condensation and segmentation of the rings. [ROcKERT.]

C. " fyefops tlnauus," illustrating Hacker's account of the tetrad-formation from elongate double rods; a group of " accessory nucleoli." [H ACKER.]

/). Germinal vesicle of an annelid (Ophryotrocka} showing nucleolus and four chromosomes. [K.ORSCHKLT.]

so that the transverse division lies in the equatorial plane, and are halved during the formation of the second polar body. The division is accordingly a " reducing division," which leaves eleven single chromosomes in the egg. Paulmier's work on Anasa and other Hemiptera ('99) gives the same result as the above in regard to the origin of the tetrads (Figs. 126, 127). The process is, however, slightly complicated by the fact that no continuous spi rem e-th read is formed, while the rings are often bent or twisted and never open out to a


ORIGIN OF THE TETRADS 253

circular form. They finally condense into true tetrads which are successively divided into dyads and monads by the two divisions; but it is an interesting fact that the order of division occurring in the copepods appears here to be reversed, the first division being the transverse and the second the longitudinal one — a result agreeing with Henking's earlier conclusion in the case of Pyrrochoris. Osterhout C97) and Calkins C97) independently discovered tetrads in the vascular cryptogams (Equisctutn, Pteris), and the last-named observer finds that in Pteris they may arise either from rings, as in Gryllotalpa or Heterocope, or from double rods as in Cyclops, the halves in the latter case being either parallel or forming a cross. This longitudinal split, occurring in the spireme, is followed by a transverse division by which the tetrad is formed. Tetrads having an essentially similar mode of origin are also described by Atkinson ('99) in Ariscema, and tetrad-formation is nearly approached in Allium according to Ishikawa C99). 1 These cases are considered at page 263.

Re'sume'. In all the foregoing cases the tetrads arise from a spireme which splits lengthwise, segments into one-half the somatic number of rods (each longitudinally divided) and each of the latter divides transversely to form the tetrad. When the ends of the daughter-chromosomes resulting from the longitudinal split remain united (as in insects) ring-forms result, and the earlier phases of tetradformation are thus identical with those of heterotypical mitosis. When the split is complete, so that the ends remain free, double rods result; while, if the daughter-chromosomes remain temporarily united at the middle or at the end, X-, Y-, and V-shaped figures may arise. In all these forms tetrad-formation is completed by the complete separation of the daughter-rods, the transverse division of each in the middle, and the condensation of the four resulting bodies into a quadruple mass. As will be shown in Section C (p. 258) the transverse division is in many forms delayed until after separation of the longitudinal halves. In such cases no actual tetrads are formed, though the result is the same.

(b) Second Type. Tetrad-formation zuit/t two Longitudinal Divisions. — The only accurately known case of this type is Ascaris y the object in which tetrads were first discovered by Van Beneden in 1883. Carnoy ('86, 2) reached the conclusion that the tetrads in some other nematodes (Op//iostomum t Ascaris clavata, A. lumbricoides) arose by a double longitudinal splitting of the primary chromatin-rods.

1 Vom Rath ('93, '59) has endeavoured to show that a process involving the formation of true tetrads occurs in the salamander and the frog, but the later and more accurate studies of Meves ('96) seem to leave little doubt that this was an error, and that the tetrads observed in these forms are not of normal occurrence, as Hemming ('87) had earlier concluded. €/. p. 259.


254


REDUCTION OF THE CHROMOSOMES


In the first of his classical celt-studies Boveri ('87, 1) reached the same result through a careful study of Ascaris megalocephala, showing that each tetrad appears in the germinal vesicle in the form of four parallel rods, each consisting of a row of chrom at in -granules (Fig. 117, A-C). He believed these rods to arise by the double longitudinal splitting of a single primary chromatin- rod, each cleavage being a



A. Resting spermatogonium with single plasmosome ; irial plate of dividing s]>ermatogonium : twenty large a [jcrmatogonium- division. /'-/. Prophases of first malt; ingle chromalin-ilucleolus. /'. Segmented split spireme. /. /. Concentration of the rings to form tetrads.


nam. [PAULMIER.]

1 two chromatin-nucleoli. B. Equatwo small chromosomes. C. Final lion-division. D. £. Synapsis, with C. H. Formation of the tetrad-rings.


preparation for one of the polar bodies. In his opinion, therefore, the formation of the polar bodies differs from ordinary mitosis only in the fact that the chromosomes split very early, and not once, but twice, in preparation for two rapidly succeeding divisions without an intervening resting period. He supported this view by further observations in 1890 on the polar bodies of Sagitta and several gasteropods, in which he again determined, as he believed, that the tetrads


ORIGIN OF THE TETRADS


255


arose by double longitudinal splitting. An essentially similar view of the tetrads was taken by Hertwig in 1890, in the spermatogenesis of Ascaris, though he couid not support this conclusion by very convincing evidence. In 1893, finally, Brauer made a most thorough and apparently exhaustive study of their origin in the spermatogenesis of Ascaris, which seemed to leave no doubt of the correctness of Boveri's result. Every step in the origin of the tetrads from the reticulum of the resting spermatocytes was traced with the most painstaking care. In the early prophases of the first division the nuclear reticulum breaks up more or Jess completely into granules, which



i8 iH

Fig. 137. — Maturat ion-divisions in an insect, Anaia. [Paui.mier.] A. Primary spermatocyte in metaphase. B. Equatorial plate, showing ten large tetrads and one small one ; " odd chromosome " al o, C. Separation of the dyads. D. Telophase, which is also a prophase of the second division. E. Secondary spermatocyte; division of the dyads; small dyad shown undivided. F. Final anaphase ; small dyad near the lower chromosome-group. - (The figures are numbered from left to right. For later stales, see Fig. 8a.)

become in part aggregated in a mass at one side of the nucleus {"synapsis," p. 276), from which delicate threads extend through the remaining nuclear space (Fig. 120, A). Even at this period the granules of the threads are divided into four parts. As the process proceeds the chromatin resolves itself into a single spireme-thread, consisting of four parallel rows of granules, which break in two to form the two tetrads (var. diva/ens), or is directly converted into a single tetrad (var. utiivalens) (Fig. 120). From these observations Brauer concludes that each tetrad arises from a rod, doubly split lengthwise by a process initiated at a very early period through the


256 REDUCTION OF THE CHROMOSOMES

double fission of the chromatin-granules. If this be correct, there can be no reduction in Wcismann's sense ; for the four products of each primary chromatin-granule are equally distributed among the four daughter-cells. A similar conclusion, based on much more incomplete evidence, was reached by Brauer ('92) in the phyllopod Branchipus.

Brauer's evidently conscientious figures very strongly sustain his conclusion, which, reinforced by the earlier work of Hertwig and Boveri, has until now seemed to rest upon an unassailable basis. The recent work of Sabaschnikoff ('97) nevertheless raises the possibility of a different interpretation. Brauer himself justly urges that the essence of the process lies in the double fission of the chromatingranules to which the formation of chromosomes is secondary. 1 Everything, therefore, turns on the manner in which the quadruple granules arise ; and Sabaschnikoff's work gives some ground for the view that they may arise, not by a double fission, but in some other way.

According to this author there is a period (in the oogenesis) at which the nuclear threads wholly disappear, the entire chromatin being broken up into granules. From this state the granules emerge in quadruple form to arrange themselves in the doubly split spireme exactly as Brauer describes : and a few observations are given (regarding the size and arrangement of the granules) which suggest the possibility that the quadruple granules may arise by the conjugation either of four separate granules or of two pairs of double granules. Since there is ground for the view that tetrads may arise by the conjugation of chromosomes (see following section), there is no a priori objection to such a conclusion. Could it be sustained, the maturationdivisions of Ascaris would in fact involve a true reduction in Weismann\s sense ; for despite the fact that the chromosomes are only longitudinally divided, the four longitudinal constituents of each tetrad would not be equivalent with respect to the granules, and it is the reduction of the latter (" ids ") that forms the essence of Weismann's hypothesis (p. 245). Another consideration, suggested to me by Professor T. H. Morgan, opens still another possibility, which seems well worthy of test by further research. As already stated (p. 88). the long chromosomes of Ascaris are plurivalcnt, since in all but the germ-cells each breaks up into a much larger number of smaller chromosomes (Fig. 73, p. 148). If, therefore, the latter correspond to the chromosomes of other forms in which tetrads occur (c.%. Cyclops or Ar/emia), the so-called "tetrad" of Ascaris is a compound body: and the true process of reduction must be sought in the origin of the smaller elements of which it is composed, which are, perhaps, directly comparable with Sabaschnikoff's i; granules." Until the questions thus opened have been further studied, the case for Ascaris must remain open : and it is perhaps worth suggesting that a new point of view may here be found for further study also of reduction in the vertebrates. 2

1 Cf. p. 113.

2 Bodies closely resembling tetrads arc sometimes formed in mitosis, where no reduction should occur. Thus, R. Hertwig ('95) has observed tetrads in the first cleavage-spindle of echinoderm-eggs after treatment with dilute poisons (p. 306). Klinckowstr8m figures them in the second polar spindle of Prostheccraus eggs, while Moore ('95) describes in the elasmobranchs small ring-shaped chromosomes, not only in the first but also in the second spermatocyte- divisions, concluding that no reduction occurs in either division.


ORIGIN OF THE TETRADS 2 $7

(c) The Formation of Tetrads by Conjugation. — A considerable number of observers have maintained that reduction may be effected by the union or conjugation of chromosomes that were previously separate. This view agrees in principle with that of Riickert, Hacker, and Vom Rath ; for the bivalent chromosomes assumed by these authors may be conceived as two conjugated chromosomes. It seems to be confirmed by the observations of Born and Fick on Amphibia and those of Riickert on selachians {Pristiurus) ; for in all these cases the number of chromatin-masses at the time the first polar body is formed is but half the number observed in younger stages of the germinal vesicle. In Pristiurus there are at first thirtysix double segments in the germinal vesicle. At a later period these give rise to a close spireme, which then becomes more open, and is found to form a double thread segmented into eighteen double segments ; i.e. the reduced number. In this case, therefore, the preliminary pseudo-reduction is almost certainly effected by the union of the original thirty-six double chromosomes, two by two. The most specific accounts of such a mode of origin have, however, been given by Calkins (earthworm) and Wilcox (grasshopper). The latter author asserts ('95) that in Caloptcnus the spireme of the first spermatocyte gives rise without longitudinal division to twenty-four chromosomes (double the somatic number). These then become associated in pairs, and still later the twelve pairs conjugate two and two to form six tetrads. There is, therefore, no longitudinal splitting of the chromosomes. The a priori improbability of such a conclusion is increased by the studies of Paulmier on the Hemiptera, which demonstrate the occurrence of a longitudinal division in a number of these forms and confirm the original studies of Vom Rath on Gryllotalpa}

The second case, which is perhaps better founded, is that of the earthworm (Lumbricus terrcstris), as described by Calkins ('95, 2), whose work was done under my own direction. Calkins finds that the spireme splits longitudinally and then divides transversely into 32 double segments. These then unite, two by two, to form 16 tetrads. The 32 primary double segments therefore represent chromosomes of the normal number that have split longitudinally,

i.e. — -p etc., and the formula for a tetrad is - , or — -. Such a b a o a\x

a tetrad, therefore, agrees as to its composition with the formulas

of Hacker, Vom Rath, and Riickert, and agrees in mode of origin

with the process described by Riickert in the eggs of Pristiurus.

While these observations are not absolutely conclusive, they never 1 Montgomery, who has denied the occurrence of a longitudinal division in Pentatoma ('98, i), has subsequently found such a division in the nearly related if not identical genus Euchistis ('99). s


25 8 REDUCTION OF THE CHROMOSOMES

theless rest on strong evidence, and they do not stand in actual contradiction of what is known in the copepods and vertebrates. The possibility of such a mode of origin in other forms must, I think, be held open.

Under the same category must be placed Korschelt's unique results in the egg-reduction of the annelid Ophryotrocha ('95), which are very difficult to reconcile with anything known in other forms. The typical somatic number of chromosomes is here four. The same number of chromosomes appear in the germinal vesicle (Fig. 125, D). They are at first single, then double by a longitudinal split, but afterward single again by a reunion of the halves. The four chromosomes group themselves in a single tetrad, two passing into the first polar body, while two remain in the egg, but meanwhile each of them again splits into two. Of the four chromosomes thus left in the egg two are passed out into the second polar body, while the two remaining in the egg give rise to the germ-nucleus. From this it follows that the formation of the first polar body is a reducing division — a result which agrees with the earlier conclusions of Henking on Pyrrochoris, and with those of Paulmier on the Hemiptera.


C. Reduction without Tetrad-formation

As already stated (p. 246), the formation of actual tetrads is of relatively rare occurrence, being thus far certainly known only in the arthropods, nematodes, and some annelids. In the greater number of cases the two divisions of the primary chromatin-masses {i.e. of the primary oocyte or spermatocyte) are separated by a considerable interval, during which the first maturation cell-division takes place or is initiated, and hence no actual tetrads are formed. This obviously differs only in degree from tetrad-formation, the latter occurring only when the two divisions are simultaneous or occur in rapid succession.

In the cases now to be considered the length of the pause between the maturation-divisions varies considerably, and in some forms (vertebrates, flowering plants) it is so prolonged that the nucleus is partially reconstructed. In all, or nearly all, these cases the first maturationdivision is of the hetcrotypical form, the chromosomes having the form of rings and arising by a process that agrees in most of its features with that leading to tetrad-formation. There is here, however, exactly the same contradiction of results as in the case of tetrad-formation described at page 247, and a bewildering confusion of the subject still exists. In brief, it may be stated that most observers of reduction of this type in the lower animals (flat-worms, annelids, mollusks) have found one transverse and one longitudinal, division ; most of those


REDUCTION WITHOUT TETRAD-FORMATION


259


who have studied the vertebrates find two longitudinal divisions; while opinion regarding the plants is still divided.

{a) Animals. — In the gephyrean Thalassema and the mollusk Zirp/uea (Figs. 128-130) Griffin ('99) finds that the rings, arising as described above, place themselves in the equator of the spindle with the longitudinal division in the equatorial plane. They are then drawn out toward the spindle-poles from the middle point, first assuming the form of a double cross, then of elongated ellipses, and finally break into two daughter-U's or -Vs. The first division is therefore longitudinal. During the late anaphase the V's break at the apex, the two limbs come close together, so as to give the decep


A




B




Fig. 128. — Diagrams of reduction in the types represented by Thalassema (A) and Solamandra (/?). In both the first division is heterotypical. The second division (6) is transverse in the first and longitudinal in the second.

tive appearance of a longitudinal split, and are separated by the second division (following immediately upon the first without intervening resting stage). The latter is therefore a transverse division (Fig. 130). An essentially similar result, though less completely worked out, is independently reached by Bolles Lee (*97) in Helix ; by Klinckowstrom ('97) in the turbellarian Prosthccerceus ; and by Francotte('97)and Van der Stricht ('98, 1) in Thysanzoon. Klinckowstrom shows that there is much variation in the way in which the rings open out and break apart, though the result is the same in all. In case of the vertebrates, Flemming ('87) long since described and figured typical tetrads in the salamander, but regarded them as "anomalies." Vom Rath's later conclusion ('93, '95) that they are


260 REDUCTION OF THE CHROMOSOMES

normal tetrads has not been sustained by the still more recent work of Mevcs ('96), whose careful studies, together with those of Moore, Lenhossek, and others, thus far give no evidence of tetrad-formation, and seem opposed to the occurrence of reducing divisions in the vertebrates. Meves's work in the main confirms the earlier results of Flemming, except that he shows that, as in so many other animals, only two generations of spermatocytes exist. At the first division the nuclear reticulum resolves itself into twelve (the reduced number) segments, which split lengthwise, the halves remaining united to form elongated rings (Figs. 27, 37). These do not, however, con


tain

A. A few moments after entrance of the spcrmatoii forming. B. Early prophase of first polar mitosis with c wall. D, Cenrralspindleeslablished; elimination of nuclei laler stage viewed from above. F. First polar spindle est


dense into tetrads, but break apart during the first division at the points corresponding with the ends of the united halves. The first div'^ionSs th ere fore an equation -division. As the V-shaped halves separate theya&ain split lengthwise (Fig. 131), each of the secondary spermatocytes receiving twelve double V's or dyads. In the telophases and ensuing renting stage, however, all traces of this splitting are lost, the nuclei partially returning to the resting stage, but retaining traces of a spireme-like arrangement (Fig. 131). In the second division twelve double V's reappear, showing a longitudinal division which Flemming and Meves believe to be directly related to that


REDUCTION WITHOUT TETRAD-FORMATION


seen during the foregoing anaphases. There is therefore no evidence of a transverse division. McGregor ('99) describes a nearly similar process in Amphiuma, where the longitudinal division of the



Pig- >30. — Maturalion in Ihe lamellibranch Zitfhaa 4-E, Zirfkaa; F-I, Thalaiitma.


in Thalaiitma. [GUI


A. Unfcrtiliied egg. ring-shaped and cross-shaped chromosomes. B, Prophase of first polar mitosis. C. First polar spindle ; double crosses. D. Slightly later stage. E. The double crosses have broken apart (equation-division). G. Ensuing stage; daughter-V's broken apart at the apex. H. Telophase of first, early prophase of second, division; limbs of the v's separate bul closely opposed. F. Later prophase of second division. /. Second polar spindle in metaphase.

daughter-V's is seen with the greatest clearness throughout the anaphases.

The weak point in both the foregoing cases is the fact that all traces of the second longitudinal division are lost during the ensuing


REDUCTION OF THE CHROMOSOMES


resting period ; and I do not think that even the observations of Flemming ('97), who has published the fullest evidence in the case, completely establish the occurrence of a subsequent longitudinal divi


ng. 131. — {Compare Fig. 17). Maturation-division! in Salamc the others from Meves.]

A. First division in metap base, showing heterotype rings, B. Anaphase; longiim ting of the daughter-loops. C. Telophase. D. Ensuing pause. E. Early prophase division with longitudinally divided segmented spireme. F. Later prophase. G. Mel second division.


[/; from Flrmminc,


sion of the chromosomes in the second mitosis. In Desmognathns, however, where the resting stage is less complete, Kingsbury ('99) finds the longitudinal split in the persistent chromosomes of the


REDUCTION WITHOUT TETRAD-FORMATION 263

pause following the first division; and he believes this to be the same division as that seen during the anaphase. Carnoy and Le Brun ('99) reach the same result in the formation of the polar bodies in Triton, though their general account of the heterotypical mitosis differs very considerably from that of other authors, the rings being stated to arise by a double instead of a single longitudinal split These observers describe the rings of the early anaphase as having almost exactly the same double cross-form as those in Thalassema or Zirphcea (Griffin, '99), but believe them to arise in a manner nearly in accordance with Strasburger's abandoned view of 1895, 1 and with Guignard's ('98, 2) and Gr^goire's ('99) latest results on the flowering plants, the ring being stated to arise by a double longitudinal splitting, as explained at page 265.

In the elasmobranch Scyllium Moore C95) finds twelve (the reduced number) ring-shaped chromosomes at the first division. These closely resemble tetrads ; but a resting stage follows, and the second division is likewise stated to be of the heterotypical form. Both divisions are stated to be equation-divisions — a conclusion well supported in case of the first, but so far from clear in the second that a careful reexamination of the matter is highly desirable.

In mammals the first division is of the heterotypical form (Hermann, '89, Lenhossek, '98), though the rings are much smaller than in the salamander, recalling those seen in arthropods. No true tetrads are, however, formed, and the two divisions are separated by a resting period. The character of the second division is undetermined, though Lenhossek believes it to be heterotypical, like the first.

(6) Plants. — It is in the flowering plants, where reduction likewise occurs, as a rule, without true tetrad-formation, that the contradiction of results reaches its climax ; and it must be said that until further research clears up the present confusion no definite result can be stated. The earlier work of Strasburger and Guignard indicated that no reducing division occurred, the numerical reduction being directly effected by a segmentation of the spireme-thread into half the somatic number of chromosomes. Thus these observers found in the male that the chromosomes suddenly appeared in the reduced number (twelve in the lily, eight in the onion) at the first division of the pollen-mother-cell, and in the female at the first division of the mother-cell of the embryo-sac. The subsequent phenomena differ in a very interesting way from those in animals, owing to the fact that the two maturation-divisions are followed in the female by one and in the male by two or more additional divisions, in both of which the reduced number of chromosomes persists. In the male the two maturation-divisions give rise to four pollen-grains, in the female to

1 Cf. p. 269.


264 REDUCTION OF THE CHROMOSOMES

the four primary cells of the embryo-sac (Fig. 132); and these two divisions undoubtedly correspond to the two maturation-divisions in animals. In the female, as in the animals, only one of the four resulting cells gives rise to the egg, the other three corresponding to the polar bodies in the animal egg, though they here continue to divide, and thus form a rudimentary prothallium. 1 The first-men


Plg. 131. — General view of ihe malurat ion-divisions in flowering plants. [MOTHER.] A-C, in the male ; D-F, in Ihe female. A. The two secondary spermatocytes (pollen-mother. cells] just after the first division {Lilium). P. Final anaphase of second division [Podophyllum).

C. Resulting telophase, which by division of Ihe cytoplasmic mass produces four pollen-grains.

D. Embryo-sac after completion of Ihe first nuclear division {IJIium). E. The same after the second division. F. The upper four cells resulting from the third division (cf. Fig. 106) : 0, ovum ; /, upper polar cell; s, synergidai. (For further details, see Figs. 133, 134.)


1 Of Ihese three cells one divides to form the " synergida?," the other two divide to form three "antipodal sells" (which like Ihe syncrgid.-c finally degenerate) and a "lower polar cell." The latter sooner or later conjugates with the " upper polar cell " (the sister-cell of the egg) to form the " secondary embryo-sac -nucleus," by the division of which Ihe endosperm-cells arise. Of the whole group of eight cells thus arising only the egg contribute*


REDUCTION WITHOUT TETRAD-FORMATION 265

tioned cell, however, does not directly become the egg, but divides once, one of the products being the egg and the other the " upper polar cell" (Fig. 132, F), which contributes to the endosperm-formation (see footnote, and compare page 218).

In the male the two maturation-divisions are in the angiosperms followed by two others, one of which separates a " vegetative " from a " generative " cell, while the second divides the generative nucleus into two definite germ-nuclei. In the gymnosperms more than two such additional divisions take place. In these later divisions, both in the male and in the female (with the exception noted in the footnote below), the reduced number persists, and the principal interest centres in the first two or maturation-divisions. Strasburger and Guignard found in Lilium that while both these divisions differed in many respects from the mitosis of ordinary vegetative cells, neither involved a transverse or reducing division, the chromosomes undergoing a longitudinal splitting for each of the maturation-divisions. Further investigations by Farmer ('93), Belajeff C94), Dixon ('96), Sargant ('96, '97), and others, showed that the first division is often of the heterotypical form, the daughter-chromosomes in the late-metaphase having the form of two V's united by their bases (<>). Despite the complication of these figures, due to torsion and other modifications, their resemblance to the ring-shaped bodies observed in the first maturation-division of so many animals is unmistakable, as was first clearly pointed out by Farmer and Moore ('95).

Botanists have differed, and still differ, widely in their interpretation both of the origin and subsequent history of these bodies upon which the question of reduction turns. According to Strasburger's ('95) first account their origin has nothing in common with that of the tetrad-rings, since they were described as arising by a double longitudinal splitting of a primary rod, the halves then separating first from one end along one of the division-planes, and then from the other end along the other plane, meanwhile opening out to form a ring such as is shown in Fig. 133. (This process, somewhat difficult to understand from a description, will be understood from the diagram, Fig. 135, E-L) The four elements of the ring are then distributed without further division by the two ensuing maturation-divisions ; and the process, except for the peculiar opening out of the ring, is

to the morphological formation of the embryo. It is a highly interesting fact that the number of chromosomes shown in the division of the lower of the two nuclei (i.e. the mothernucleus of the antipodal cells and lower polar-cell) formed at the first division of the embryo-sac-nucleus is inconstant, varying in the lily from 12, 16, 20, to 24 (Guignard, '91, 1), in which respect they contrast with the descendants (egg, synergidre) of the upper nucleus, which always show the reduced number (Mottier, '97, 1), i.e. in Lilium twelve. Thisexception only emphasizes the rule of the constancy of the chromosome-number in general; for these cells are destined to speedy degeneration.


266


REDUCTION OF THE CHROMOSOMES


essentially in agreement with the facts described in Ascaris, and involves no reduction-division. Essentially the same result is reached by Guignard ('98) in his latest paper on Naias, and by Gregoire ('99) in the Liliaceas.

Strasburger twice shifted ground in rapid succession. First ('97, 2), with Mottier ('97, 1 ), he somewhat doubtfully adopted a view agreeing




\F, Strasbubgeb and MOT


e of firs! division ; chromatined. Ft. Slightly later stage (split spireme) in the nucleus r prophase (pollen-mo I her-cel I. Podophyllum) t

F. First maturation-spindle {Fritillaria, male). G. Dive


s (£'


Je).


essentially with the interpretation of Vom Rath, Riickert, etc. (p. 247). The primary rods split once, and bend into a V, the branches of which often come close together, and may be twisted on themselves, thus giving the appearance of the second longitudinal split described in Strasburger's paper of 1895. The two halves of the split U then separate, opening out from the apex, to form the o -figure. In the


REDUCTION WITHOUT TETRAD-FORMATION 267

second division the limbs of the daughter-V's again come close together, remaining, however, united at one end, where they were believed finally to break apart during the second division. The latter was, therefore, regarded as a true reduction-division, the apparent longitudinal split being merely the plane along which the halves of the V come into contact (Fig. 134, C, D).

The two accounts just given represent two extremes, the first agreeing essentially with Ascaris, the second with the copepods or insects. When we compare them with others, we encounter a truly bewildering confusion. Strasburger and Mottier ('97) themselves soon abandoned their acceptance of the reducing division, returning to the conclusion that in both sexes {Lilium, Podophyllum) both divisions involve a longitudinal splitting of the chromosomes (Figs. 133, 134). In the first division the longitudinally split spireme segments into twelve double rods, which bend at the middle to form double V's, with closely approximated halves. Becoming attached to the spindle by the apex, the limbs of each separate to form a -figure. At telophase the daughter-V's shorten, thicken, and join together to form a daughter-spireme consisting of a single contorted thread. This splits lengthwise throughout its whole extent, and then segments into double chromosomes, the halves of which separate at the second division (Fig. 135, L-M). The latter, therefore, like the first, involves no reducing division. This result agrees in substance with the slightly earlier work of Dixon C96) and of Miss Sargant ('96, '97), whose account of the origin of the -figure of the first division differs, however, in some interesting details. It is also in harmony with the general results of Farmer and Moore ('95), of Gr^goire ('99), and of Guignard ('98), who, however, describes the first division nearly in accordance with Strasburger's account of 1895, as stated above. On the other hand, Ishikawa (pollen-mother-cells of Allium, '97) and especially Belajeff (pollen-mother-cells of Iris, '98) conclude that the second division is a true transverse or reducing division. 1 Ishikawa described the first division as being nearly similar to the ring-formation in copepods, the four elements of the ring being often so condensed as nearly to resemble an actual tetrad. In the early anaphases the daughter-V's break at the apex ; and, although in the later anaphases the limbs reunite, Ishikawa is inclined to regard the transverse division as being a preparation for the second mitosis. Belajeffs earlier work ('94) on Lilium gave an indecisive result, though one on the whole favourable to a reducing division. In his latest paper, however ('98, 1), Belajeff takes more positive ground, stating that after the examination of a large number of forms he has found

1 Schaffner ('97, 2) reaches exactly the reverse result in Lilium philadelphicum, i.e. the first division is transverse, the second longitudinal.


REDUCTION OF THE CHROMOSOMES


in the pollen-mother-cells of Iris a much more favourable object of investigation than Lilitim, Frititlaria, and the other forms on which most of the work thus far has been done, and one in which the second division takes place with " admirable clearness " ; he also gives interesting additional details of the first division in this and other forms. In the first division the spireme splits lengthwise, and then breaks into chromosomes, which assume the shape of a V, Y, or X (Fig. 135, N—Q). The two limbs of these bodies do not, as might be



Showing


[ft. STRASBURUKR i


ucleus of secondary spermatocyte (PodaphylliLm). B. Prophase of sec male) with longitudinally divided chromatin -threads. B. Corresponding stage /-'. Meiaphase of second division (Fodcphyllam, male). G. Initial anaphase (/. C. D. illustrate Motlier's earlier conclusions. C. Second division (Lilimm. male) imcs bent together so as to simulate a split. D. Slightly later stage (Fritillana, 1 siagc supposed to result from breaking apart of the limbs of the U at point of flexu


supposed, represent sister-chromosomes (resulting from the longitudinal division of the spireme) attached by one end or at the middle, since each X, Y, or V is double, consisting of two similar superimposed halves. Belajeff, therefore, regards these figures as longitudinally divided bivalent chromosomes, having the value of tetrads, each limb being a longitudinally split single chromosome. The double V's, Y's, and X's take up a position with the apex (or one end of the X) attached to the spindle, and the longitudinal division in the equatorial plane. The halves then progressively diverge from the


REDUCTION WITHOUT TETRAD-FORMATION 269

point of attachment, thus giving rise to -shaped, <>- -shaped, or xx -shaped figures, all of which in the end assume the -shape. This part of the process is in the main similar to that described by Strasburger and Mottier, and the daughter-V's diverge in the same way as these authors describe. The second division, however, differs radically from their account, since no splitting of the spireme-thread occurs. The chromosomes reappear in the V-, Y-, and X-forms, but are undivided^ and only half as thick as in the first division. Passing to the equator of the spindle, the V- and Y-forms break apart at the apex, while the X-forms separate into the two branches of the X, the daughter-chromosomes having the form of rods slightly bent at the outer end to form a J-figure (Fig. 135, R-T). This division is, accordingly, a transverse or reducing one, which " corresponds completely to the reduction-division in the animal organism " ('98, 2, p. 33.) Atkinson ('99) reaches the same general result in Trillium, stating very positively that no longitudinal division occurs in the second mitosis, and believing that the daughter-V's of the first (heterotypical) mitosis retain their individuality throughout the ensuing pause, and break apart at the apex (reducing division) in the second mitosis. This observer finds further that in Ariscema the heterotypical rings of the first mitosis condense into true tetrads, by one longitudinal and one transverse division, but believes that in this case it is the first division that effects the reduction, as in the insects.

Such confusion in the results of the most competent observers of reduction in the flowering plants is itself a sufficient commentary on the very great difficulty and uncertainty of the subject ; and it would be obviously premature to draw any positive conclusions until further research shall have cleared up the matter. 1

1 Strasburger's new book, entitled Uber Reduktiomtheilung, Spindelbildung, Centrosomen und Cilienbildner im Pflanzcnreich (Jena, 1900), is received while this work is in press, too late for analysis in the text. In this treatise the author gives an exhaustive review of the entire subject, contributing also many new and important observations on Lilium, Iris, Podophyllum, Tradescantia, Allium, Ijxrix, and several other forms. The general result of these renewed researches leads Strasburger to return, in the main, to his conclusions of 1895, w ' tn which agree, as stated above, the results of Guignard and Gregoire; and, in a careful critique of Belajeff's work, he shows how the results of this observer may be reconciled with his own. The essence of Strasburger's interpretation is as follows. In the prophases of the first division the chromosomes first undergo a longitudinal division, shorten to form double rods, and then again split lengthwise in a plane at right angles to the first. The following stages vary even in the same species (Lilium); and here lies the explanation of much of the divergence between the accounts of different observers. (1) In the typical case, the chromosomes are placed radially, with one end next the spindle ; and, during the metaphase, they open apart along the first division-plane, from the spindle outwards, to form h- -shaped figures. These figures meanwhile open apart from the free end inwards along the second division-plane. Thus arise the characteristic < > -shaped figures, the daughter-V's having separated along the first (equatorial) division-plane, while the two limbs of each V have resulted, not through bending, but from a second (axial) split (Fig. 135, E-I/). The



^r


'. Illtu


- Diagrams illustrating diffcrem liiiivc mitoses (hi'terotypical forr tiurgers


T

accounts of reduction in Ihe flowering plants. ) in Pica. [BEI.AJEFF.] ('95) and the later one of Guignard, of Ihe first


in-division. E. Doubly split rod. F. Metaphtte, in profile. G. The ; showing ihe hctcrotvpc ring. H. I. Opening out and breaking apart of the ring.

JT-.lt. 1-aliT iMMHiunl of Strasburger and Motlier {.if. tigs. 133. .34). J. longitudinally split. V-shaped rhmiimvmu^ of first division. A'. Opening out oi the ring, L. Prophase of second division, showing longitudinally split segmented spireme. M. Initial anaphase of second division,

A'-(). First division. [Bki.ajf.ff.] A*. Lung itu din ally split chromosomes, viewed in the equatorial plane. 0. The same viewed in the axis of the spindle. /'. Separation of the daughterchromosomes. Q. Anaphase, all the chromosomes assuming the V-form.

K-T. Second division in frit. [Hki.MKH-'.J A', l-^uaiorial plate, limbs of X's and V*s brraking apart (reducing division). S. Slightly later stage, with daughter-chromosomes si ill untied at one end. /: Anaphase.

270


PECULIARITIES OF REDUCTION IN THE INSECTS 2J\

Rtsumt. In reduction without tetrad-formation the spireme segments into half the somatic number of chromosomes, which split lengthwise and open out to form rings for the first (heterotypical) mitosis. According to one set of observers, including Flemming, Meves, McGregor, Kingsbury, Moore, Klinckowstrom, Van der Stricht, Francotte, Griffin, Belajeff, Farmer, Dixon, Strasburger, Sargant, Mottier, Ishikawa, and Atkinson, the ring arises by a single longitudinal division. According to another group, including Carnoy, Le Brun, Guignard, and Gr^goire, the ring arises through a double longitudinal division, one representing the axial and the other the equatorial plane of the o -figure. The second group of observers regard both maturation-divisions as longitudinal. Among the first group, Flemming, Meves, McGregor, Kingsbury, Moore, Farmer, Dixon, Strasburger, Sargant, and Mottier likewise believe both divisions to be longitudinal, the daughter-V's or their products again splitting lengthwise for the second division ; while Klinckowstrom, Van der Stricht, Francotte, Griffin, Belajeff, Ishikawa, and Atkinson believe one of them to be transverse, the daughter-V's breaking apart at the apex, and thus giving the reducing division of Weismann. 1


D. Some Peculiarities of Reduction in the Insects

We may here briefly consider some interesting observations which show that in some cases the nuclear substance may be unequally distributed to the germ-nuclei. Henking ('90) discovered that in the second spermatocyte-division of Pyrrochoris one of the <% chromosomes " passes undivided into one of the daughter-cells (spermatids) which receives twelve chromatin-elements while its sister receives but eleven. (The number of chromosomes in the spermatogonia, and of rings in the first spermatocyte-division is twenty-four). This anomalous process is confirmed with interesting additional details by Paulmier ('99) in Anasa, and obviously related phenomena are described by Montgomery ('99, 1 ) in Pcntatoma, and by McClung ('99) in Xiphidium.

breaking apart of the V's at the apex, as described by Belajeff, is, therefore, not a transverse division, but merely the completion of the second longitudinal division. (2) In a second and exceptional type, the chromosomes are placed tangentially to the spindle, and the halves separate from the middle, again producing O -shaped figures. These, however, are not of the same nature as those arising in the first case, since they are formed by a bending out of each daughter-chromosome at the middle to form the V, and not by the second longitudinal split. The effect of the latter is in this case to render each daughter-V in itself double, precisely as in the salamander. The difference between the two types results merely from the difference of position of the chromosome with respect to the spindle, and the final result is the same in both, i.e. two longitudinal divisions and no reducing one.

This highly important work brings very strong evidence against the occurrence of transverse or reducing divisions in the higher plants, and seems to explain satisfactorily most of the differences of interpretation given by other olwervers. It will be interesting to see whether a similar interpretation is possible in the case of mollusks, annelids, and arthropods, where the early stages, in many cases, so strikingly resemble those occurring in the plants.

1 Cf. footnote on page 269.


272 REDUCTION OF THE CHROMOSOMES

In Pentatoma the number of chromosomes in the spermatocyte is fourteen. During the final anaphases of the last division, one of the fourteen daughter-chromosomes assumes a different staining-capacity from the others, and becomes a •* chromatin-nucleolus " which fragments into several smaller bodies during the ensuing resting-stage. During each of the succeeding spermatocyte-divisions appear seven chromosomes and a single small chromatin-nucleolus, and both of these kinds of bodies are halved at each division, so that each spermatid receives seven chromosomes and a single chromatin-nucleolus. 1 In Xiphidium a body called by McClung the accessory chromosome," and believed by him to correspond to the "chromatinnucleolus " of Pentatoma, appears in the early prophases of the last spermatogoniumdivision while the remaining chromatin still forms a reticulum. In the equatorial plate this lies outside the ring of chromosomes, but divides like the latter. The same body appears in the ensuing resting-stage, and during both of the spermatocytedivisions. In these it lies, as before, outside the chromosome-ring, and differs markedly from the other chromosomes, but divides like the latter, each of the halves passing into one of the spermatids, where it appears to form an important part of the sperm-nucleus.

Despite the peculiarities described above, the chromatin, as a whole, seems to be equally distributed in both Pentatoma and Xiphidium. In Anasa, however, Paulmier's studies ('98, ^99), made in my laboratory, give a result agreeing with that of Henking, and suggest some very interesting further questions. The spermatogonianuclei contain two nucleolus-like bodies, aiid give rise to twenty-two chromosomes, of which two are smaller than the others (Fig. 126). In the first spermatocyte-division appear eleven tetrads. Ten of these arise from rings like those of Gryllotalpa, etc. The eleventh, which is much smaller than the others, seems to arise from a single nucleolus-like body of the spermatocyte-nucleus, and by a process differing considerably from the others. All of these bodies are halved to form dyads at the first division. In the second spermatocyte-division (Fig. 127) the larger dyads divide to form single chromosomes in the usual manner. The small dyad, however, fails to divide, passing over bodily into one of the spermatids. In this case, therefore, half of the spermatids receive ten single chromosomes, while the remainder receive in addition a small dvad.

A comparison of the foregoing results indicates that the small tetrad (dyad) corresponds to the extra chromosome observed by Henking in Pyrrochoris, and perhaps also to the "accessory chromosome" of Xiphidium. Whether it corresponds to the " chromatin-nucleolus " of Pentatoma is not yet clear. The most remarkable of these strange phenomena is the formation of the small tetrad, which seems to be a non-essential element, since it does not contribute to all the spermatozoa. Paulmier is inclined to ascribe to it a vestigial significance, regarding it as a "degenerating" chromosome which has lost its functional value, though still undergoing in some measure its original morphological transformation ; in this connection it should be pointed out that the spermatocyte-nucleolus, from which it seems to be derived, is represented in the spermatogonia by two such nucleoli, just as the single small tetrad is represented by two small chromosomes in the spermatogonia-mitoses. The real meaning of the phenomenon is, however, wholly conjectural.


E. The Early History of the Germ-nuclei

There are many peculiarities in the early history of the germnuclei, both in plants and animals, that have a special interest in con 1 On this latter point Montgomery's observations do not seem quite decisive.


EARLY HISTORY OF THE GERM-NUCLEI 273

nection with the reduction-problem ; and some of these have raised some remarkable questions regarding the origin of reduction. A large number of observers are now agreed that during the growthperiod preceding the maturation-division (p. 236), in both sexes, the nucleus of the mother-cell (spermatogonium, oogonium), both in plants and in animals, passes through some of the changes preparatory to reduction at a very early period. Thus, in the egg the primary chroma tin-rods are often present in the very young ovarian eggs, and from their first appearance are already split longitudinally. 1 Hacker ('92, 2) made the interesting discovery that in some of the copepods {Cant/iocamptus, Cyclops) these double rods could be traced



Pig. 136. — Longitudinal section through the ovary of the copepod Canlhoiamptm. [Hiic

eg. The youngest germ-cells or oogonia (dividing al eg. 1 ) : a. upper pan of the growlii-sone ; ot. oftcyte. or growing ovarian egg; if. fully formed egg. with double chroma tin-rods.

back continuously to a double spireme-thread, following immediately upon the division of the last generation of oogonia, and that at no period is a true reticulum formed in the germinal vesicle (Fig. 136). In the following year Riickert ('93, 2) made a precisely similar discovery in the case of selachians. After division of the last generation of oogonia the daughter-chromosomes do not give rise to a reticulum, but split lengthwise, and persist in this condition throughout the entire growth-period of the egg. Riickert therefore concluded that the germinal vesicle of the selachians is to be regarded as a " daughter-spireme of the oogonium (Ur-ei) grown to enormous dimensions, the chromosomes of which are doubled and arranged in


274 REDUCTION OF THE CHROMOSOMES

pairs." * In this case their number seems to be at first the somatic number (thirty-six), which is afterward halved by conjugation of the elements two and two (Riickert), as in Lutnbricus (Calkins). It is, however, certain that in many cases (insects, copepods) the double rods first appear in the reduced number, and the observations of Vom Rath C93) and Hacker ('95, 3) give some reason to believe that the reduced number may in some forms be present in the earlier progenitors of the germ-cells, the former author having found but half the normal number in some of the embryonic cells of the salamander, while Hacker ('95, 3) finds that in Cyclops brevicomis the reduced number of chromosomes (twelve) appears in the primordial germ-cells which are differentiated in the blastula-stage (Fig. 74). He adds the interesting discovery that in this form the somatic nuclei of the cleavagestages show the same number, and hence concludes that all the chromosomes of these stages are bivalent. As development proceeds, the germ-cells retain this character, while the somatic cells acquire the usual number (twenty-four) — a process which, if the conception of bivalent chromosomes be valid, must consist in the division of each bivalent rod into its two elements. We have here a wholly new light on the historical origin of reduction ; for the pseudo-reduction of the germ-nuclei seems to be in this case a persistence of the embryonic condition, and we may therefore hope for a future explanation of the process by which it has in other cases been deferred until the penultimate cell-generation, as is certainly the fact in Ascaris?

This leads to the consideration of some very interesting recent discoveries regarding the relation of reduction to the alternation of generations in the higher plants. As already stated (p. 263), Strasburger, Guignard, and other observers have found that in the angiosperms the two maturation-divisions are in both sexes followed by one or more divisions in which the reduced number persists. The cells thus formed are generally recognized as belonging to the vestiges of the sexual generation (prothallium) of the higher cryptogams, the pollengrains (or their analogues in the female) corresponding to the asexual spores of the archegoniate cryptogams. We should, therefore, expect to find reduction in the latter forms occurring in the two corresponding divisions, by which the " tetrad " of spores is formed (as was first pointed out by Hartog, '91). Botanists were thus led to the surmise, first expressed by Overton in 1892, that the reduced number would be found to occur in the prothallium-cells derived from those spores.

1 '92, 2, p. 51.

2 It may be recalled that in Ascaris Boveri proved that the primordial germ-cells have the full number of chromosomes, and Ilertwig clearly showed that this number is retained up to the last division of the spermatogonia. Ishikawa ('97) finds that in Allium the reduced number (eight) appears in the mitosis of the " Urpollenzellen " preceding the pollen-mothcr-cclls. This is, however, contradicted by Motticr ('97, 2).


EARLY HISTORY OF THE GERM-NUCLEI 27$

This surmise quickly became a certainty. Overton himself discovered ('93 ) that the cells of the endosperm in the gymnosperm Ccratozamia divide with the reduced number, namely eight ; and Dixon observed the same fact in Pinus at the same time. In the following year Strasburger brought the matter to a definite conclusion in the case of a fern (Osmunda), showing that all the cells of the prothallium, from the original spore-mot her-cc 11 onwards to the formation of the germ-cells, have one-half the number of chromosomes found in the asexual generation, namely twelve instead of twentyfour ; in other words, the reduction takes place in the formation of the spore from which the sexual generation arises, many cell-generations before the germ-cells are formed, indeed before the formation of the body from which these cells arise. Similar facts were determined by Farmer in Pallavicinia, one of the Hepaticae, where all of the nuclei of the asexual generation (sporogonium) show eight chromosomes during division, those of the sexual generation (thallus) four. It now seems highly probable that this will be found a general rule.

The striking point in these, as in Hacker's observations, is that the numerical reduction takes place so long before the fertilization for which it is the obvious preparation. Speculating on the meaning of this remarkable fact, Strasburger advances the hypothesis that the reduced number is the ancestral number inherited from the ancestral type. The normal, i.e. somatic, number arose through conjugation by which the chromosomes of two germ-cells were brought together. Strasburger does not hesitate to apply the same conception to animals, and suggests that the four cells arising by the division of the oogonium (egg plus three polar bodies) represent the remains of a separate generation, now a mere remnant included in the body in somewhat the same manner that the rudimentary prothallium of angiosperms is included in the embryo-sac. This may seem a highly improbable conclusion, but it must not be forgotten that so able a zoologist as Whitman expressed a nearly related thought, as long ago as 1878 : " I interpret the formation of polar globules as a relic of the primitive mode of asexual reproduction'* l Strasburger's view is exactly the reverse of this in identifying the polar bodies as the remains of a sexual generation ; and as Hacker has pointed out ('98, p. 102), it is difficult to reconcile with the fact that true reduction appears to occur already in the unicellular organisms (p. 277). The hypothesis is nevertheless highly suggestive and one which suggests a quite new point of view for the study not only of maturation but also of the whole problem of sexuality.

We may now return to the consideration of some details. In a considerable number of forms, though not in all, the early prophase is

1 '78, p. 262.


276 REDUCTION OF THE CHROMOSOMES

characterized, especially in the male, by a more or less complete concentration of the chromatin-substance at one side of the nucleus. This stage, to which Moore has given the name synapsis (Fig. 120, A), sometimes occurs when the spireme thread is already split {Ascaris, Liliiim), sometimes before the division is visible (insects). In either case the chromatin-segments emerge from the synapsis stage longitudinally divided and in the reduced number, a fact which gives ground for the conclusion that the synapsis is in some way concerned with the rearrangement of the chromatin-substance involved in the numerical reduction. During the synapsis the nucleolus remains quite distinct from the chromatin, and in many cases it afterward persists beside the tetrads, in the formation of which it takes no part, to be cast out into the cytoplasm (Fig. 124) or to degenerate in situ during the first maturation-division.

A suggestive phenomena, described by several observers, 1 is the casting out of a large part of the nuclear reticulum of the germinal



[CAK


Fig.^.-Typesofmali spindle with tetrads, in lit OV and l.kliRUM.] C. First polar spindle of At


pindles in the ft [HACKER].


[FiJRST.]


i polaj spindle


vesicle at the time the polar bodies are formed (Figs. 97, 128). In these cases {Asterias, Polyehazrus, Thalassema, Nereis) only a small fraction of the chromatin-substance is preserved to form the chromosomes, the remainder degenerating in the cytoplasm. 2

As a final point wc must briefly consider the varying accounts of the achromatic maturation-figures in the female already briefly referred to at page 85. In many forms {e.g. in turbellarians, nemertines, annelids, mollusks, cchinoderms) the polar amphiasters are of quite typical form, with large asters and distinct ccntrosomes nearly similar to those of the cleavage-fignres. In others, however (nematodes, arthropods, tunicates, vertebrates), the polar spindles differ markedly from those of the cleavage-figures, being described by many authors as entirely devoid of asters and even in some cases of centrosomes (Fig. 137).

ardiner ("98), Gtiffin ('99).

i-suhstanee in the clasmobta rich egg, p. 338.


REDUCTION IN UNICELLULAR FORMS 277

There can be no doubt that these polar spindles differ from the usual type, and that they approach those recently described in the mitosis of the higher plants, but it is doubtful whether the apparent absence of asters and centrosomes is normal. In Ascaris, the first polar spindle arising by a direct transformation of the germinal vesicle (Fig. 117) has a barrel-shape, with no trace of asters. At the poles of the spindle, however, are one or two deeply staining granules (Fig. 137), which have been identified as centrosomes by Hacker C94) and Erlanger ('97, 4), but by Fiirst ('98) are regarded as central granules, the whole spindle being conceived as an enlarged centrosome. 1 For the reasons stated at page 314, I believe the former to be the correct interpretation. 2 Spindles without centrosomes have been described in the eggs of tunicates (Julin, Hill, Crampton), in Amphioxus (SoboXXz), in some species of copepods (Hacker), and in some vertebrates (Diemyctylus, Jordan ; mouse, Sobotta). In Amphioxus (Sobotta) and Triton (Carnoy and LeBrun) complete asters are not. formed, but fibrillae apparently corresponding to astral rays and converging to the spindle-poles are found outside the limits of the spindle (Fig. 137). In the guinea-pig, according to Montgomery ('98), centrosomes and asters are present in the first polar spindle, but absent in the second. The evidence is on the whole rather strong that the achromatic figure in these cases approaches in form that seen in the higher plants ; but it is an open question whether the appearances described may not be a result of imperfect fixation.

F. Reduction in Unicellular Forms

Although the one-celled and other lower forms have not yet been sufficiently investigated, we have already good ground for the conclusion that a process analogous to the reduction of higher types regularly recurs in them. In the conjugation of Infusoria, as already described (p. 223), the original nucleus divides several times before union, and only one of the resulting nuclei becomes the conjugating germ-nucleus, while the others perish, like the polar bodies. The numerical correspondence between the rejected nuclei or "corpuscules de rebut" has already been pointed out (p. 227). Hertwig could not count the chromosomes with absolute certainty, yet he states ('89) that in Paramcecium caudatum, during the final division, the number of spindle-fibres and of the corresponding chromatic elements is but 4-6, while in the

1 Cf. p. 312.

2 Sala C94) and Fiirst have shown that occasionally the polar spindles of Ascaris are provided with large typical asters, and thus resemble those of annelids or mollusks. Sala believed this to be an effect of lowered temperature, but Fiirst's observations are unfavourable to this conclusion.


2 7 8


REDUCTION OF THE CHROMOSOMES


earlier divisions the number is approximately double this(8-9). This observation makes it nearly certain that a numerical reduction of chromosomes occurs in the Protozoa in a manner similar to that of the higher forms ; but the reduction here appears to be deferred until



A B C

Fig. 138. — Conjugation and formation of the polar bodies in Attinipkrys. [SCHMIDINN.] A. Union of tbe gametes: first polar spindle. B. Fusion of the cell-bodies; a single polar


bodyni


r the periphery of each. C Fusion of them


the final division. In the gregarines Wolters ('91) has observed the formation of an actual polar body as a small cell segmented off from each of the two conjugating animals soon after their union ; but the number of chromosomes was not determined. Sc haudinn ('96, 2) has observed a like process in Actinephtys, each of the gametes segmenting off a single polar body, after which the germnuclei fuse (Fig. 138). It is possible, as R. Hertwig ('98) points out, that in both these forms a second polar body may have been overlooked, owing perhaps to its rapid disintegration. In Actinosphtzrittm, according to R. Hertwig ('98), the nucleus of each gamete divides twice in rapid succession to form two polar bodies (nuclei), which degenerate, after



REDUCTION IN UNICELLULAR FORMS


which the germ-nuclei unite (Fig. 139). Whether a reduction in the number of chromosomes occurs in these cases was not determined.'



A. Soon after union, f< distinct. C. Fusion of the F. Second cleavage. G. 1


Conjugation of Cfosttrivm. [Ki.h

hroTnatopliorcs. li. C hro mat oph ores reduced lo two, nuclei ii. D. First cleavage of the lygole. E. Resulting a-cell stage.

ing stage, each cell bi-nucleate. H. St-paration of the cells; one o: the nuclei in eacn enlarging to form the permanent nucleus, the other (probably representing a polar body) degenerating.

1 AiHnospharium forms one of the most extreme known cases of in-breeding; for the gametes are sitlcr-cills which immediately reunite after forming the polar bodies. The general facta are as follows: The mother animal, containing very numerous nuclei, become* encysted, and a very large number of the nuclei degenerate. The body then segments into


280 REDUCTION OF THE CHROMOSOMES

Adelea (one of the Coccidiae) is a very interesting case, for according to Siedlecki C99) polar bodies or their analogues are formed in both sexes. The gametes are here of very unequal size. Upon their union the smaller male cell divides twice to form apparently equivalent spermatozoids, of which, however, only one enters the ovum, while three degenerate as polar bodies. These two divisions are of different type ; the first resembles true mitosis, while the second is of simpler character and is believed by Siedlecki to effect a reduction in the number of chromosomes. In the meantime the nucleus of the macrogamete moves to the surface and there expels a portion of its chromatin, after which union of the nuclei takes place. Interesting facts have been observed in unicellular plants which indicate that the reduction may here occur either before (diatoms) or after (desmids) fusion of the conjugating nuclei. In the former (Rhopalodind) Klebahn ('96) finds that each nucleus divides twice, as in many Infusoria, giving rise to two large and two small nuclei. Each of the conjugates then divides, each daughter-cell receiving one large and one small nucleus. The four resulting individuals then conjugate, two and two, the large nuclei fusing while the small (polar bodies) degenerate. The comparison of this case with that of the Infusoria is highly interesting. In the desmids on the other hand {Closterium and Cosmarium, Fig. 140), according to Klebahn ('92), the nuclei first unite to form a cleavagenucleus, after which the zygote divides into two. Each of the new nuclei now divides, one of the products persisting as the permanent nucleus, while the other degenerates and disappears. Chmielewski asserts that a similar process occurs in Spirogyra. Although the numerical relations of the chromosomes have not been determined in these cases, it appears probable that the elimination of a nucleus in each cell is a process of reduction occurring after fertilization.


G. Maturation of Parthenogenetic Eggs

The maturation of eggs that develop without fertilization is a subject of special interest, partly because of its bearing on the general theory of fertilization, partly because it is here, as I believe, that one of the strongest supports is found for the hypothesis of the individuality of chromosomes. In an early article by Minot {'77) on the

a number (five to twelve) of "primary cysts," each containing one of the remaining nuclei. Each primary cyst divides by mitosis to form two gametes ("secondary cysts "), which, after forming the polar bodies, reunite, their nuclei fusing to form a single one. The resulting cell soon creeps out of the cyst-wall and assumes the active life, its nucleus meanwhile multiplying to produce the multinuclear condition characteristic of the adult animal. What is here the physiological motive for the formation of the polar bodies, and how shall it be explained under the Weismann hypothesis?


MATURATION OF PARTHENOGENETIC EGGS 28 1

theoretical meaning of maturation, the suggestion is made that parthenogenesis may be due to failure on the part of the egg to form the polar bodies, the egg-nucleus thus remaining hermaphrodite, and hence capable of development without fertilization. This suggestion forms the germ of all later theories of parthenogenesis. Balfour ('8o) suggested that the function of forming polar cells has been acquired by the ovum for the express purpose of preventing parthenogenesis, and a nearly similar view was afterward maintained by Van Beneden. 1 These authors assumed accordingly that in parthenogenetic eggs no polar bodies are formed. Weismann ('86) soon discovered, however, that the parthenogenetic eggs of Polyphemus (one of the Daphnidae) produce a single polar body. This observation was quickly followed by the still more significant discovery by Blochmann ('88) that in Aphis the parthenogenetic eggs produce a single polar body, while the fertilized eggs produce two, Weismann was able to determine the same fact in ostracodes and Rotifera, and was thus led to the view 2 which later researches have entirely confirmed, that it is the second polar body that is of special significance in parthenogenesis. Blochmann observed that in insects the polar bodies were not actually thrown out of the egg, but remained embedded in its substance near the periphery. At the same time Boveri ('87, 1) discovered that in Ascaris the second polar body might in exceptional cases remain in the egg and there give rise to a resting-nucleus indistinguishable from the egg-nucleus or sperm-nucleus. He was thus led to the interesting suggestion that parthenogenesis might be due to the retention of the second polar body in the egg and its union with the egg-nucleus. "The second polar body would thus, in a certain sense, assume the role of the spermatozoon, and it might not without reason be said : " Parthenogenesis is the result of fertilisation by the second polar body." 3

This conclusion received a brilliant confirmation through the observations of Brauer ('93) on the parthenogenetic egg of Artetnia, though it appeared that Boveri arrived at only a part of the truth. Blochmann ('88-89) had found that in the parthenogenetic eggs of the honey-bee two polar bodies are formed, and Platner discovered the same fact in the butterfly Liparis ('89) — a fact which seemed to contradict Boveri's hypothesis. Brauer's beautiful researches resolved the contradiction by showing that there are two types of parthenogenesis which may occur in the same animal. In the one case Boveri's conception is exactly realized, while the other is easily brought into relation with it.

(a) In both modes typical tetrads are formed in the germ-nucleus to the number of eighty-four. In the first and more frequent case

1 '83, p. 622. a Essay VI., p. 359. • /.<■., p. 73.


282 REDUCTION OF THE CHROMOSOMES

(Fig. 141) but one polar body is formed, which removes eighty-four dyads, leaving eighty-four in the egg. There may be an abortive attempt to form a second polar spindle, but no division results, and the eighty-four dyads give rise to a reticular cleavage-nucleus. From



Pig. 141. — First type of m


aluralion


in ihe parthenojjtnc


A. The nrsl polar spindle- 1 first polur hoily . H4 dyads rema F. Appearance of Ihe egg-uenl of Ihe cleavage-figure; ihe equa


lie equal

in in the


■ml 'lister, fs. G. 1

'<• consists of 84 apf


o the CRg-nnclcus. showi


this arise eighty-four thread-like chromosomes, and the same number appears in later cleavage-stages,

(A) It is the second and rarer mode that realizes Bovcri's conception (Fig. 142). Hoth polar bodies are formed, the first removing eighty-four dyads and leaving the same number in the egg. In the formation of the second, the eighty-four dyads are halved to form


MATURATION OF PAKTHENOGENETIC EGGS


283


two daughter-groups, each containing eighty-four single chromosomes, Both these groups remain in the egg, and each gives rise to a single reticular nucleus, as described by Bovcri in Ascaris. These two nuclei place themselves side by side in the cleavage-figttre, and give rise each to eighty-four chromosomes, precisely like two germ-nuclei in ordinary fertilization. The one hundred and sixty-eight chromosomes split



Fig. 14a. — Second type of maluralii . Formation of second polar body


il plate


1 two groups of 84 each.


lengthwise, and are distributed in the usual manner, and reappear in the same number in later stages. In other words, the second polar body here plays the part of a sperm-nucleus precisely as maintained by Boveri.

In all individuals arising from eggs of the first type, therefore, the somatic number of chromosomes is eighty-four; in all those arising from eggs of the second type, it is one hundred and sixty-eight This


284 REDUCTION OF THE CHROMOSOMES

difference is clearly due to the fact that in the latter case the chromosomes are single or univalent, while in the former they are bivalent (actually arising from dyads or double chromosomes). The remarkable feature, on which too much emphasis cannot be laid, is that the numerical difference should persist despite the fact that the mass, and, as far as we can see, the quality, of the chromatin is the same in both cases. In this fact we must recognize a strong support, not only of Hacker's and Vom Rath's conception of bivalent chromosomes, but also of the more general hypothesis of the individuality of chromosomes (Chapter VI.).

1 . Accessory Cells of the Testis

It is necessary to touch here on the nature of the so-called " Sertoli-cells." or supporting cells of the testis in mammals, partly because of the theoretical significance attached to them by Minot, partly because of their relations to the question of amitosis in the testis. In the seminiferous tubules of the mammalian testis, the parentcells of the spermatozoa develop from the periphery inwards toward the lumen, where the spermatozoa are finally formed and set free. At the periphery is a layer of cells next the basement-membrane, having flat, oval nuclei. Within this, the cells are arranged in columns alternating more or less regularly with long, clear cells, containing large nuclei. The latter are the Sertoli-cells \ or supporting cells ; they extend nearly through from the basement-membrane to the lumen, and to their inner ends the young spermatozoa are attached by their heads, and there complete their growth. The spermatozoa are developed from cells which lie in columns between the Sertolicells, and which undoubtedly represent spermatogonia, spermatocytes, and spermatids, though their precise relationship is, to some extent, in doubt. The innermost of these cells, next the iumen, are spermatids, which, after their formation, are found attached to the Sertoli-cells, and are there converted into spermatozoa without further division. The deeper cells from which they arise are spermatocytes, and the spermatogonia lie deeper still, being probably represented by the large, rounded cells.

Two entirely different interpretations of the Sertoli-cells were advanced as long ago as 1 87 1, and both views still have their adherents. Von Ebner ('71) at first regarded the Sertoli-cell as the parent-cell of the group of spermatozoa attached to it. and the same view was afterward especially advocated by Biondi ('85 ) and by Mi not (92), the latter of whom regarded the nucleus of the Sertoli-cell as the physiological analogue of the polar bodies, i.e. as containing the female nuclear substance C92, p. 77). According to the opposing view, first suggested by Merkel C71). the Sertolicell is not the parent-cell, but a nurse-cell, the spermatozoa developing from the columns of rounded cells, and becoming secondarily attached to the Sertoli-cell, which serves merely as a support and a means of conveying nourishment to the growing spermatozoa. This view was advocated by Brown ('85), and especially by Benda C87). In the following year ('88), von Ebner himself abandoned his early hypothesis and strongly advocated Benda's views, adding the very significant result XhaXfour spermatids arise from each spermatocyte, precisely as was afterward shown to be the case in Ascaris, etc. The very careful and thorough work of Benda and von Ebner, confirmed by that of Lcnhosse'k ('98, 2), leaves no doubt that mammalian spermatogenesis conforms, in its main outlines, with that of Ascaris, the salamander, and other forms, and that Biondi's account is untenable. Minors theoretical interpretation of the Sertoli-cell, as the physiological equivalent of the polar bodies, therefore collapses.


SUMMARY AND CONCLUSION 285

2. A mitosis in the Early Sex-cells

Whether the progenitors of the germ-cells ever divide amitotically is a question of high theoretical interest. Numerous observers have described amitotic division in testis-cells. and a few also in those of the ovary. The recent observations of Meves ('91 ). Vom Rath C93), and others leave no doubt whatever that such divisions occur in the testis of manv animals. Vom Rath maintains, after an extended investigation. that all cells so dividing do not belong in the cycle of development of the germ-cells ('93, p. 164) : that amitosis occurs only in the supporting or nutritive cells (Sertoli-cells, etc.), or in such as are destined to degenerate, like the "residual bodies" of Van Beneden. Meves has, however, produced strong evidence C94) that in the salamander the spermatogonia may, in the autumn, divide by amitosis, and in the ensuing spring may again resume the process of mitotic division, and give rise to functional spermatozoa. On the strength of these observations Flemming C93) himself now admits the possibility that amitosis may form part of a normal cycle of development. 1

H. Summary and Conclusion

The one fact of maturation that stands out with perfect clearness and certainty amid all the controversies surrounding it is a reduction of the number of chromosomes in the ultimate germ-cells to one-half the number characteristic of the somatic cells. It is equally clear that this reduction is a preparation of the germ-cells for their subsequent union, and a means by which the number of chromosomes is held constant in the species. With a few exceptions the first indication of the numerical reduction appears through the segmentation of the spiremethread, or the resolution of the nuclear reticulum, into a number of masses one-half that of the somatic chromosomes. In nearly all higher animals this process first takes place two cell-generations before the formation of the definitive germ-cells, and the process of reduction is completed by two rapidly succeeding " maturation-divisions," giving rise to four cells, all of which become functional in the male, while in the female only one becomes the cgg 9 while the other three — the polar bodies or their analogues — are cast aside. During these two divisions each of the original chromatin-masses gives rise to four chromosomes, of which each of the four daughter-cells receives one ; hence, each of the latter receives one-half the somatic number of chromosomes. In the higher plants, however, the two maturationdivisions are followed by a number of others, in which the reduced number of chromosomes persists, a process most strikingly shown in the pteridophytes, where a separate sexual generation (prothallium) thus arises, all the cells of which show the reduced number.

Two general types of maturation may be distinguished according to the manner in which the primary chromatin-masses divide. In one,

1 For more recent literature on this subject see Meves, Zelltheilung, in Merkel and Bonnet's Ergebnisse, VIII., 1898.


286 REDUCTION OF THE CHROMOSOMES

typically represented by Ascaris and the arthropods, each of these masses divides into four to form a tetrad, thus preparing at once for two rapidly succeeding divisions, which are not separated by a reconstruction of the daughter-nuclei during an intervening resting period. In the other, examples of which are given by the flowering plants and the spermatogenesis of the Amphibia, no true tetrads are formed, the primary chromatin-masses dividing separately for each of the maturation-divisions, which are separated by a period in which the nuclei regress toward the resting state, though often not completely returning to the reticular condition. These two types differ, however, only in degree, and with few exceptions they agree in the fact that during the prophases of the first division the chromatin-bodies assume the form of rings, the mitosis thus being of the heterotypical form, and each ring having the prospective value of four chromosomes.

Thus far the phenomena present no difficulty, and they give us a clear view of the process by which the numerical reduction of the chromosomes is effected. The confusion of the subject arises, on the one hand, from its complication with theories regarding the individuality of the chromosomes and the functions of chromatin in inheritance, on the other through conflicting results of observation on the mode of tetrad-formation and the character of the maturation-divisions. Regarding the latter question nearly all observers are now agreed that one of these divisions, usually the first, is a longitudinal or equationdivision, essentially like that occurring in ordinary mitosis. The main question turns upon the other division, which has been shown in some cases to be transverse and not longitudinal, and thus separates what were originally different regions of the spireme-thread or nuclear substance. The evidence in favour of such a division seems at present well-nigh demonstrative in the case of insects and copepods, and hardly less convincing in the turbellarians, annelids, and mollusks. On the other hand, both divisions are regarded as longitudinal by most of those who have investigated the phenomena in Ascaris and in the vertebrates, and by some of the most competent investigators of the flowering plants.

The evidence as it stands is so evenly balanced that the subject is hardly yet ripe for discussion. The principle for which Weismann contended in his theory of reducing division has received strong support in fact ; yet should it be finally established that numerical reduction may be effected either with or without transverse division, as now seems probable, not only will that theory have to be abandoned or wholly remodelled, but we shall have to seek a new basis for the interpretation of mitosis in general. Weismann's theory is no doubt of a highly artificial character; but this should not close our eyes to the great interest of the problem that it attempted to solve.


LITERA TURE 287

The existing contradiction of results has led to the opinion, expressed by a number of recent writers, that the difference between longitudinal or transverse division is of minor importance, and that the entire question of reduction is a barren one. This opinion fails to reckon with the facts on which rests the hypothesis of the individuality of chromosomes (Chap. VI.); but these facts cannot be left out of account. We must find a common basis of interpretation for them and for the phenomena of reduction ; yet how shall we reconcile them with reduction by longitudinal division only ? I cannot, therefore, share the opinion that we are dealing with a barren problem. The peculiarities of the maturation-mitoses are obviously correlated in some way with the numerical reduction, and the fact that they differ in so many ways from the characters of ordinary mitosis gives ground to hope that their exhaustive study will throw further light not only on the reduction-problem itself but also on mitosis in general and on still wider problems relating to the individuality of the chromosomes and the morphological organization of the nucleus. It is indeed very probable that Weismann's theory is but a rude attempt to attack the problem, and one that may prove to have been futile. The problem itself cannot be ignored, nor can it be dissociated from the series of kindred problems of which it forms a part


LITERATURE. V 1

Van Beneden, E. — Recherches sur la maturation de Toeuf, la fecondation et la division

cellulaire: Arch. Biol. % IV. 1883. Boveri, Th. — Zellenstudien, I., III. Jena, 1887-90. See also " Befruchtung "

(List IV.). Brauer, A. — Zur Kenntniss der Spermatogenese von Ascaris megalocephala : Arch.

mik. Anat., XLIL 1893. Id. — Zur Kenntniss der Reifung der parthenogenetisch sich entwickelnden Eies von

Artemia Salina: Arch. mik. Aflat., XLI1I. 1894. Guignard, L. — Le deVeloppement du pollen et la reduction chromatique dans le

Naias: Arch. Anat. Mic. % II. 1899. (Full literature on reduction in plants.) Griffin, B. B. — See Literature, IV. Hacker, V. — Die Vorstadien der Eireifung (General Review) : Arch. mik. Anat. t

XLV. 2. 1895. Id. — Ober weitere Obereinstimmungen zwischen den Fertpflanzungsvorgangen der

Thiere und Pflanzen : Biol. Centralb.s XVII. 1897. Id. — Cber vorbereitende Theilungsvorgange bei Thieren und Pflanzen : Verh.

deutsch. Zool. Ges. % VIII. 1898. Id. — Die Reifungserscheinungen : Merkel und Bonnets Ergebnisse. VIII. 1898. Hertwig, 0. — Vergleich der Ei- und Samenbildung bei Nematoden. Eine Grund lage flir cellulare Streitfragen : Arch. mik. Anat., XXX V I. 1890. Mark, E. L. — (See List IV.) Peter, K. — Die Bedeutung der Nahrzellen im Hoden : Arch. mik. Anat., LI II. 1898.

1 See also Literature, IV., p. 231.


288 REDUCTION OF THE CHROMOSOMES

Platner, 6. — Uber die Bedeutung der Richtungskorperchen : Biol. Centralb., VIII. 1889.

Vom Rath, 0. — Zur Kenntniss der Spermatogenese von Gryllotalpa vulgaris: Arch, mik. Anat.s XL. 1892.

Id. — Neue Beitrage zur Frage der Chromatinreduktion in der Samen- und Eireife : Arch. mik. A fiat., XLVI. 1895.

Riickert, J. — Die Chromatinreduktion der Chromosomenzahl im Entwicklungsgang der Organismen : Ergebn. d. Anal. u. Entwick.* III. 1893 (1894).

Strasburger, E. — (Jber periodische Reduktion der Chromosomenzahl im Entwicklungsgang der Organismen : Biol. Centralb., XIV. 1894.

Id. — Reduktionstheilung, Spindelbildung, etc. : Jena, Fischer, 1900.


CHAPTER VI SOME PROBLEMS OF CELL-ORGANIZATION

" Wir miissen dcshalb den lebenden Zellen, abgesehen von der Molecularstructur der organischen Verbindungen, welche sie enthalt, noch eine andere und in anderer Wcise complicirte Structur zuschreiben, und diese es ist, welche wir mit dem Namen Organization bezeichnen." Brucke. 1

" Was diese Zelle eigentlich ist, dariiber existieren sehr vcrschiedene Ansichten."

Hackel. 2

The remarkable history of the chromatic substance in the maturation of the germ-cells forces upon our attention the problem of the ultimate morphological organization of the nucleus, and this in its turn involves our whole conception of protoplasm and the cell. The grosser and more obvious organization is revealed to us by the microscope as a differentiation of its substance into nucleus, cytoplasm, and the like. But, as Strasburger has well said, it would indeed be a strange accident if the highest powers of our present microscopes had laid bare the ultimate organization of the cell. Brucke insisted more than thirty years ago that protoplasm must possess a far more complicated morphological organization than is revealed to us in the visible structure of the cell, repeating, though without accepting, an earlier suggestion of Henle's('4i) that the cell might be composed of more elementary vital units ranking between the molecule and the cell. Many biological thinkers since Briicke's time have in one form or other accepted this conception, which indeed lies at the root of nearly all recent attempts to analyze exhaustively the phenomena of cell-life. Without attempting to follow out the history of opinion in detail or to give any extended review of the various theories, 8 it may be pointed out that this conception was based both on theoretical a priori grounds and on the observed facts of cell-structure. On the former basis it was developed by Herbert Spencer 4 in his theory of " physiological units " by which he endeavoured to explain the phenomena of regeneration, development, and heredity ; while Nageli ('84) developed on the same general lines his theory of miccllce which

1 Eiementarorganismen % 1 861, p. 386.

2 Anthropogcnie, 1 89 1, p. 104.

8 For an exhaustive review see Yves Delage, La structure du protoplasma et les theories sur rherediti. Paris, 1895. 4 Principles of Biology, 1864.

U 289


29O SOME PROBLEMS OF CELL-ORGANIZATION

has been so widely accepted by botanists. In the meantime Darwin l introduced a new element into the speculative edifice in his celebrated hypothesis of pangenesis, where for the first time appear the two assumptions of specific differences in the ultra-microscopic corpuscles ("gemmules") and the power of self-propagation by division. Darwin did not, however, definitely maintain that protoplasm was actually built of such bodies. The latter hypothesis was added by De Vries ('89), who remodelled the theory of pangenesis on this assumption, thus laying the basis for the theories of development which reached their climax in the writings of Hertwig and Weismann.

The views of Spencer and Darwin were based on purely theoretical grounds derived from the general phenomena of growth and inheritance. 2 Those of Nageli, De Vries, Wiesner, Altmann, and others were more directly based on the results of microscopical investigation. The view was first suggested by Henle ('41), and at a later period developed by Bechamp and Estor, by Maggi and especially by Altmann, that the protoplasmic granules might be actually organic units or bioblasts, capable of assimilation, growth, and division, and hence to be regarded as elementary units of structure standing between the cell and the ultimate molecules of living matter. By Altmann, especially, this view was pushed to an extreme limit, which lay far beyond anything justified by the known facts; and the theory of genetic continuity expressed by Redi in the aphorism "omne vivttm ex vivo,' 9 reduced by Virchow to " omnis cellula e cellula" finally appears in the writings of Altmann as " omne granu lit m e granulo" / 8

Altmann's premature generalization rested upon a very insecure foundation and was received with just scepticism. Except in the case of plastids, the division of the cytoplasmic granules was and still remains a pure assumption, and furthermore many of Altmann's "granules" (zymogen-granules of gland-cells, etc.) are undoubtedly metaplasmic bodies. 4 Yet the beautiful discoveries of Schimper ('85 > and others on the origin of plastids in plant-cells give evidence that these cells do in fact contain large numbers of bodies, other than the nuclei, that possess the power of growth and division. The division of the chlorophyll-bodies, observed long ago by Mohl, was shown by Schmitz and Schimper to be their usual if not their only mode of origin ; and Schimper was able to trace them back to minute colourless plastids, scarcely larger than " microsomes," that are present in large numbers in the protoplasm of the embryonic cells and of the &gg f and give rise not only to chlorophyll-bodies but also to the amyloplasts or starch-formers and the chromoplasts or pigment-bodies. While it still remains doubtful whether the plastids arise solely by division or also

1 Variation of Animals and Plants, 1868. a Cf Introduction, p. 12.

  • Die EUmentarorganismen, Leipsic, 1 894, p. 155. * Cf Lazarus, '98.


THE NATURE OF CELL-ORGANS 29 1

by new formation (as now seems to be the case with the centrosome), the foregoing observations on the plastids give a substantial basis for the hypothesis that protoplasm may be built of minute dividing bodies which form, its ultimate structural basis. It was these facts, taken in connection with the phenomena of particulate inheritance and variation (Galton), that led De Vries and his followers to the fundamental assumption of "pangens," "plasomes," " biophores," and the like as final protoplasmic units ; l but these were conceived not as the visible granules, plastids, etc., but as much smaller bodies, lying far beyond the limits of present microscopical vision, through the growth or aggregation of which the visible structures arise. This assumption has been harshly criticised ; yet when we recall that in one form or another it has been accepted by such men as Spencer, Darwin, Beale, Hackel, Michael Foster, Nageli, De Vries, Wiesner, Roux, Weismann, Oscar Hertwig, Verworn, and Whitman, and on evidence drawn from sources so diverse, we must admit that despite its highly speculative character it is not to be lightly rejected. In the present chapter we may inquire how far the known facts of cell-structure speak for or against this hypothesis, incidentally considering a number of detailed questions of cell-morphology which have not hitherto found a place.

A. The Nature of Cell-organs

The cell is, in Briicke's words, an elementary organism, which may by itself perform all the characteristic operations of life, as is the case with the unicellular organisms, and in a sense also with the germ-cells. Even when the cell is but a constituent unit of a higher grade of organization, as in multicellular forms, it is no less truly an organism, and in a measure leads an independent life, even though its functions be restricted and subordinated to the common life. It is true that the earlier conception of the multicellular body as a colony of one-celled forms cannot be accepted without certain reservations. 2 Nevertheless, all the facts at our command indicate that the tissue-cell possesses the same morphological organization as the egg-cell, or the protozoan, and the same fundamental physiological properties as well. Like these the tissue-cell has its differentiated structural parts or organs, and we have now to inquire how these cell-organs are to be conceived.

1 The following list includes only some of the various names that have been given to these hypothetical units by modern writers: Physiological units (Spencer); gem mules (Darwin); pangens (De Vries); plasomes (Wiesner); micella (Nageli); plastittules (Hackel and Elssberg); inotagmata (Kngelmann); biophores (Weismann); bioblasts (Beale); somacuUs (Foster) ; idioblasts (Hertwig); idiosomes (Whitman); biogens (Verworn); microzymas (Bechamp and Estor); gemma (Haacke). These names are not strictly synonymous, nor do all of the writers cited assume the power of division in the units. a Cf. p. 58.


292 SOME PROBLEMS OF CELL-ORGANIZATION

The visible organs of the cell fall under two categories, according as they are merely temporary structures, formed anew in each successive cell-generation out of the common structural basis, or permanent structures whose identity is never lost, since they are directly handed on by division from cell to cell. 1 To the former category belong, in general, such structures as cilia, pseudopodia, and the like ; to the latter, the nucleus, perhaps also the centrosomes, and the plastids of plant-cells. A peculiar interest attaches to the permanent cell-organs. Closely interrelated as these organs are, they nevertheless have a remarkable degree of morphological independence. Tkey assimilate food, grow, divide, and perform their own characteristic actions like coexistent but independent organisms, of a lower grade than the cell, living together in colonial or symbiotic association. So striking is this morphological and physiological autonomy in the case of the green plastids or chromatophores that neither botanists nor zoologists are as yet able to distinguish with absolute certainty between those that form an integral part of the cell, as in the higher green plants, and those that are actually independent organisms living symbiotically within it, as is probably the case with the yellow cells of Radiolaria. Even so acute an investigator as Watase* ('93, 1) has seriously propounded the view that the nucleus itself — or rather the chromosome — should be regarded as a distinct organism living in symbiotic association with the cytoplasm, but having had, in an historical sense, a different origin. This rather fantastic view has not found much favour, and even were it true would teach us nothing of the origin of the power of division, which must for the present be taken as an elementary process forming one of the primary data of biology. Yet we may still inquire whether the power of division shown by such protoplasmic masses as plastids, chromosomes, centrosomes, nucleoli, and nuclei may not have its root in a like power residing in ultimate protoplasmic units of which they are made up. Could we accept such a view, we might much more easily meet some puzzling cytological difficulties. For under this assumption the difference between transient and permanent cellorgans would become only one of degree, depending on the degree of cohesion between their structural components ; and we could thus conceive, for example, how such a body as a centrosome might form, persist by division for a number of generations, and finally disintegrate. In connection with this it may be pointed out that even such a typical permanent organ as the nucleus does not persist as such during the ordinary form of division ; for it loses its boundary and many of its other structural characters, becoming resolved into a group of separate chromosomes. What persists is here not the structural unit, but the characteristic substance which forms its essential constituent, and

1 Cf. footnote, p. 30.


STRUCTURAL BASIS OF THE CELL 293

a large part even of this substance may be lost in the process. The term "persistent organ" is therefore used in rather a figurative sense, and if too literally understood may easily mislead us.

With the foregoing considerations in mind let us turn to the actual structural relation of the cell-organs.

B. Structural Basis of the Cell

In Chapter I. some of the reasons have been given for the conclusion that none of the obvious structural features of protoplasm (fibrillae, alveoli, granules, and the like) can be regarded as necessary or universal ; and we may now inquire whether there is any evidence that such structures may have such a common structural basis as De Vries's theory assumes. I shall here take as a point of departure my observations on the structure of protoplasm in echinoderm-eggs, already briefly reviewed at page 28. The beautiful alveolar structure of these eggs is entirely of secondary origin, and all the visible structural elements arise during the growth of the eggs by the deposit and subsequent enlargement of minute spherical bodies, all apparently liquid drops, in a homogeneous or finely granular basis which is itself a liquid. Some of these spheres enlarge to form the alveolar spheres, while the homogeneous basis or continuous substance remains as the interalveolar material. Others remain much smaller to constitute the " microsomes " scattered through the interalveolar walls ; and these bodies, like the alveolar spheres, are perfectly visible in life, as well as in section ; they are therefore not coagulation- products or artifacts. From these three elements arise all the other structures observed in these eggs, deutoplasm-spheres (Ophiura) and pigment-bodies (Arbacia) being formed by further enlargement and chemical alteration of the alveolar spheres, while astral rays and spindle-fibres are differentiated out of the inter-alveolar material and microsomes. 1 These various elements show a continuous gradation in size from the largest deutoplasm-spheres down to the smallest visible granules, the latter being the source of all the larger elements, and in their turn emerging into view from the " homogeneous " basis. Clearly, then, none of these bodies can be regarded as the ultimate structural units ; for the latter, if they exist, must lie in a region at present inaccessible to the microscope. This fact, however, no more disproves their existence than it does that of molecules and atoms. It only shows the futility of such attempts as those of Altmann and his predecessors to identify " granules " or "microsomes " as final morphological units, and compels us to turn to indirect instead of direct evidence. It may, however, again be pointed out that it would be quite irrational to conclude that the smalt

1 Cf. Wilson, '99.


294 SOME PROBLEMS OF CELL-ORGANIZATION

est visible granules first come into existence when they first come within view of the microscope. The " homogeneous " substance must itself contain or consist of granules still smaller. The real question is not whether such ultra-microscopical bodies exist, but whether they are permanent organized bodies possessing besides the power of growth also the power of division. This question can be only indirectly approached ; and we shall find it convenient to do so by beginning at the opposite end of the series, through a reconsideration of the phenomena of nuclear division.

C. Morphological Composition of the Nucleus

I. The Chromatin

(a) Hypothesis of the Individuality of the Chromosomes. — It may now be taken as a well-established fact that the nucleus is never fortned de novo, but always arises by the division of a preexisting nucleus. In the typical mode of division by mitosis the chromatic substance is resolved into a group of chromosomes, always the same in form and number in a given species of cell, and having the power of assimilation, growth, and division, as if they were morphological individuals of a lower order than the nucleus. That they are such individuals or units has been maintained as a definite hypothesis, especially by Rabl and Boveri. As a result of careful study of mitosis in epithelial cells of the salamander, Rabl ('85) concluded that the chromosomes do not lose their individuality at the close of division, but persist in the chromatic reticulum of the resting nucleus. The reticulum arises through a transformation of the chromosomes, which give off anastomosing branches, and thus give rise to the appearance of a network. Their loss of identity is, however, only apparent. They come into view again at the ensuing division, at the beginning of which "the chromatic substance flows back, through predetermined paths, into the primary chromosome-bodies " (Kernfaden), which reappear in the ensuing spireme-stage in nearly or quite the same position they occupied before. Even in the resting nucleus, Rabl believed that he could discover traces of the chromosomes in the configuration of the network, and he described the nucleus as showing a distinct polarity having a "pole" corresponding with the point toward which the apices of the chromosomes converge {i.e. toward the centrosome), and an " antipole " (Gegenpol) at the opposite point {i.e. toward the equator of the spindle) (Fig. 22). Rabl's hypothesis was precisely formulated and ardently advocated by Boveri in 1887 and 1888, and again in 1891, on the ground of his own studies and those of Van Beneden on the early stages of Ascaris. The hypothesis was supported


MORPHOLOGICAL COMPOSITION OF THE NUCLEUS


295


by extremely strong evidence, derived especially from a study of abnormal variations in the early development of Ascaris % the force of which has, I think, been underestimated by the critics of the hypothesis. Some of this evidence may here be briefly reviewed. In some cases, through a miscarriage of the mitotic mechanism, one or both of the chromosomes destined for the second polar body are accidentally left




Fig. 143. — Evidence of the individuality of the chromosomes. Abnormalities in the fertilization of Ascaris. [BOVERI.J

A. The two chromosomes of the egg-nucleus, accidentally separated, have given rise each to a reticular nucleus ($, $) ; the sperm-nucleus below (<?). B. Later stage of the same, a single chromosome in each egg-nucleus, two in the sperm-nucleus. C An egg in which the second polar body has been retained ; p.b* the two chromosomes arising from it ; 9 * ne egg-chromosomes ; d the sperm-chromosomes. D. Resulting equatorial plate with six chromosomes.


in the egg. These chromosomes give rise in the egg to a reticular nucleus, indistinguishable from the egg-nucleus. At a later period this nucleus gives rise to the same number of chromosomes as those that entered into its formation, i.e. either one or two. These are drawn into the equatorial plate along with those derived from the germnuclei, and mitosis proceeds as usual, the number of chromosomes being, however, abnormally increased from four to five or six (Fig. 143,


296 SOME PROBLEMS OF CELL-ORGANIZATION

C, Z>). Again, the two chromosomes left in the egg after removal of the second polar body may accidentally become separated- In this case each chromosome gives rise to a reticular nucleus of half the usual size, and from each of these a single chromosome is afterward formed (Fig. 143, A, B). Finally, it sometimes happens that the two germ-nuclei completely fuse, while in the reticular state, as is normally the case in sea-urchins and some other animals (p. 188). From the cleavage- nucleus thus formed arise four chromosomes.

The same general result is given by the observations of Zur Strassen ('98) on the history of giant embryos in Ascaris. These embryos arise by the fusion, either before or after the fertilization, of previously separate eggs, and have been shown to be capable of development up to a late stage. Not only in the first but also in some, at least, of the later mitoses, such eggs show an increased number of chromosomes proportional to the number of nuclei that have united. Thus in monospermic double eggs (variety bivalent) the number is six instead of four; in dispermic double eggs the number is increased to eight (Fig. 144).

These remarkable observations show that whatever be the number of chromosomes entering into the formation of a reticular nucleus, the same number afterward issues from it — a result which demonstrates that the number of chromosomes is not due merely to the chemical composition of the chromatin-substance, but to a morphological organization of - Giam-embryo of Auaris, the nucleus. A beautiful confirmation sing from a double- Q f tn j s conclusion was afterward made tgR. showing eight , _ . ,, , , , ., ,,

■t Strom*). by Boven ( 93, 95, i ) and Morgan ( 95,

4), in the case of echinoderms, by rearing larvae from enucleated egg-fragments, fertilized by a single spermatozoon (p. 194). All the nuclei of such larva? contain but half the typical number of chromosomes, — i.e. in Echinus nine instead of eighteen, — since all are descended from one germ-nucleus instead of two !

Equally striking is the remarkable fact, described at page 275, that all of the cells in the sexual generation (oophore) of the higher cryptogams show half the number of chromosomes characteristic of the sporophyte, the explanation being that while reduction occurs at the time of spore-formation, the spores develop without fertilization, the reduced chromosome- number persisting until fertilization occurs



MORPHOLOGICAL COMPOSITION OF THE NUCLEUS


297


long afterward. Attention may be again called to the surprising case of Anemia, described at page 281, which gives a strong argument in favour of the hypothesis.

In addition to the foregoing evidence, Van Beneden and Boveri were able to demonstrate in Ascaris that in the formation of the spireme the chromosomes reappear in the same position as those which entered into the formation of the reticulum, precisely as Rabl



Pif. MS- — Evidence of the individuality of the chromosomes in the egg of Ascarii. [BOVRM.) E. Anaphase of the first cleavage. F. Two-cell stage with lolled nuclei, the lobes formed by the ends of the chromosome*. C, Early prophase of the ensuing division ; chromosomes re-forming, ccntrosomes dividing. //. Later prophase, the chromosomes lying with their ends in the same position as before; centrosomes divided.

maintained. As the long chromosomes diverge, their free ends are always turned toward the middle plane (Fig. 31), and upon the reconstruction of the daughter-nuclei these ends give rise to corresponding lobes of the nucleus, as in Fig. [45, which persist throughout the resting state. At the succeeding division the chromosomes reappear exactly in the same position, their ends lying in the nuclear lobes as before (Fig. 145, G, H\ On the strength of these facts Boveri concluded that the chromosomes must be regarded as " individuals" or " elementary organisms," that have an independent existence in the


SOME PROBLEMS OF CELL-ORGANIZATION


cell. During the reconstruction of the nucleus they send forth pseudo podia which anastomose to form a network in which their identity is lost to view. As the cell prepares for division, however, the chromosomes contract, withdraw their processes, and return to their "resting state," in which fission takes place. Applying this conclusion to the fertilization of the egg, Boveri expressed his belief that



Fig. 146.— Independence of paternal _ Cyclops. [A-C. from ROCKKkT; D, from H

A. Firs! cleavage- figure in chromosomes. B. Resulting I«  Mill in double groups. D. Bias


, , independence of paternal and 1

with double nuclei. C. Second cleavage; chror h double nuclei from the eight-cell stage of C. in


" wc may identify every chromatic element arising from a resting nucleus with a definite element that entered into the formation of that nucleus, from which the remarkable conclusion follows that in all cells derived in the regular course of division front the fertilised egg, one-half of the chromosomes are of strictly paternal origin, the other half of maternal." 1

I'ol.p. 410.


MORPHOLOGICAL COMPOSITION OF THE NUCLEUS 299

Boveri's hypothesis has been criticised by many writers, especially by Hertwig, Guignard, and Brauer, and I myself have urged some objections to it. Recently, however, it has received a support so strong as to amount almost to a demonstration, through the remarkable observations of Ruckert, Hacker, Herla, and Zoja on the independence of the paternal and maternal chromosomes. These observations, already referred to at page 208, may be more fully reviewed at this point. Hacker ('92, 2) first showed that in Cyclops strenuns, as in Ascaris and other forms, the germ-nuclei do not fuse, but give rise to two separate groups of chromosomes that lie side by side near the equator of the cleavage-spindle. In the two-cell stage (of Cyclops tenuicornis) each nucleus consists of two distinct though closely united halves, which Hacker believed to be the derivatives of the two respective germ-nuclei. The truth of this surmise was demonstrated three years later by Ruckert ('95, 3) in a species of Cyclops, likewise identified as C. strenutis (Fig. 146). The number of chromosomes in each germ-nucleus is here twelve. Ruckert was able to trace the paternal and maternal groups of daughter-chromosomes not only into the respective halves of the daughter-nuclei of the two-cell stage, but into later cleavage -stages. From the bilobed nuclei of the two-cell stage arise, in each cell, a double spireme and a double group of chromosomes, from which are formed bilobed or double nuclei in the four-cell stage. This process is repeated at the next cleavage, and the double character of the nuclei was in many cases distinctly recognizable at a late stage when the germ-layers were being formed.

Finally Victor Herla's ('93) and Zoja's ('95, 2) remarkable observations on Ascaris showed that in Ascaris not only the chromatin of the germ-nuclei, but also the paternal and maternal chromosomes, remain perfectly distinct as far as the twelve-cell stage — certainly a brilliant confirmation of Boveri's conclusion. Just how far the distinction is maintained is still uncertain, but Hacker's and Riickert's observations give some ground to believe that it may persist throughout the entire life of the embryo. Both these observers have shown that the chromosomes of the germinal vesicle appear in two distinct groups, and Ruckert suggests that these may represent the paternal and maternal elements that have remained distinct throughout the entire cycle of development, even down to the formation of the egg !

Leaving aside all doubtful cases (such as the above suggestion of Riickert's), the well-determined facts form an irresistible proof of the general hypothesis ; and it is one with which every general analysis of the cell has to reckon. I believe, however, that the hypothesis has received an unfortunate name ; for, except in a few special cases, 1

1 Cf. p. 273.


300


SOME PROBLEMS OF CELL-ORGANIZATION


almost no direct evidence exists to show that the chromosomes persist as " individuals " in the chroma tin- reticulum of the resting cell. The facts indicate, on the contrary, that in the vast majority of cases the identity of the chromosomes is wholly lost in the resting nucleus, and the attempts to identify them through the polarity or other morphological features of the nuclear network have on the whole been futile. It is therefore an abuse of language to speak of a persistent " individ


— Hybrid fertiliiatie

W.WKM&W. [HfiKLA.]

A. The germ-nuclei shortly before ur as given rise lo one chromosome (J).


i still shown in the primordial germ-cell o:


uality " of chromosomes. But this verbal difficulty should not blind us to the extraordinary interest and significance of the facts. It is difficult to suppose that the tendency of the chromatin to resolve itself into a particular number of chromosomes is directly due to its chemical or molecular structure, or is analogous to crystallization ; for in the chromatin of the same species, or even in that of the same egg, this tendency varies, not with chemical, but with purely morphological


MORPHOLOGICAL COMPOSITION OF THE NUCLEUS 301

conditions, i.e. with the number of chromosomes that enter the nucleus. Neither can we assume that it is due merely to the total mass of the chromatin in each case ; for this varies in different nuclei of the same species, or even in the nucleus of the same cell at different periods (as in the egg-cell), yet the same number of chromosomes is characteristic of all. Indeed, we seek in vain for an analogy to these phenomena and can only admit our entire inability to explain them. No phenomena in the history of the cell more clearly indicate the existence of a morphological organization which, though resting upon, is not to be confounded with, the chemical and molecular structure that underlies it ; and this remains true even though we are wholly ignorant what that organization is.

(6) Composition of the Chromosomes. — We owe to Roux l the first clear formulation of the view that the chromosomes, or the chromatinthread, consist of successive regions or elements that are qualitatively different (p. 244). This hypothesis, which has been accepted by Weismann, Strasburger, and a number of others, lends a peculiar interest to the morphological composition of the chromatic substance. The facts are now well established ( 1 ) that in a large number of cases the chromatin-thread consists of a series of granules (chromomeres) embedded in and held together by the linin-substance, (2) that the splitting of the chromosomes is caused by the division of these more elementary bodies, (3) that the chromatin-grains may divide at a time when the spireme is only just beginning to emerge from the reticulum of the resting nucleus. These facts point unmistakably to the conclusion that these granules are perhaps to be regarded as independent morphological elements of a lower grade than the chromosomes. That they are not artifacts or coagulation-products is proved by their uniform size and regular arrangement in the thread, especially when the thread is split. A decisive test of their morphological nature is, however, even more difficult than in the case of the chromosomes ; for the chromatin-grains often become apparently fused together so that the chromatin-thread appears perfectly homogeneous, and whether they lose their individuality in this close union is undetermined. Observations on their number are still very scanty, but they point to some very interesting conclusions. In Boveri's figures of the eggmaturation of Ascaris each element of the tetrad consists of six chromatin-discs arranged in a linear series (Van Beneden's figures of the same object show at most five) which finally fuse to form an apparently homogeneous body. In the chromosomes of the germ-nuclei the number is at least double this (Van Beneden). Their number has been more carefully followed out in the spermatogenesis of the same animal (variety bivalens) by Brauer. At the time the chromatin-grains

1 Bedeutung der Kcrnthcilung$figuren> 1883, p. 15.


302 SOME PROBLEMS OF CELL-ORGANIZATION

divide, in the reticulum of the spermatocyte-nucleus, they are very numerous. His figures of the spireme-thread show at first nearly forty granules in linear series (Fig. 120, B). Just before the breaking of the thread into two the number is reduced to ten or twelve ( Fig. 120, C). Just after the division to form the two tetrads the number is four or five (Fig. 120, D) y which finally fuse into a homogeneous body. 1

It is certain, therefore, that the number of chromomeres is not constant in a given species, but it is a significant fact that in Ascaris the final number, before fusion, appears to be nearly the same (four to six) both in the oogenesis and the spermatogenesis. The facts regarding bivalent and plurivalent chromosomes (p. 87) at once suggest themselves, and one cannot avoid the thought that the smallest chromatin-grains may successively group themselves in larger and larger combinations of which the final term is the chromosome. Whether these combinations are to be regarded as " individuals " is a question which can only lead to a barren play of words. The fact that cannot be escaped is that the history of the chromatin-substance reveals to us, not a homogeneous substance, but a definite morphological organization in which, as through an inverted telescope, we behold a series of more and more elementary groups, the last visible term of which is the smallest chromatin-granule, or nuclear microsome, beyond which our present optical appliances do not allow us to see. Are these the ultimate dividing units, as Brauer suggests (p. 113)? Here again we may well recall Strasburger's warning, and hesitate to identify the end of the series with the limits reached t>y our best lenses. Somewhere, however, the series must end in final chromatic units which cannot be further subdivided without the decomposition of chromatin into simpler chemical substances ; and these units must be capable of assimilation, growth, and division without loss of their specific character. It is in these ultimate units that we must seek the " qualities," if they exist, postulated in Roux's hypothesis ; but the existence of such qualitative differences is a physiological assumption that in no manner prejudices our conclusion regarding the ultimate morphological composition of the chromatin.

D. Chromatin, Linin, and Cytoplasm

What, now, is the relation of the chromatin-grains to the linin-network and the cytoplasm ? Van Beneden long ago maintained 2 that

1 Eisen ('99) finds that the chromosomes of the spermatogonia of Batrachoseps always consist of six " chromomeres," each of which consists of three smaller granules or " chromioles." The latter persist as the chromatin-granules of the resting nucleus; and it is through their successive aggregation that the chromomeres and chromosomes are formed.

2 '83, pp. 580, 583.


CHROMATIN, LIN IN, AND CYTOPLASM 303

the achromatic network, the nuclear membrane, and the cell-meshwork have essentially the same structure, all consisting of microsomes united by connective substance, and being only " parts of one and the same structure." But, more than this, he asserted that the chromatic and achromatic microsomes might be transformed into one another, and were therefore of essentially the same morphological nature. " They pass successively, in the course of the nuclear evolution, through a chromatic or an achromatic stage, according as they imbibe or give off the chromophilous substance." l Both these conclusions are borne out by recent researches. Heidenhain ('93, '94), confirmed by Reinke and Schloter, finds that the nuclear network contains granules of two kinds differing in their staining-capacity. The first are the basi-chromatin granules, which stain with the true nuclear dyes (basic tar-colours, etc.), and are identical with the " chromatin-granules " of other authors. The second are the oxychromatin-granules of the linin-network, which stain with the plasma-stains (acid colours, etc.), and are closely similar to those of the cytoreticulum. These two forms graduate into one another, and are conjectured to be different phases of the same elements. This conception is furthermore supported by many observations on the behaviour of the nuclear network as a whole. The chromatic substance is known to undergo very great changes in staining-capacity at different periods in the life of the nucleus (p. 338), and is known to vary greatly in bulk. In certain cases a very large amount of the original chromatic network is cast out of the nucleus at the time of the division, and is converted into cytoplasm. And, finally, in studying mitosis in sea-urchin eggs I found reason to conclude ('95, 2) that a considerable part of the linin-network, from which the spindle-fibres are formed, is actually derived from the chromatin.

From the time of the earlier writings of Frommann ('65, '67), Arnold ('67), Heitzmann ('73), and Klein ('78). down to the present, an increasing number of observers have held that the nuclear reticulum is to be conceived as a modification of the same structural basis as that which forms the cytoplasm. The latest researches indicate, indeed, that true chromatin (nuclein) is confined to the nucleus. 2 But the whole weight of the evidence now goes to show that the lininnetwork is of the same nature as the cell-meshwork, and that the achromatic nuclear membrane is formed as a condensation of the same substance. Many investigators, among whom may be named Frommann, Leydig, Klein, Van Beneden, Carnoy, and Reinke, have described the fibres of both the intra- and extra-nuclear network as terminating in the nuclear membrane ; and the membrane itself is described by these and other observers as being itself reticular in structure, and by some (Van Beneden) as consisting of closely crowded

1 U. p. 583. 2 Cf. Hammarsten C95).


304 SOME PROBLEMS OF CELL-ORGANIZATION

microsomes arranged in a network. The clearest evidence is, however, afforded by the origin of the spindle-fibres in mitotic division ; for it is now well established that these may be formed either inside or outside the nucleus, and at the close of mitosis the central portion of the spindle appears always to give rise to a portion of the cytoplasm lying between the daughter-nuclei. In such a case as that of the sea-urchin (see above) we have, therefore,. evidence of a direct transformation of chromatin into linin-substance, of the latter into spindlefibres, and, finally, of these into cytoplasm.

When all these facts are placed in connection, we find it difficult to escape the conclusion that no definite line can be drawn between the cytoplasmic granules at one extreme and the chromatin-granules at the other. And inasmuch as the latter are certainly capable of growth and division, we cannot deny the possibility that the former may themselves have, or arise from elements having like powers. But while we may take this as a fair working hypothesis, we should clearly recognize that the base of well-determined fact on which it rests is approached by a circuitous route ; that in case of most of the cytoplasmic granules there is not the slightest evidence that they multiply by division ; and that even though some of them may have such powers, we cannot regard them as the ultimate structural units, for the latter must be bodies far more minute.

E. The Centrosome

From our present point of view the centrosome possesses a peculiar interest as a cell-organ which may be scarcely larger than a cytomicrosome, yet possesses specific physiological properties, assimilates, grows, divides, and may persist from cell to cell without loss of identity. Nearly all observers of the centrosome have found it lying in the cytoplasm, outside the nucleus; but apart from the Protozoa (p. 94) there is at least one well-established case in which it lies within the nucleus, namely, that of Ascaris, where Brauer made the interesting discovery that /;/ one variety (univalens) the centrosome lies inside the nucleus y in the other variety (bivalens) outside — a fact which proves that its position is non-essential (cf. Figs. 120 and 148).

An intra-nuclear origin of the centrosome has also been asserted by Julin ('93) in the primary spermatocytes of Styleopsis y by Riickert ('94) in the eggs of Cyclops, Mathews ('95) in those of Asterias, Carnoy and Le Brun ('97, 2) in Ascaris, Van der Stricht C98) in the eggs of Thysanozobn, by R. Hertwig ('98) in Actinosph<zrium y Calkins ('98, 1) in Noctiluca, and Schaudinn ('96, 3) in spore-producing buds of Acanthocystis, though in the last-named form the centrosome of the vegetative forms is extra-nuclear (p. 92).


THE CENTROSOME


305


As already stated, 1 it is still undetermined whether a true centrosome may ever arise de novo, but the evidence in favour of such a possibility has of late rapidly increased. Carnoy ('86) long since showed that the egg of Asearis, during the formation of the polar bodies, sometimes showed numerous accessory asters scattered through the cytoplasm. Reinke ('94) described somewhat similar asters in peritoneal cells of the salamander, distinguishing among them three orders of magnitude, the largest containing distinct centrosomes or " primary centres," while the smaller contained " secondary " and " tertiary " centres, the last named being single




F


G


Fig. 148. — Mitosis with inlra-nu clear centres


ome, in the spermatocytes of


ctphala, var. mmivaUni. [Brauek.]



A. Nucleus containing a quadruple group or


tetrad of chromosomes (/), nu


cenlrosome (t). B.C. Division of the cenlrosom


e. D.E.F.G. Formation of ll


centrosomes escaping from the nucleus in G.





microsomes at the nodes of the cytoreticulum. By successive aggregations of the tertiary and secondary centres arise true centrosomes as new formations. Watase ('94-95) also finds in the egg of Macrobdella, besides the normal aster containing an undoubted centrosome, numerous smaller asters graduating downwards to such "tertiary asters " as Reinke describes with a microsome at the centre of each, and on this basis concludes that the true centrosome differs from a microsome only in degree and may arise de novo. Mottier ('97, 2) finds in pollen- mother-cells numerous minute " cyto-asters " having no direct relation to the spindle-formation (Fig. 133). Again Juel

1 Cf. pp. 53, 214.


306 SOME PROBLEMS OF CELL-ORGANIZATION

('97) finds that an isolated chromosome, accidentally separated from the equatorial plate (pollen-m other-cells of Hcnterocallis), may give rise to a small vesicular nucleus which may subsequently divide by mitosis, though it is quite out of relation to the spindle-poles of the preceding mitosis (Fig. 149). Strong evidence of the same character as the last is given by the facts in the hehozobn Acanthocystis, as shown by Schaudinn ('96, 3), the ordinary vegetative cells containing a persistent extra-nuclear centrosome, while in the bud-formation of the swarm-spores a centrosome is formed de novo, without relation to that of the mother-cell, inside the nucleus of the bud (Fig. 41).

The strongest case in favour of the independent origin of centrosomes is, however, given by the observations of Mead on Ckcetopterus ('98) and the remarkable experiments of R. Hertwig ('95, '96) and



Morgan ('96, 1; '99, i)on theeggs of echinoderms and other animals. When eggs of C/iaJtofterHs are taken from the body-cavity and placed in sea-water, a multitude of small asters appear in the cytoplasm, two of which are believed to persist as those of the polar spindle, while the others degenerate (Fig. 150). Mead is therefore convinced that the polar centrosomes arise in this case separately and de novo. 1 R. Hertwig showed that when unfertilized eggs of sea-urchins (Sfrongylocetitrotits, Echinus) are kept for some time in sea-water or treated with dilute solutions of strychnine the nuclei undergo some of

1 A numher of other authors (eg. Griffin, Tkalassima, Coe, Ccrt/ira/ului) have likewise fouiul the first polar asters widely separated at their first appearance. On the other hand, Mathews ('05), whose preparations 1 have seen, limls the polar ccntrusomes in Asteriai close together, and Krancotte ('97, '98) has demonstrated that in Cyrlnperas and Prosthae raajlhey arise by the division of a single primary centrusome. The same is stated by Gardiner ('98) to he the case in Polyth<rrHs. It should be noted, further, thai Mead could find no undoubted centrosomes save in the " primary " or definitive polar asters.


THE CENTKOSOME 307

the changes of mitosis, the chromatin-network giving rise to a group of chromosomes and a spindle, or more frequently a fan-shaped half-spindle, arising from the achromatic substance. In some cases not only a complete spindle appeared but also asters at the poles, though no centrosomes were observed (Fig. 151). Morgan's experiments along the same lines were mainly performed upon the seaurchin Arbacia, but included also the eggs of Asterias, Sipunculus, and Cerebratulus (Figs. 150, 151). In these eggs numerous asters may arise in the cytoplasm, if they are allowed to He some time in sea


>w;.i



^:


RgNf^


Fig. 150. — Formation dc novo (1) of centrosomes. [A. W,MeaI>; C, Moroan.]

A. Unfertilized egg of Chatopttnis wilh " secondary asters " developed a few minutes after the

f gR i* placed in sea-water. /(. Slightly later stage with two definitive polar asters and centrosomes.

C. Large " sun " (transformed polar aster) containing numerous small " secondary asters " and

centrosomes, from unferiiliied egg of Ctribrotulm after aa hours in 1.5 % sodium chloride

solution.

water or treated by weak solutions of sodium or magnesium chloride. These asters often contain deeply staining, central granules indistinguishable from the centrosomes of the normal asters ; and, what is of high interest, such of them as lie near the nucleus take part in the irregular nuclear division that ensues, forming centres toward which the chromosomes pass. These divisions continue for some time, the chromosomes being irregularly distributed through the egg, and giving rise to nuclei of various sizes apparently dependent upon the number of chromosomes each receives. After a variable number of such


SOME PROBLEMS OF CELL-ORGANIZATION


divisions the asters disappear, yet the irregular nuclear divisions continue, nuclear spindles with distinct centrosomes being formed at each division, but apparently without relation to the older asters, and they



■ ;.•■■ *.



. after tff, hours in 1.5% solut


1 asters in unfertilized echinoderm-eges. [A, B.

on of sodium chloride, then 5 hours in sea-water: formed after 6% hours in NaCI. C-E. Echinus afier treatment with 0.5 '!/, stry^hninc-so'iit-iin. showing various forms of astral formations (fanshaped asler. half spindle, and complete mitotic figure).

are believed by Morgan to arise dc novo from the egg substance. 1 In the meantime irregular cleavage of the egg occurs, though no embryo is produced. 2 Loeb, however, in the remarkable experiments

1 '99. p. 479

  • Morgan makes Ihe important observation, which harmonizes with that of Boreri,

reported at page toK, that the divisions ocrur with rcspttt to the number and position of Iht nutlet, not oj the asters, concluding that the former must therefore play an essential rile as centres of division, and that the activity of (he asters is in itself not sufficient to account for division of the cytoplasm.


THE CENTROSOME 309

referred to at page 215, finds that after treatment with magnesium chloride unfertilized sea-urchin eggs (Ariacta) may give rise to perfect Pluteus larvae — a result which if well founded seems to place the new formation of true centrosomes beyond question.

Taken together, these researches give strong ground for the conclusion that true {i.e. physiological) centrosomes may arise de novo from either the cytoplasmic or the nuclear substance and may play the usual rdle (whatever that may be) in mitosis. If this conclusion be sustained by future research, we shall no longer be able to accept Van Beneden's and Boveri's conception of the centrosome as a persistent organ in the same sense as the nucleus ; but on the other hand we shall have gained important ground for further inquiry into the nature and source of that power of division which is so characteristic of living things and upon which the law of genetic continuity rests.

Morphology of the Centrosome. — In its simplest form (Fig. 152, A) the centrosome appears under the highest powers as nothing more than a single granule of extraordinary minuteness which stains intensely with iron-haematoxylin, and can scarcely be distinguished from the cyto-microsomes except for the fact that it lies at the focus of the astral rays. In this form it always appears at the centre of the very young sperm-asters during fertilization (Figs. 97, 99), in the early phases of ordinary mitosis ( Figs. 27, 32), and in some cases also in the resting cell, for example, in leucocytes and connective tissue corpuscles (Figs. 8, 49), where, however, it is often triple or quadruple. In the course of division the centrosome often increases in size and assumes a more complex form, becoming also surrounded by various structures involved in the aster-formation. The relation of these structures to the centrosome itself has not yet been fully cleared up and there is still much divergence of opinion regarding the cycle of changes through which the centrosome passes. It is, therefore, not yet possible to give a very consistent account of the centrosome, still less to frame a satisfactory morphological definition of it.

It is convenient to take up as a starting-point Boveri's (*88) account of the centrosomes in the egg of Ascaris, supplemented by Brauer's ('93) description of those in the spermatocytes of the same animal. During the early prophases of the first cleavage Boveri found the centrosome as a minute granule which steadily enlarges as the spindle forms, until shortly before the metaphase it becomes a rather large, well-defined sphere in the centre of which a minute central granule or centriole appears (Fig. 152, B, C). From this time onward the centrosome decreases in size until in the daughter-cells it is again reduced to a small granule which divides into two and goes through a similar cycle during the second cleavage and so on. The centrosome is at all stages surrounded by a clear zone (" Heller Hof ") in which


3io


SOME PROBLEMS OF CELL-ORGANIZATION


the astral rays are thinner and stain less deeply than farther out. Brauer's account is substantially the same, though no definite " Heller Hof " was found, and the astral rays were traced directly in to the boundary of the centrosome. He added, however, two important observations, viz. ( i ) that the central granule is visible at every period ; and (2) division of the centrosome is prccedal by division of the central granule (Fig. 148) — an observation recently extended by Boveri to the division of the egg-centrosome. 1 Van Beneden and Neyt ('87), on the other hand, gave a quite different account of the



e enclosing a central granuli

surrounded by a " Heller Hof; tx. Iloveri's account of the centrosome of the Ascaris egg. D. Central granule surrounded by a

tx. polar spindles of ThysanotoSn, Van der Stricht. E. Ceniral granule ("centrosome") surrounded by medullary and cortical radial lones, each bounded by a microsome-circle ; tx. polar spinille of Unit, LIUie. P. Van Bent- don's representation of aster of the Aiearil egg '. I'l« the last.

granules surrounded by a ■'Heller Hof; tx. the echinoilenn-egg. //. "Centrosome" (central granule) surrounded by a vague larger body lying in a reticulated centrosphere ; tx. Thataatma. [GKIKP1N.]

structures at the centre of the aster. The " corpuscule central " (usually assumed by later writers to be the centrosome), described as a "mass of granules," is surrounded by two well-defined astral zones, formed as modifications of the inner part of the aster, and constituting the "attraction-sphere." These are an inner "medullary zone," and an outer " cortical zone," each bounded by a very distinct layer of microsomes (Fig. 152, F).

• Reported by Flint, '9S, p. III.


THE CENTROSOME 311

The discrepancy between these results on the part of the two pioneer investigators of the centrosome has led to great confusion in the terminology of the subject, which has not yet been fully cleared away. Many of the observers who followed Boveri (Flemming, Hermann, Van der Stricht, Heidenhain, etc.) found the centrosome, in various cells, as a much smaller body than he had described, often as a single or double minute granule, staining intensely with iron-haematoxylin. Heidenhain ('93, '94) and Driiner ('94, '95) found further that the asters in leucocytes and other forms often show several concentric circles of microsomes, and that the sphere bounded by the innermost circle often stains more deeply than the outer portions and may appear nearly or quite homogeneous (Fig. 156). To this sphere, with its contained central granule or granules Heidenhain applies the term microcentrum C94, p. 463), while Kostanecki and Siedlecki suggest the term microsphere ('96, p. 217). Still later Kostanecki and Siedlecki ('97) found that even in Ascaris, as in other forms, sufficient extraction of the colour (iron-haematoxylin) reduces the centrosome to a minute granule to which the astral rays converge, and which is presumably identical with Boveri's "central granule." Heidenhain ('93, '94) found that in leucocytes the central granule is often double, triple, or even quadruple, while in giant-cells of certain kinds there are numerous deeply staining granules (Fig. 14). He therefore proposed to restrict the term centrosome to the individual granules, whatever be their number, applying the term microcentrum to the entire group ('94, p. 463).

With these facts in mind we can gain a clear view of the manner in which both the confusion of terminology and the contradiction of results has arisen. Brauer ('93 ) found in Ascaris (see above) that division of the central granule precedes division of the "centrosome" and therefore suggested that only the former is equivalent to Van Beneden's "corpuscule central," while the body called "centrosome" by Boveri is really the medullary astral zone, the " Heller Hof " being the cortical zone. This is substantially the same conclusion reached by Heidenhain, Rawitz, Lenhoss£k, Kostanecki and Siedlecki, Erlanger, Van der Stricht, Lillie, and several others. The confusion of the subject is owing, on the one hand, to the fact that those who have accepted this conclusion continue to use the word centrosome in two quite different senses, on the other hand to the fact that the conclusion is itself repudiated by Boveri ('95), MacFarland C97), and Furst C98).

As regards the terminology we find that most recent writers agree with Heidenhain, Kostanecki and Siedlecki, in restricting the word centrosome to the minute, deeply staining granules, whether one or more, at the centre of the aster. On the other hand, Brauer, Fran


312


SOME PROBLEMS OF CELL-ORGANIZATION


cotte, Van der Stricht, Meves, and others apply the term to the central granule or granules plus the surrounding sphere (" centrosome " of Boveri), which they regard as equivalent to the medullary zone of Van Beneden, the " corpuscule central " of the last-named author being identified with the central granule or "centriole" of Boveri, though the latter structure is considerably smaller than the former as described by Van Beneden.

The matter of fact turns largely on the question whether the astral rays traverse the larger sphere to the central granule. That such is the case in Ascaris is positively asserted by Kostanecki and Siedlecki, ('97) and as positively denied by Furst ('98) with whose observations



HIT //■/;■/




>l#


Fakland.]

A. Mitotic figure, formation of first polar body. B. Inner aster granule double within the " centrosome." C. Elongation of c" polar spindle.


those of MacFarland ('97) on gasteropod-eggs agree. On the other hand, in the turbellarians the observations of Francotte ('97, '98) and Van der Stricht ('98, 1) seem to leave no doubt that the larger sphere ("centrosome"), here very sharply defined and staining deeply in iron-hrematoxylin, is traversed by well-defined astral rays converging to the central corpuscle, and both these observers agree further that both the corpuscle and the sphere divide to persist as the "cetttrosomes" of the daughter-cells — a result in conformity with Van Beneden's conclusion in the case of Ascaris.

LilHe's valuable observations on the polar asters of Unto ('98) afford, I believe, conclusive evidence as to the nature of the sphere. In the


THE CENTROSOME


313


earlier stages the aster has exactly the structure described by Van Beneden in Ascaris, except that the innermost body {i.e. the " corpuscule central ") is a single minute granule. This Is surrounded by typical medullary and cortical zones, through both of which the


SSa-PJ^v




Kg- 154- A. Alter o( the first polar (enrosphere) and cortical (ectc . entosphere bounded \>y continuous of central spindle within and Irum tV


B. Late anaphase oi . C, D. Prophases <

of the old cntospheri


D

esof Unie. [LlLLtB.]

>some) surrounded by medullary


rays pass (Fig. 152, E, Fig. 154). The inner sphere, consisting of a dense and deeply staining substance, has at first a typical radiate structure and is bounded by a microsome-circle. In later stages (late anaphase) the central granule divides into two and afterward into four or more granules, of which, however, only one or two actually


314 SOME PROBLEMS OF CELL-ORGANIZATION

persist. The inner sphere is now bounded by a definite membrane, and its radiate structure becomes obscure, the astral rays extending only to the boundary of the sphere, though a few rays persist within it (Fig. 154, B). It is clear from this that the inner sphere and central granule pass through phases that bridge the gap between Van Beneden's and Boveri's descriptions. Lillie's observations fully sustain the conclusion that the central granule ("centriole" of Boveri) corresponds to the " corpuscule central" of Van Bcnedcn, and the inner sphere {medullary zone) to Boveri's "centrosome." A comparison of the polar aster of Unio with that of Thysanozoon, as described by Van der Stricht ('98), leaves hardly room for doubt that the cortical zone represents Boveri's " Heller Hof " ; for in both forms the rays of the cortical zone are much thinner and lighter than the more peripheral portions, thus giving a clear zone, which in Unio is bounded by only a fairly definite microsome-circle and in Thysanozoon by none. Lastly, we must recognize the justice of the view urged by Kostanecki, Griffin, Mead, Lillie, Coe, and others, that the term centrosotne should be applied to the central granule and not to the sphere surrounding it (medullary zone), despite the fact that historically the word was first applied by Boveri to the latter structure. For in both Diaulula (MacFarland) and Unio (Lillie) the second polar spindle arises from the substance of the inner sphere, while the central granule, becoming double, gives rise to the centrosomes at its poles. By following Boveri's terminology, therefore, MacFarland is driven to the strange conclusion that the second polar spindle is nothing other than an enormously enlarged " centrosome " — a result little short of a reductio ad absurdum when we consider that in Ascaris the polar spindle arises by a direct transformation of the germinal vesicle (p. 277). The obvious interpretation is that the central granule is the only structure that should be called a centrosome, the surrounding sphere being a part of the aster, or rather of the attraction-sphere. Thus regarded, the origin of the spindle in Diaulula presents nothing anomalous and a similar interpretation may be placed on the polar spindles of Ascaris as described by Fiirst C98). 1

1 In echinoderms the concurrent results of Reinke ('95), Boveri ('95), myself (*96-'97), show that the " centrosome " is a well-defined sphere containing a large group (ten to twentv) of irregularly scattered, deeply staining granules. I have shown in this case that in the early prophases there is but one such granule, which then becomes double and finally multiple, forming a pluricorpuscular centrum (Fig. 52) not unlike that described by Heidenhain in giant-cells. Kostanecki, who asserts that the centrosome of echinoderms is a single granule ('96, 1, '96, 2, p. 24S), has not sufficiently studied the later phases of mitosis. Cf. also Erlanger ('98). The centrosomes described in nerve cells by Lenhossek ('95) are apparently of somewhat similar type. Until the facts are more fully known the exact nature of these " centrosomes " remains an open question. Lillie's observations on Unio show that here, too (first polar spindle), the centrosome divides to form a considerable number of


THE CENTROSOME 315

The genesis of the concentric spheres surrounding the centrosome will be considered in the following section. We may here only emphasize the remarkable fact that the centres of the dividing system are bodies which are in many cases so small as to lie almost at the limits of microscopical vision, and which in the absence of the surrounding structures could not be distinguished from other protoplasmic granules. Full weight should be given to this fact in every estimate of the centrosome theory, and it is no less interesting in its bearing upon the corpuscular theory of protoplasm.

Watas£ ('93, '94) made the very interesting suggestion that the centrosome is itself nothing other than a microsome of the same morphological nature as those of the astral rays and the general meshwork, differing from them only in size and in its peculiar powers. 1 Despite the vagueness of the word " microsome," which has no well-defined meaning, Watas^'s suggestion is full of interest, indicating as it does that the centrosome is morphologically comparable to other elementary bodies existing in the cytoplasmic structure, and which, minute though they are, may have specific chemical and physiological properties.

An interesting hypothesis regarding the historical origin of centrosome is that of Biitschli ('91) and R. Hertwig (92), who suggest that it may be a derivative of a body comparable with the micro-nucleus of Infusoria, which has lost its chromatin but retained the power of division ; and the last-named author has suggested further that the so-called " archoplasmic loops" discovered by Platner in pulmonates may be remnants of the chromatic elements. A similar view has been advocated by Heidenhain ('93, '94) and Lauterborn C96). Heidenhain regards central spindle and centrosomes as forming essentially a unit ("microcentrum ,, ) homologous with the micro-nucleus of the Infusoria, the centrodesmus (p. 79) representing a part of the original achromatic elements. The metazoan nucleus is compared to the protozoan macro-nucleus. The improbability of a direct derivation of the Metazoa from Infusoria, urged by Boveri ('95) and Hertwig C96), has led Lauterborn ('96) to the view that the metazoan centrosome and nucleus are respectively derivatives of two equivalent nuclei, such as Schaudinn ('95) describes in Amoeba binucUata, the "Nebenkorper" of Paramceba (cf. p. 94), being regarded as an intermediate step, and the micro-nucleus of Infusoria a side-branch. R. Hertwig ('96), on the other hand, regards the metazoon centrosome as a derivative of an intra-nuclear body such as the '* nucleolo-centrosome " of Euglena (p. 91), which has itself arisen through a condensation of the general achromatic substance. With this view Calkins ('98), on the whole, agrees ; but he regards it as probable that the " nucleolo-centrosome "

granules of which one or two remain as the persistent centrosome, while others are converted into microsomes or other cytoplasmic structures. It is probable that something similar occurs in the echinoderms.

1 The microsome is conceived, if I understand Watase rightly, not as a permanent morphological body, but as a temporary varicosity of the thread, which may lose its identity in the thread and reappear when the thread contracts. The centrosome is in like manner not a permanent organ like the nucleus, but a temporary body formed at the focus of the astral rays. Once formed, however, it may long persist even after disappearance of the aster, and serve as a centre of formation for a new aster.


316 SOME PROBLEMS OF CELL-ORGANIZATION

of Euglena and Amoeba and the sphere of Noctiluca and Paramaba are to be compared with the attraction-sphere, while the centrosome may have had a different origin.

It appears to me that none of these views rests upon a very substantial basis, and they must be taken rather as suggestions for further work than as well-grounded conclusions.


F. The Archoplasmic Structures I . Hypothesis of Fibrillar Persistence

The asters and attraction-spheres have a special interest for the study of cell-organs ; for they are structures that may divide and persist from cell to cell or may lose their identity and re-form in successive cell-generations, and we may here trace with the greatest clearness the origin of a cell-organ by differentiation out of the structural basis. Two sharply opposing views of these structures have been held, represented among the earlier observers on the one hand by Boveri, on the other by Butschli, Klein, Van Beneden, and Carnoy. The latter observers held that the astral rays and spindle-fibres, and hence the attraction-sphere, arise through a morphological rearrangement of the preexisting protoplasmic meshwork, under the influence of the centrosome. This view, which may be traced back to the early work of Fol ('73) and Auerbach ('74)> was first clearly formulated by Butschli ('76), who regarded the aster as the optical expression of a peculiar physico-chemical alteration of the protoplasm primarily caused by diffusion-currents converging to the central area of the aster. 1 An essentially similar view is maintained in Butschli's recent great work on protoplasm, 2 the astral " rays " being regarded as nothing more than the meshes of an alveolar structure arranged radially about the centrosomes (Fig. 10, B). The fibrous appearance of the astral rays is an optical illusion, for they are not fibres, but flat lamellae forming the walls of elongated closed chambers. This view has recently been urged, especially by Erlanger ('97, 4, etc.), who sees in all forms of asters and spindles nothing more than a modified alveolar structure.

The same general conception of the aster is adopted by most of those who accept the fibrillar or reticular theory of protoplasm, the astral rays and spindle-fibres being regarded as actual fibres forming part of the general network. One of the first to frame such a conception was Klein {'78), who regarded the aster as due to " a radial arrangement of what corresponds to the cell-substance," the latter

1 For a very careful review of the early views on this subject, see Mark, Li max, 1881. 2 '92, 2, pp. 158-169.


THE ARCHOPLASMIC STRUCTURES 317

being described as having a fibrillar character. 1 The same view is advocated by Van Beneden in 1883. With Klein, Heitzman, and Frommann he accepted the view that the intra-nuclear and extranuclear networks were organically connected, and maintained that the spindle-fibres arose from both. 2 "The star-like rays of the asters are nothing but local differentiations of the protoplasmic network. 8 . . . In my opinion the appearance of the attraction-spheres, the polar corpuscle (centrosome), and the rays extending from it, including the achromatic fibrils of the spindle, are the result of the appearance in the egg-protoplasm of two centres of attraction comparable to two magnetic poles. This appearance leads to a regular arrangement of the reticulated protoplasmic fibrils and of the achromatic nuclear substance with relation to the centres, in the same way that a magnet produces the stellate arrangement of iron filings." 4

This view is further developed in Van Beneden's second paper, published jointly with Neyt ('87). " The spindle is nothing but a differentiated portion of the asters." 6 The aster is a " radial structure of the cell-protoplasm, whence results the image designated by the name of aster." 6 The operations of cell-division are carried out through the " contractility of the fibrillae of the cell-protoplasm and their arrangement in a kind of radial muscular system composed of antagonizing groups." 7

An essentially similar view of the achromatic figure has been advocated by many later workers. Numerous observers, such as Rabl, Flemming, Carnoy, Watas£, Wilson, Reinke, etc., have observed that the astral fibres branch out peripherally into the general meshwork and become perfectly continuous with its meshes, and tracing the development of the aster, step by step, have concluded that the rays arise by a direct progressive modification of the preexisting structure. The most extreme development of this view is contained in the works of Heidenhain ('93, '94), Buhler ('95), Kostanecki and Siedlecki ('97), which are, however, only a development of the ideas suggested by Rabl in a brief paper published several years before. Rabl ('89, 2) suggested that neither spindle-fibres nor astral rays really lose their identity in the resting cell, being only modified in form to constitute the mitome or filar substance (meshwork), but still being centred in the centrosome. Fission of the centrosome is followed by that of the latent spindle-fibres (forming the lininnetwork); hence each chromosome is connected by pairs of daughter 1 It is interesting to note that in the same place Klein anticipated the theory of fibrillar contractility, both the nuclear and the cytoplasmic reticulum being regarded as contractile (/.<:., p. 417).

2 '83. P- 592. 4 '83. P. 55°- 6 lc * P- 2 75 •'83, p. 576. 6, 87, p. 263. 7 Lc. t p. 280.


318 SOME PROBLEMS OF CELL- ORGANIZATION

fibres with the respective centrosomes. Heidenhain, adopting the first of these assumptions, builds upon it an elaborate theory of cellpolarity and cell-division already considered in part at pages 103-105. Sometimes the astral rays (" organic radii n ) retain their radial arrangement throughout the life of the cell (leucocytes, Fig. 49) ; more commonly they are disguised and lost to view in the cytoplasmic mesh work. All, however, are equal in length and in tension — assumptions based on the one hand on the occurrence of concentric circles of microsomes in the aster, on the other hand on the analogy of the artificial model described at page 104. Btihler C95) and Kostanecki and Siedlecki C97) likewise unreservedly accept the view that besides the centrosome the entire system of " organic radii," including astral rays, mantle-fibres, and central spindle-fibres, persists in the resting cell in modified form, and is centred in the centrosome. Kostanecki finally ('97 ) takes the last step, logically necessitated by the foregoing conclusion, and apparently supported also by the crossing of the astral rays opposite the equator of the spindle and the relations of their peripheral ends, concluding that the monocentric astral system is converted into the dicentric system (amphiaster) by longitudinal fission of the rays} Thus the entire mitome of the mother-cell divides into equal halves for daughter-cells ; and since the radii consist of microsomes, each of these must likewise divide into two. 2

Could this tempting hypothesis be established, Roux's interpretation of nuclear division (p. 224) could be extended also to the cytoplasm ;. and the aster- and amphiaster-formation, with the spireme-formation, might be conceived as a device for the meristic division of the entire cell-substance — a result which would place upon a substantial basis the general corpuscular theory of protoplasm. Unfortunately, however, the hypothesis rests upon a very insecure foundation : first, because it is based solely upon the fibrillar theory of protoplasm ; second, because of the very incomplete direct evidence of such a splitting of the rays ; third, because there is very strong evidence that in many cases the old astral rays wholly disappear, to be replaced by new ones. 3 We may best consider this adverse evidence in connection with a general account of the opposing archoplasm-hypothesis.

2. The A re hop las m Hypothesis

Entirely opposed to the foregoing conception are the views of Boveri and his followers, the starting point of which is given by

1 '97, p. 680.

2 This view had been definitely stated also by O. Schultze in 1890.

3 There is, however, no doubt that the aster as a whole does, in some cases, divide into two — for instance, in the echinoderm-egg, Fig. 95.


THE ARCHOPLASMIC STRUCTURES 319

Boveri's celebrated archoplasm-hypothesis. Boveri has from the first maintained that the amphiastral fibres are quite distinct from the general cell-meshwork. In his earlier papers he maintained ('88, 2) that the attraction-sphere of the resting cell is composed of a distinct substance, "archoplasm" consisting of granules or microsomes aggregated about the centrosome as the result of an attractive force exerted by the latter. From the material of the attraction-sphere arises the entire achromatic figure, including both the spindle-fibres and the astral rays, and these have nothing to do with the general reticulum of the cell. They grow out from the attraction-sphere into the reticulum as the roots of a plant grow into the soil, and at the close of mitosis are again withdrawn into the central mass, breaking up into granules meanwhile, so that each daughter-cell receives one-half of the entire archoplasmic material of the parent-cell. Boveri was further inclined to believe that the individual granules or archoplasmic microsomes were " independent structures, not the nodal points of a general network," and that the archoplasmic rays arose by the arrangement of these granules in rows without loss of their identity. 1 In a later paper on the sea-urchin this view underwent a considerable modification through the admission that the archoplasm may not preexist as formed material, but that the rays and fibres may be a new formation, crystallizing, as it were, out of the protoplasm about the centrosome as a centre, but having no organic relation with the general reticulum ; though Boveri still held open the possibility that the archoplasm might preexist in the form of a specific homogeneous substance distributed through the cell, though not ordinarily demonstrable by reagents. 2 In this form the archoplasm-theory approaches very nearly that of Strasburger, described below.

There are three orders of facts that tell in favour of Boveri's modified theory: first, the existence of persistent archoplasm-masses or attraction-spheres from which the amphiasters arise ; second, the origin of amphiasters in alveolar protoplasm ; and, third, the increasing number of accounts asserting the replacement of the old asters by others of quite new formation. In at least one case, namely, that of Noctiluca % the entire achromatic figure is formed from a permanent attraction-sphere lying outside the nucleus and perfectly distinct from the general cell-meshwork. 8 Other cases of this kind are very rare, and in most cases the attraction-sphere sooner or later disintegrates, 4 but in the formation of the spermatozoa we have many examples of archoplasmic masses (Nebenkern, attraction-sphere, idiozome), which apparently consist of a specific substance having a special relation to the achromatic figure.

1 '88, 2, p. 80. 8 Ishikawa, '94, '98; Calkins, '9$, 2.

  • '95, 2, p. 40. * Cf. p. 323.


320


SOME PROBLEMS OF CELL-ORGANIZATION


The amphiastral formation in alveolar protoplasm gives very clear evidence against the theory of fibrillar persistence. Here the fibrillar rays can be seen growing out through the walls of the alveoli ' quite distinct from, though embedded in, them. At the close of mitosis every trace of the fibrillar formation may disappear, e.g. in echinoderm-eggs after formation of the polar bodies, the protoplasm retaining only a typical alveolar structure.



Fig. 155. — Siages in the first cleavage of Ihe egg in Cttebral«l«i (A -C, Cof.) and Thalaistma

{D-F. Griffin).

A. Firs! appearance of rhe clcavaj>e-ccntrosome at the poles of Ihe fused perm-nuclei : cleavageasirrs iorminj; within i!ie ricgeneralinj; sperm -asters. B. Final anaphase of first cleavage, showing persistent eentrosomes anil new aslirs forming. C. Immediately after division. D-F. Three stages of Ihe laic anaphase in Thalnsstatii, showing formation of new asters wiihin the old. {Cf. Fig. 99.)

The strongest evidence against fibrillar persistence is, however, given by recent studies on mitosis, showing on the one hand that the new astral centres do not coincide with the old ones, on the other that the old rays degenerate /;/ situ, to be replaced by new ones. Aside from many earlier observers, who believed the entire aster to disappear at the close of mitosis, the first to assert the wholly new » Cf. Reinke ('95). Wilson ('99).


THE ARCHOPLASMIC STRUCTURES 32 1

formation of the rays was Drtiner, who maintained in the case of the mitosis of salamander testis-cells, that " not a single fibre of the astral system of the mother-cell is carried over unchanged into the organism of the daughter-cell" ('95, p. 309). The same conclusion was soon afterward supported by Braus ('95) in the case of the cleavagemitoses of Triton. The most convincing evidence of this fact has been given by studies on the maturation and fertilization of the egg by Griffin ('96, '99), Mac Far land C97), Lillie ('99), and Coe C99), all of whom find that the new astral centres, arising by division of the centrosome, move away from the old position, to which, however, the old rays still converge while the new asters are independently forming (Fig. 155). This is shown with especial clearness in the egg of Ccrebratulus (Coe), where the peripheral portions of the old asters persist until the new amphiaster is completely formed. This observation seems conclusively to overturn Kostanecki's hypothesis of the persistence and division of the rays, and together with the work of MacFarland gives a very strong support to Boveri's later view.

It still remains an open question whether the rays actually arise from the substance of the centrosome, from a specific surrounding archoplasm, or by differentiation out of the general substance of the meshwork. The first of these possibilities has been urged in a very interesting way by Watase ('94), who believes that the centrosome " spins out the cytoplasmic filaments " l of the spindle and aster, and that ordinary microsomes may in like manner spin out the fibrillae of ordinary cytoplasmic networks. 2 This view is sustained by the mode of origin of the axial filament in the spermatozoa and that of the cilia in plant spermatozoids. It is, on the other hand, opposed by the almost infinitesimal bulk of the centrosome as compared with that of the aster that may form about it, and by the formation of the spindles in higher plants in the apparent absence of centrosomes. On the whole, the facts do not seem at present to warrant the acceptance of Watas^'s ingenious hypothesis, and the most probable view is that of Druner and Boveri, that the rays are differentiated out of the walls of the meshwork. In cases where the protoplasm is reticular or fibrillar the differentiation of the rays may be indistinguishable from a mere rearrangement of the thread-work; in alveolar protoplasm they may be seen as new formations, while in either case the material of the old aster may be more or less directly utilized in the building of the new. The feature common to all is the periodic activity either of the centre itself or of the surrounding protoplasm, and the coincidence or non-coincidence of the new aster with the old is apparently a secondary matter.

1 /.r., p. 283.

8 See the same paper for a suggestive comparison of the astral fibrillae to muscle-fibres. Y


322 SOME PROBLEMS OF CELL-ORGANIZATION

In its original form the archoplasm hypothesis, as stated by Boveri, was developed with reference only to the material of the spindlefibres and astral rays. Later writers have greatly extended the conception on the basis of Boveri's earlier view that archoplasm is a specific form of protoplasm, possessing specially active properties. Strasburger ('92-98), whose views have already been considered in part, believes the protoplasm to consist of, or to show a tendency to differentiate itself into, two distinct substances, namely, a specially active fibrillar kinoplastn and a less active alveolar trophoplasm. The former gives rise to the mitotic fibrillae, constitutes the peripheral cell layer, or Hautschicht, from which the membrane arises, forms the substance of the centrosomes, and gives origin to the contractile substance of cilia and flagella. The kinoplasm is thus mainly concerned with the motor phenomena of the cell, the trophoplasm with those of nutrition ; and this physiological difference is morphologically expressed in the fact that the former has in general a fibrillar structure, the latter an alveolar. Beyond this the two forms of protoplasm show a difference of staining-reaction, the kinoplasmic fibrillae staining deeply with gentian-violet and iron-haematoxylin, while the trophoplasm is but slightly stained.

Prenant ('98, '99) still further extends the hypothesis, adopting the view that the " ergastoplasmic " (Gamier) fibrillae of gland-cells l are equivalent to the kinoplasmic or archoplasmic fibrillae of the mitotic figure, and to the fibrillae of nerve- and muscle-fibres as well. He is thus led to the conception of a dominating or " superior " cytoplasm (including "archoplasm," "kinoplasm," "ergastoplasm"), which arises by differentiation out of the general cytoplasm, plays the leading role in the elaboration of active cell-elements ("cytosomes"), such as mitotic, neural, and glandular fibrillae, and finally, its rdle accomplished, may disappear. Under the same category with the foregoing structures are placed the centrosome, attraction-sphere, mid-body, idiozome, Nebenkern, and yolk-nucleus.

Such a generous expansion of the archoplasm-hypothesis brings it perilously near to a rcductio ad absurdum ; for the step is not a great one to the identification of the " superior protoplasm " with the active cell-substance in general, which would render the whole hypothesis superfluous. Physiologically, we can draw no definite line of demarcation between the more and the less active protoplasmic elements, and it may further be doubted whether such a boundary exists even between the latter and the metaplasmic substances. 2 It is further quite unjustifiable to infer physiological likeness from similarity in staining-reaction 3 or in fibrillar structure. For these reasons the hypothesis of " superior protoplasm " seems one of doubtful utility.

1 Cf. the pancreas, p. 44. 2 Cf. p. 29. 8 Cf. p. 335.


THE ARCHOPLASMIC STRUCTURES 323

In its more restricted form, however, the archoplasm or kinoplasm hypothesis is of high interest as indicating a common element in the origin and function of the mitotic fibrillae, the centrosome and midbody, and the contractile substances of cilia, flagella, and musclefibres. The main interest of the hypothesis seems to me to lie in the definite genetic relations that have been traced between the archoplasmic structures of successive cell-generations (as is most clearly shown in the phenomena of maturation and fertilization). It has been pointed out at various places in the preceding chapters l how many apparently contradictory phenomena in cell-division, fertilization, and related processes can be brought into relation with one another under the assumption of a specific substance, carried by the centrosome or less definitely localized, which gives the stimulus to division, which is concerned in the formation of the mitotic figure and of contractile elements, and which may be transmitted from cell to cell without loss of its specific character. There seems, however, to be clear evidence that such substance (or substances), if it exists, is not to be regarded as being necessarily a permanent constituent of the cell, but only as a phase, more or less persistent, in the general metabolic transformation of the cell-substance. 2

3. The Attraction-sphere

As originally used by Van Beneden 8 the term attraction-sphere was applied (in Ascaris) to the central mass of the aster surrounding the " corpuscule central " and consisting of medullary and cortical zones, as already described (p. 310). The cortical zone is bounded by a distinct circle of microsomes from which the astral rays proceed ; and at the close of cell-division the rays were stated to fade away, leaving only the attraction-sphere, which, like the centrosome, was regarded as a permanent cell-organ. Later researches have conclusively shown that the attraction-sphere cannot be regarded as a permanent organ, since in many cases it disintegrates and disappears. This occurs, for example, in the early prophases of mitosis in the testis-cells of the salamander, 4 where the sphere breaks up and scatters through the cell as the new amphiaster forms (Fig. 27). A very interesting case of this kind occurs in the cleavage of the ovum in Crepidula, as described by Conklin C99). The spheres here persist for a considerable period after division (Fig. 192), but have no direct relation to those of the ensuing division, finally disappearing in situ. The new spheres are formed about the centrosomes, which Conklin believes to migrate out of the old spheres (somewhat as occurs in the spermatid, p. 167) to their new position. The interesting point here is that the old sphere

1 Cf. pp. in, 215. 2 6y.p. 171. 8 '83, p. 548.

4 Drtiner, '95, Rawitz, '96, Meves, '96.


324 SOME PROBLEMS OF CELL-ORGAXIZATIOX

takes up such a position as to pass entirely into one of the granddaughter-cells, while the new sphere-substance is equally distributed between them and in its turn passes into one of the cells of the ensuing division. 1

In Crepidula, as in Ascaris, the attraction-sphere represents only the central part (centrosphere)of the aster. In some cases, however, e.g. in leucocytes, the entire aster may persist, and the term attraction-sphere has by some authors been applied to the whole structure. Later workers have proposed different terminologies, which are at present in a state of complete confusion. Fol ('91 ) proposed to call the centrosome the astrocentre, and the spherical mass surrounding it (attraction-sphere of Van Beneden) the astrosphere. Strasburger accepted the latter term but proposed the new word ccntrosphcre for the astrosphere and the centrosome taken together. 2 A new complication was introduced by Boveri ('95 ), who applied the word "astrosphere " to the entire aster exclusive of the centrosome, in which sense the phrase " astral sphere " had been employed by Mark in 188 1 . The word " astrosphere " has therefore a double meaning and would better be abandoned in favour of Strasburger's convenient term centra sphere, which may be understood as equivalent to the ** astrosphere " of Fol.

Besides these terms we have HeidenhahVs microcentrum (p. 311), equivalent to the centrosome or group of centrosomes at the centre of the aster, with its surrounding sphere : 3 Kostanecki's and Siedlecki's microsphere* applied to the central region of the aster surrounding the centrosome whether bounded by a distinct microsome-circle or not ; 4 Erlanger's centrop/asm, equivalent to microsphere ; 5 Ziegler's cctosphere and entosphere % applied to the cortical and medullar}' zones respectively : and M eves' s idiozome % applied to the attraction-sphere " of the spermatids. 6 This profusion of technical terms has arisen through the desire to avoid ambiguity in the use of the term *• attraction-sphere," which, like the word *" Nebenkern " (p. 163 \, has been applied to bodies of quite different origin and fate. If we adhere to Van Beneden's original use of the term it must be confined to the body surrounding the centrosome, forming a part of. or directly derived from, an aster, and giving rise wholly or in part to the succeeding aster. Meves('96), Rawitz (96X Erlanger ( '07, 21 and others have, however, clearly shown that the "attraction-sphere" surrounding the centrosome (in testis-cells)may not only contain other material derived from the cytoplasm, e.g. the %i centrodeutop'asm " of Erlanger. but may take no direct part in the succeeding aster- formation, disintegrating and scattering through the cell as the new aster forms ( Fig. 2JV In

  • Cf. p. 424- * '94- P- 403. * 96. 3. p. S.

1 '9i p. 5- « 9k p. 217. * "97. 4- P- 5*5


THE ARCHOPLASMIC STRUCTURES 325

other cases a sphere closely simulating an attraction-sphere may arise in the cytoplasm without apparent relation to the centrosomes or to the preceding aster, e.g. the yolk-nucleus or the sphere from which the acrosome arises in mammalian spermatogenesis. 1 To call such structures "attraction-spheres" or " archoplasm-masses " is to beg an important question ; and in all such doubtful cases the simple word sphere should be used. 2 In case of the aster itself we may, for descriptive purposes, employ Strasburger's convenient and non-committal term centrosphere, to designate in a somewhat vague and general way the central mass of the aster surrounding the centrosome, leaving its exact relation to Van Beneden's attraction-sphere to be determined in each individual case. Where the centrosphere shows two concentric zones (medullary and cortical), they may be well designated with Ziegler as entosphere (" centrosome " of Boveri) and ectosphere.

As regards the structure of the centrosphere, two well-marked types have been described. In one of these, described by Van Beneden in Ascaris, by Heidenhain in leucocytes, by Druner and Braus in dividing cells of Amphibia, and by Francotte, Van der Stricht, Lillie, Kostanecki, and others, in various segmenting eggs, the centrosphere has a radiate structure, being traversed by rays which stretch between the centrosome and the peripheral microsome-circle (Fig. 152, D, E t F), when the latter exists. In the other form, described by Vejdovsky in the eggs of Rhynchelmis, by Solger and Zimmermann in pigment-cells, by myself in Nereis, by Riickert in Cyclops, by Mead in Chaetoptems, Griffin in Tfialassema, Coe in Cerebratulus, Gardiner in Po/ychcerus, and many others, the centrosphere has a non-radial reticular or vesicular structure, in which the centrosomes lie (Figs. 152, H> 155). Kostanecki and others have endeavoured to show that such structures are artifacts, insisting that in perfectly fixed material the astral rays always traverse the centrosphere to the centrosome. This interpretation is, however, contradicted by the fact that the new asters developing in the centrospheres during the anaphases and telophases of such forms as Thalassema or Cerebratulus (Figs. 99, 155) show perfect fixation of the rays. The reticular centrosphere almost certainly arises as a normal differentiation of the interior of the aster, which, as Griffin ('96) has suggested, probably marks the beginning of the degeneration of the whole astral apparatus, to make way for the newly developing system.

The radial centrosphere is in Ascaris divided into cortical and medullary zones, as already described (p. 310), the aster being bounded by a distinct circle of microsomes. The true interpretation of these zones was given through HeidenhahVs beautiful studies on the asters in leucocytes, and the still more thorough later work of Druner on the sper 1 Cf. p. 170. 2 Cf. Lenhossck, '98.


326 SOME PROBLEMS OF CELL-ORGANIZATION

matocyte-divisions of the salamander. In leucocytes (Fig. 49) the large persistent aster has at its centre a well-marked radial sphere bounded by a circle of microsomes, as described by Van Beneden, but without division into cortical and medullary zones. The astral rays, however, show indications of other circles of microsomes lying outside the centrosphere. Driiner found that a whole series of such concentric circles might exist (in the cell shown in Fig. 1 56 no less than nine), but that the innermost two are often especially distinct, so as to mark off a centrosphere composed of a medullary and a cortical zone precisely as described by Van Beneden. These observations show conclusively that the centrosphere of the radial type is merely the innermost portion of the aster, which acquires a boundary through the especial development of a ring of microsomes, or otherwise, and which often further acquires an intense staining-capacity so as to appear like acentrosome ^tart^uS^'tf d^^^k <P- 3'3)- I" Tkystni0soif*(Vm der

circles of microsome!). The area within the Stricht) Only a Single ring of microsecond drclc probably represents the "altrac- somes exists, and this lies at the Uoii-spherc of Van Eteneden. '

boundary between the medullary and cortical zones (Fig. 152, D), the latter differing from the outer region only in the greater delicacy of the rays and their lack of stain ing-capacity, thus producing a " Heller Hof." In other cases, no " microsome-circles " exist ; but even here a clear zone often surrounds the centrosome (e.g. in P/tysa, t. Kostanecki and Wierzejski), like that seen in the cortical zone of ThysauosoSn.

There are some observations indicating that the entosphere (medullary zone) may be directly derived from the centrosome (central granule). This is the conclusion reached by Lillie in the case of Vnio referred to above, where, during the prophases of the second polar spindle, the central granule enlarges and breaks up into a group of granules from which the new entosphere is formed. Van der Stricht ('98) reaches a similar conclusion in case of the first polar spindle of Tliysanosoon. We may perhaps give the same interpretation to the large pluricorpiisciilar centrum of echinoderms (p. 314). This observation may be used in support of the probability that the astral rays



Pig. Ijjfi. — Spermatogonium der. [DHUNEK.J

The nucleus lies below. Abov


SUMMARY AND CONCLUSION 327

may be actually derived from the centrosome (p. 321) ; but Lillie finds in some cases that in the same mitosis the entosphere is formed by a different process, arising by a differentiation of the cytoplasm around the central granule. The former case, therefore, may be interpreted to mean simply that the centrosome may give rise to other cytoplasmic elements (as has already been shown in the formation of the spermatozoon, p. 172), the material of which may then contribute either directly or indirectly to the building of the aster ; and the facts do not come into collision with the view that the astral rays are in general formed from the cytoplasmic substance.

G. Summary and Conclusion

A minute analysis of the various parts of the cell leads to the conclusion that all cell-organs, whether temporary or " permanent," are local differentiations of a common structural basis. Temporary organs, such as cilia or pseudopodia, are formed out of this basis, persist for a time, and finally merge their identity in the common basis again. Permanent organs, such as the nucleus or plastids, are constant areas in the same basis, which never are formed de novo, but arise by the division of preexisting areas of the same kind. These two extremes are, however, connected by various intermediate gradations, examples of which are the contractile vacuoles of Protozoa, which belong to the category of temporary organs, yet in many cases are handed on from one cell to another by fission, and the attraction-spheres and asters, which may either persist from cell to cell or disappear and re-form about the centrosome. There is now considerable evidence that the centrosome itself may in some cases have the character of a permanent organ, in others may disappear and re-form like the asters.

The facts point toward the conclusion, which has been especially urged by De Vries and Wiesner, that the power of division, not only of the cell-organs, but also of the cell as a whole, may have its root in a like power on the part of more elementary masses or units of which the structural basis is itself built, the degree of permanence in the cellorgans depending on the degree of cohesion manifested by these elementary bodies. If such bodies exist, they must, however, in their primary form, lie beyond the present limits of the microscope, the visible structures arising by their enlargement or aggregation. The cell, therefore, cannot be regarded as a colony of "granules or other gross morphological elements. The phenomena of cell-division show, however, that the dividing substance tends to differentiate itself into several orders of visible morphological aggregates, as is most clearly shown in the nuclear substance. Here the highest term is the plurivalent chromosome, the lowest the smallest visible dividing basichromatin-grains,


328 SOME PROBLEMS OF CELL-ORGANIZATION

while the intermediate terms are formed by the successive aggregation of these to form the chromatin-granules of which the dividing chromosomes consist. Whether any or all of these bodies are " individuals " is a question of words. The facts point, however, to the conclusion that at the bottom of the series there must be masses that cannot be further split up without loss of their characteristic properties, and which form the elementary morphological units of the nucleus. In case of the cytoplasm the evidence is far less satisfactory. Could Rabl's theory of fibrillar persistence, as developed by Heidenhain and Kostanecki, be established, we should indeed have almost a demonstration of panmeristic division in the cytoplasm. At present, however, the facts do not admit the acceptance of that theory, and the division of the visible cytoplasmic granules must remain a quite open question. Yet we should remember that the dividing plastids of plant-cells are often very minute, and that in the centrosome we have a body, no larger in many cases than a " microsome," which is positively known to be in some cases a persistent morphological element, having the power of growth, division, and persistence in the daughter-cells. Probably these powers of the centrosome would never have been discovered were it not that its staining-capacity renders it conspicuous and its position at the focus of the astral rays isolates it for observation. When we consider the analogy between the centrosome and the basichromatin-grains, when we recall the evidence that the latter graduate into the oxychromatin-granules, and these in turn into the cytomicrosomes, we must admit that Briicke's cautious suggestion that the whole cell might be a congeries of selfpropagating units of a lower order is sufficiently supported by fact to constitute a legitimate working hypothesis.


LITERATURE. VI l

Van Beneden, E. — (See List IV.)

Van Beneden and Julin. — La segmentation chez les Ascidiens et ses rapports avec

l'organisation de la larve: Arch. Biol., V. 1884. Boveri, Th. — Zellenstudien. (See List IV.)

Briicke, C — Die Elementarorganismen : Wiener Siiz.-fier.. XLIV. 1861. Butschli, 0. — Protoplasma. (See List I.) Delage, Yves. — La structure du protoplasma. et les thdories sur rhe'rddite'. Paris*

^95Hacker, V. — Uber den heutigen Stand der Centrosomenfrage : Vcrh. d. deutsch.

Zool. Ges. 1894. Heidenhain, M. — (See List I.) Herla, V. — Etude des variations de la mitose chez Tascaride megalocephale : Arch.

BioLXllh 1893.

1 See also Literature, I., II., IV., V.


LITERATURE 329

Morgan, T. H. — The Action of Salt-solutions on the Fertilized and Unfertilized

Eggs of Arbacia and Other Animals. Arch. Entw.* VIII. 3. 1898. Kostanecki, K. — Ueber die Bedeutung der Polstrahung wahrend der Mitose. Arch.

mik. Ana/., XLIX. 1897. Nussbaum, M. — Uber die Teilbarkeit der lebendigen Materie : Arch. mik. Anat.,

XXVI. 1886. Prenant, A. — Sur le protoplasma supeVieure (archiplasme. kinoplasme. ergastro plasme) : Journ. Ana/, et Phys., XXIV.-V. 1898-99. (Full Literature-lists.) Rabl, C. — Uber Zellteilung: Morph. Jahrb., X. 1885. Anat. Anzeiger,W. 1889. Ruckert, J. — (See List IV.)

De Vries, H. — Intracellular Pangenesis: Jena % 1889.

Watasl, S. — Homology of the Centrosome : Journ. Morph., VIII. 2. 1893. Id. — On the Nature of Cell-organization : Woods Holl Biol. lectures. 1893. Wiesner, J. — Die Elementarstruktur und das Wachstum der lebenden Substanz :

W'ien, 1892. Wilson, Edm. B. — Archoplasm, Centrosome, and Chromatin in the Sea-urchin Egg :

Journ. Morph., Vol. XI. 1895.


CHAPTER VII

SOME ASPECTS OF CELL-CHEMISTRY AND CELL-PHYSIOLOGY

" Les phenomenes fonctionnels ou de depense vitale aura tent done leur sitge dans U protof las me cellulaire.

" Le noyau est un appareil de synthase org ant que, l' instrument de la production, U germe de la cellule." Claude Bernard;


A. Chemical Relations of Nucleus and Cytoplasm

It is no part of the purpose of this work to give even a sketch of general cell-chemistry. I shall only attempt to consider certain questions that bear directly upon the functional relations of nucleus and cytoplasm and are of especial interest in relation to the process of nutrition and through it to the problems of development. It has often been pointed out that we know little or nothing of the chemical conditions existing in living protoplasm, since every attempt to examine them by precise methods necessarily kills the protoplasm. We must, therefore, in the main rest content with inferences based upon the chemical behaviour of dead cells. But even here investigation is beset with difficulties, since it is in most cases impossible to isolate the various parts of the cell for accurate chemical analysis, and we are obliged to rely largely on the less precise method of observing with the microscope the visible effects of dyes and other reagents. This difficulty is increased by the fact that both cytoplasm and karyoplasm are not simple chemical compounds, but mixtures of many complex substances ; and both, moreover, undergo periodic changes of a complicated character which differ very widely in different kinds of cells. Our knowledge is, therefore, still fragmentary, and we have as yet scarcely passed the threshold of a subject which belongs largely to the cytology of the future.

It has been shown in the foregoing chapter that all the parts of the cell arise as local differentiations of a general protoplasmic basis. Despite the difficulties of chemical analysis referred to above, it has been determined with certainty that some at least of these organs are the seat of specific chemical change ; just as is the case in the various organs and tissues of the organism at large. Thus, the nucleus is

1 1 eeons sur Us phenomenes de la vie, I., 1 878, p. 198.

330


CHEMICAL RELATIONS OF NUCLEUS AND CYTOPLASM 33 1

characterized by the presence of nuclein (chromatin) which has been proved by chemical analysis to differ widely from the cytoplasmic substances, 1 while the various forms of plastids are centres for the formation of chlorophyll, starch, or pigment. These facts give ground for the conclusion that the morphological differentiation of cell-organs is in general accompanied by underlying chemical specializations which are themselves the expression of differences of metabolic activity ; and these relations, imperfectly comprehended as they are, are of fundamental importance to the student of development.

1. The Proteids and their Allies

The most important chemical compounds found in the cell are the group of protein substances, and there is every reason to believe that these form the principal basis of living protoplasm in all of its forms. These substances are complex compounds of carbon, hydrogen, nitrogen, and oxygen, often containing a small percentage of sulphur, and in some cases also phosphorus and iron. They form a very extensive group of which the different members differ considerably in physical and chemical properties, though all have certain common traits and are closely related. They are variously classified even by the latest writers. By many authors (for example Halliburton, '93) the word "proteids " is used in a broad sense as synonymous with albuminous substances, including under them the various forms of albumin (eggalbumin, cell-albumin, muscle-albumin, vegetable-albumins), globulin (fibrinogin vitellin, etc.), and the peptones (diffusible hydrated proteids). Another series of nearly related substances are the albuminoids (reckoned by some chemists among the "proteids"), examples of which are gelatin, mucin, and, according to some authors also, nuclein, and the nucleo-albumins. Some of the best authorities however, among them Kossel and Hammarsten, follow the usage of Hoppe-Seyler in restricting the word proteid to substances of greater complexity than the albumins and globulins. Examples of these are the nucleins and nucleo-proteids, which are compounds of nucleinic acid with albumin, histon, or protamin. The nucleo-proteids, found only in the nucleus, are not to be confounded with the nucleo 1 It has long been known that a form of " nuclein " may also be obtained from the nucleoalbumins of the cytoplasm, eg. from the yolk of hens' eggs (vitellin). Such nucleins differ, however, from those of nuclear origin in not yielding as cleavage-products the nuclein bases (adenin, xanthin, etc.). The term " paranuclein " (Kossel) or " pseudo-nuclein " (Hammarsten) has therefore been suggested for this substance. True nucleins containing a large percentage of albumin are distinguished as tiucU&-proteids. They may be split into albumin (or albumin radicals) and nucleinic acid, the latter yielding as cleavage-products the nuclein bases. Pseudo-nucleins containing a large percentage of albumin are designated as nucleoalbumins, which in like manner split into albumin and paranucleinic or pseudo-nucleinic acid, which yields no nuclein bases. (See Hammarsten, '94.)


332


CELL-CHEMISTRY ASD CELL^PHYSJOLOGY


albumins, which are compounds of pseudo-nucleinic acid with albumin and yield no nuclein-bases (xanthin, hypoxanthin, adenin, guanin) as decomposition products.

The distribution of these substances through the cell varies greatly not only in different cells, but at different periods in the life of the same cell. The cardinal fact always, however, remains, that there is a definite and constant contrast between nucleus and cytoplasm. The latter always contains large quantities of nucleo-albumins, certain globulins, and sometimes small quantities of albumins and peptones ; the former contains, in addition to these, nuclein and nucleo-proteids y which form its main bulk, and its most constant and characteristic feature. It is the remarkable substance, nuclein, — which is almost certainly identical with chromatin, — that chiefly claims our attention here on account of the physiological rdle of the nucleus.

2. The Nuclein Series

Nuclein was first isolated and named by Miescher, in 1 871, by subjecting cells to artificial gastric digestion. The cytoplasm is thus digested, leaving only the nuclei ; and in some cases, for instance puscells and spermatozoa, it is possible by this method to procure large quantities of nuclear substance for accurate quantitative analysis. The results of analysis show it to be a complex albuminoid substance, rich in phosphorus, for which Miescher gave the chemical formula Qd^^N^PgO^- The earlier analysis of this substance gave somewhat discordant results, as appears in the following table of percentage-compositions : 1 —



Pus-cells.


Spermatozoa of Salmon.


Human Brain.



(Hoppe-Seyler.)


(Miescher.)


(v. Jaksch.)


c


495 8


36."


50.6


H


7.IO


5- ! 5


7.6


N


15.02


13-09


1318


P


2.28


559


I.89


These differences led to the opinion, first expressed by HoppeSeylcr, and confirmed by later investigations, that there are several varieties of nuclein which form a group having certain characters in common. Altmann ('89) opened the way to an understanding of the matter by showing that " nuclein " may be split up into two substances ; namely, ( 1 ) an organic acid rich in phosphorus, to which he

1 From Halliburton, '91, p. 203. [The oxygen-percentage is omitted in this table.]


CHEMICAL RELATIONS OF NUCLEUS AND CYTOPLASM 333

gave the name nuclcinic acid, and (2) a form of albumin. Moreover, the nuclein may be synthetically formed by the re-combination of these two substances. Pure nucleinic acid, for which Miescher ('96) afterward gave the formula C 40 H 54 N 14 P 4 O 27 , 1 contains no sulphur, a high percentage of phosphorus (above 9%), and no albumin. By adding it to a solution of albumin a precipitate is formed which contains sulphur, a lower percentage of phosphorus, and has the chemical characters of " nuclein. " This indicates that the discordant results in the analyses of nuclein, referred to above, were probably due to varying proportions of the two constituents; and Altmann suggested that the " nuclein " of spermatozoa, which contains no sulphur and a maximum of phosphorus, might be uncombined nucleinic acid itself. Kossel accordingly drew the conclusion, based on his own work as well as that of Liebermann, Altmann, Malfatti, and others, that "what the histologists designate as chromatin consists essentially of combinations of nucleinic acid with more or less albumin, and in some cases may even be free nucleinic acid. The less the percentage of albumin in these compounds, the nearer do their properties approach those of pure nucleinic acid, and we may assume that the percentage of albumin in the chromatin of the same nucleus may vary according to physiological conditions." 2 In the same year Halliburton, following in part Hoppe-Seyler, stated the same view as follows. The so-called " nucleins " form a series leading downward from nucleinic acid thus : —

(1) Those containing no albumin and a maximum (9-10%) of phos phorus (pure nucleinic acid). Nuclei of spermatozoa.

(2) Those containing little albumin and rich in phosphorus. Chro matin of ordinary nuclei.

(3) Those with a greater proportion of albumin — a series of sub stances in which may probably be included pyrenin (nucleoli) and plastin (linin). These graduate into

(4) Those containing a minimum (0.5 to 1%) of phosphorus —

the nucleo-albumins, which occur both in the nucleus and in the cytoplasm (vitellin, caseinogen, etc.).

Finally, we reach the globulins and albumins, especially characteristic of the cell-substance, and containing no nucleinic acid. "We thus pass by a gradual transition (from the nucleo-albumins) to the other proteid constituents of the cell, the cell-globulins, which contain no phosphorus whatever, and to the products of cell-activity, such as the proteids of serum and of egg-white, which are also principally

1 Derived from analysis of the salmon-sperm. 8 '93, p. 158.


334 CELL-CHEMISTRY AND CELL-PHYSIOLOGY

phosphorus-free." l Further, " in the processes of vital activity there are changing relations between the phosphorized constituents of the nucleus, just as in all metabolic processes there is a continual interchange, some constituents being elaborated, others breaking down into simpler products." This latter conclusion has been well established; the others, as stated by Halliburton, require some modification, on the one hand, through the results of later analyses of chromatin, on the other, because of the failure to distinguish between the nucleoproteids and the nucleo-albumins. First, it has been shown by Miescher ('96), Kossel ('96), and Mathews ('97, 2) that the chromatin of the sperm-nuclei (in fish and sea-urchins) is not pure nucleinic acid, as Altmann conjectured, but a salt of that acid, with histon, protamin, or a related substance. Thus, in the spermatozoa of the salmon, Miescher's analyses give 60.56% of nucleinic acid and 35.56% of protamin (C 16 H 28 N 9 2 ). In the herring the chromatin is a compound of nucleinic acid (over 63%) and a form of protamin called by Kossel " clupein " (C 30 H 57 N 17 O 6 ). In the sea-urchin Arbacia Mathews finds the chromatin to be a compound of nucleinic acid and " arbacin," a histon-like body. Kossel finds also that chromatin (nuclein) derived from the thymus gland, and from leucocytes, is largely a histon salt of nucleinic acid, the proportion of the latter being, however, much less than in the sperm-chromatin, while albumin is also present. In these cases, therefore, the greater part of the nucleinic acid is combined not with albumin but with a histon or protamin radical. Second, the nucleo-albumins of the cytoplasm are in no sense transitional between the nucleins and the albumins, since they contain no true nucleinic acid, but only pseudo-nucleinic acid. 2 The fact nevertheless remains that the nucleins and nucleo-proteids, though confined to the nucleus, form a series descending from such highly phosphorized bodies as the sperm-chromatin toward bodies such as the albumins, which are especially characteristic of the cytoplasm ; and that they vary in composition with varying physiological conditions. The way is thus opened for a more precise investigation of the physiological role of nucleus and cytoplasm in metabolism.

3. Staining-re action of the Nuclein Series

In bringing these facts into relation with the staining-reactions of the cell, it is necessary briefly to consider the nature of stainingreactions in general, and especially to warn the reader that in the whole field of " micro-chemistry " we are still on such uncertain ground that all general conclusions must be taken with reserve.

First, it is still uncertain how far staining-reactions depend upon chemical reaction and how far upon merely physical properties of

1, 93»P. 574- 2 C/.p. 331.


CHEMICAL RELATIONS OF NUCLEUS AND CYTOPLASM 335

the bodies stained. The prevalent view that staining-reactions are due to a chemical combination of the dye with the elements of the cell has been attacked by Gierke ('85), Rawitz C97), and Fischer ('97, '99), all of whom have endeavoured to show that these reactions are of no value as a chemical test, being only a result of surfaceattraction and absorption due to purely physical qualities of the bodies stained. On the other hand, a long series of experiments, beginning with Miescher's discovery ('74) that isolated nucleinic acid forms green insoluble salts with methyl-green, and continued by Lilienfeld, Heidenhain, Paul Mayer, and others, gives strong reason to believe that beyond the physical imbibition of colour a true chemical union takes place, which, with due precautions, gives us at least a rough test of the chemical conditions existing in the cell. 1

Second, similarity of s tain ing-rcact ion is by no means always indicative of chemical similarity y as is shown, for example, by the fact that in cartilage both nuclei and inter-cellular matrix are intensely stained by methyl-green, though chemically they differ very widely.

Third, colour in itself gives no evidence of chemical nature ; for the nucleus and other elements of the same cell may be stained red, green, or blue, according to the dye employed, and to class them as " erythrophilous," "cyanophilous," and the like, is therefore absurd.

Fourth, the character of the staining-reaction is influenced and in some cases determined by the fixation or other preliminary treatment, a principle made use of practically in the operations of mordaunting, but one which may give very misleading results unless carefully controlled. Thus Rawitz ('95) shows that certain colours which ordinarily stain especially the nucleus (saffranin, gentian-violet), can be made to stain only the cytoplasm through preliminary treatment of object with solutions of tannin, followed by tartar-emetic. In like manner Mathews ('98) shows that many of the " nuclear " tar-colours (saffranin, methyl-green, etc.) stain or do not stain the cytoplasm, according as the material has been previously treated with alkaline or with acid solutions.

The results with which we now have to deal are based mainly upon experiments with tar-colours ("aniline dyes"). Ehrlich ('79) long since characterized these dyes as "acid" or "basic," according as the colouring matter plays the part of an acid or a base in the compound employed, showing further that, other things equal, the basic dyes (methyl-green, saffranin, etc.) are especially "nuclear stains" and the acid (rubin, eosin, orange, etc.) "plasma stains." Malfatti ('91), and especially Lilienfeld ('92, '93), following out Miescher's earlier work ('74), found that albumin stains preeminently in the acid stains, nucleinic acid only in the basic ; and, further, that artifi 1 Cf. Mayer, '91, '92, '97; Lilienfeld, '93; Mathews, '98.


336 CELL-CHEMISTRY AND CELL-PHYSIOLOGY

rial nucleins, prepared by combining egg-albumin with nucleinic acid in various proportions, show a varying affinity for basic and acid dyes according as the nucleinic acid is more or less completely saturated with albumin. Lilienfeld's starting-point was given by the results of Kossel's researches on the relations of the nuclein group, which are expressed as follows : l —

Nuclco-proteid (i% of P or less), by peptic digestion splits into


Peptone Nuclein (3-4% P),

by treatment with acid splits into

, ■ ,

Albumin Nucleinic acid (9-10% P),

heated with mineral acids splits into

, . _^

Phosphoric acid Nuclein bases (A carbohydrate.)

(adenin, guanin, etc.).

Now, according to Kossel and Lilienfeld, the principal nucleoproteid in the nucleus of leucocytes is nucleo-liiston, containing about 3% of phosphorus, which may be split into a form of nticlein playing the part of an acid, and an albuminoid base, the his ton of Kossel; the nuclein may in turn be split into albumin and nucleinic acid. These four substances — albumin, nucleo-histon, nuclein, nucleinic acid — thus form a series in which the proportion of phosphorus, which is a measure of the nucleinic acid, successively increases from zero to 9-10%. If the members of this series be treated with the same mixture of red acid fuchsin and basic methyl-green, the result is as follows. Albumin (egg-albumin) is stained red, nucleo-histon greenish blue, nuclein bluish green, nucleinic acid intense green. "We see, therefore, that the principle that determines the staining of the nuclear substances is always the nucleinic acid. All the nuclear substances, from those richest in albumin to those poorest in it, or containing none, assume the tone of the nuclear {i.e. basic) stain, but the combined albumin modifies the green more or less toward blue." a Lilienfeld explains the fact that chromatin in the cell-nucleus seldom appears pure green on the assumption, supported by many facts, that the proportions of nucleinic acid and albumin vary with different physiological conditions, and he suggests further that the intense staining-power of the chromosomes during mitosis is probably due to the fact that they contain a maximum of nucleinic acid. Very interesting is a comparison of the foregoing staining-reactions with those given by a mixture of a red basic dye (saffranin) and a green acid one (" light green "). With this combination an effect is given which reverses that of the Biondi-Ehrlich mixture ; i.e. the nuclein

1 From Lilienfeld, after Kossel ('92, p. 129). a Lc. t p. 394.


CHEMICAL RELATIONS OF NUCLEUS AND CYTOPLASM 337

is coloured red, the albumin green, which is a beautiful demonstration of the fact that staining-reagents cannot be logically classified according to colour, but only according to their chemical nature, and gives additional ground for the view that staining-reactions of this type are the result of a chemical rather than a merely physical combination.

These results must be taken with some reserve for the following reasons: Mathews ('98) has shown that methyl-green and other basic dyes will energetically stain albumose, coagulated egg-albumin, and the cell-cytoplasm in or after treatment by alkaline fluids ; while conversely the acid dyes do not stain, or only slightly stain, these substances under the same conditions. This probably does not affect the validity of HeidenhahVs results, 1 since he worked with acid solutions. What is more to the point is the fact that hyaline cartilage and mucin, though containing no nucleinic acid, stain intensely with basic dyes. Mathews probably gives the clue to this reaction, in the suggestion that it is here probably due to the presence of other acids (in the case of cartilage a salt of chondroitin-sulphuric acid, according to Schmiedeberg); from which Mathews concludes that the basic dyes will, in acid or neutral solutions, stain any element of the tissues that contains an organic acid in a salt combination with a strong base. 2 Accepting this conclusion, we must therefore recognize that, as far as the cytoplasm is concerned, the basic or " nuclear " stains are in no sense a test for nuclein, but only for salts of organic acids in general. In case of the nucleus, however, we know from direct analysis that we are dealing with varying combinations of nucleinic acid, and hence, with the precautions indicated above, may draw provisional conditions from the staining-reactions.

Thus regarded, the changes of staining-reaction in the chromatin are of high interest. Heidenhain ('93, '94), in his beautiful studies on leucocytes, has correlated some of the foregoing results with the staining-reactions of the cell as follows. Leucocytes stained with the Biondi-Ehrlich mixture of acid fuchsin and methyl-green show the following reactions. Cytoplasm, centrosome, attraction-sphere, astral rays, and spindle-fibres are stained pure red. The nuclear substance shows a very sharp differentiation. The chromatic network and the chromosomes of the mitotic figure are green. The lininsubstance and the true nucleoli or plasmosomes appear red, like the cytoplasm. The linin-network of leucocytes is stated by Heidenhain to consist of two elements, namely, of red granules or microsomes suspended in a colourless network. The latter alone is called " linin " by Heidenhain. To the red granules is applied the term " oxychromatin," while the green substance of the ordinary chromatic network,

1 See below. 2 '98, pp. 451-452.

z


338 CELI^CHEMISTRY AND CELL-PHYSIOLOGY

forming the " chromatin " of Flemming, is called " basichromatin." * Morphologically, the granules of both kinds are exactly alike, 2 and in many cases the oxychromatin-granules are found not only in the " achromatic " nuclear network, but also intermingled with the basichroma tin-granules of the chromatic network. Collating these results with those of the physiological chemists, Heidenhain concludes that basichromatin is a substance rich in phosphorus (i.e. nucleinic acid), oxychromatin a substance poor in phosphorus, and that, further, " basichromatin and oxychromatin are by no means to be regarded as permanent unchangeable bodies but may change their colourreactions by combining with or giving off phosphorus." In other words, " the affinity of the chromatophilous microsomes of the nuclear network for basic and acid aniline dyes is regulated by certain physiological conditions of the nucleus or of the cell." 3

This conclusion, which is entirely in harmony with the statements of Kossel and Halliburton quoted above, opens up the most interesting questions regarding the periodic changes in the nucleus. The staining-power of chromatin is at a maximum when in the preparatory stages of mitosis (spireme-thread, chromosomes). During the ensuing growth of the nucleus it always diminishes, suggesting that a combination with albumin has taken place. This is illustrated in a very striking way by the history of the egg-nucleus or germinal vesicle, which exhibits the nuclear changes on a large scale. It has long been known that the chromatin of this nucleus undergoes great changes during the growth of the egg f and several observers have maintained its entire disappearance at one period. Riickert first carefully traced out the history of the chromatin in detail in the eggs of sharks, and his general results have since been confirmed by Born in the eggs of Triton. In the shark Pristinrus^ Riickert ('92, 1 ) finds that the chromosomes, which persist throughout the entire growth-period of the egg t undergo the following changes (Fig. 157): At a very early stage they are small, and stain intensely with nuclear dyes. During the growth of the egg they undergo a great increase in size, and progressively lose their staining-capacity. At the same time their surface is enormously increased by the development of long threads which grow out in every direction from the central axis (Fig. 157, A). As the egg approaches its full size, the chromosomes rapidly diminish in size, the radiating threads disappear, and the staining-capacity increases (Fig. 157, B). They are finally again reduced to minute, intensely staining bodies which enter into the equatorial plate of the first polar, mitotic figure (Fig. 157, C). How great the change of volume is may be seen from the following figures. At the beginning the chromosomes measure, at most, 12 /* (about ^uW m -) m length and

1 '94, P- 543- a/ ' c > P- 547- 8/ '» P- 548.


CHEMICAL RELATIONS OF NUCLEUS AND CYTOPLASM


339


\ p in diameter. At the height of their development they are almost eight times their original length and twenty times their original diameter. In the final period they are but 2 p in length and 1 fi in diameter. These measurements show a change of volume so enormous, even after making due allowance for the loose structure of the large chromosomes, that it cannot be accounted for by mere swelling or shrinkage. The chromosomes evidently absorb a large amount of




Pif . 157- — Chromosomes of the germinal »e drawn lo Ihe same scale. [RUCKERT.]

A. Al Ihe period ol maiimal site and min B. Later period (egg 13 mm. in diameter). C.


matter, combine with it to form a substance of diminished stainingcapacity, and finally give off matter, leaving an intensely staining substance behind. As Ruckert points out, the great increase of surface in the chromosomes is adapted to facilitate an exchange of material between the chromatin and the surrounding substance; and he concludes that the coincidence between the growth of the chromosomes and that of the egg points to an intimate connection between the nuclear activity and the formative energy of the cytoplasm.


340 CEU^CHEMISTRY AND CELL-PHYSIOLOGY

If these facts are considered in the light of the known stainingreaction of the nuclein series, we must admit that the following conclusions are something more than mere possibilities. We may infer that the original chromosomes contain a high percentage of nucleinic acid ; that their growth and loss of staining-power is due to a combination with a large amount of albuminous substance to form a lower member of the nuclein series, probably a nucleo-proteid ; that their final diminution in size and resumption of staining-power is caused by a giving up of the albumin constituent, restoring the nuclein to its original state as a preparation for division. The growth and diminished staining-capacity of the chromatin occurs during a period of intense constructive activity in the cytoplasm ; its diminution in bulk and resumption of staining-capacity coincides with the cessation of this activity. This result is in harmony with the observations of Schwarz and Zacharias on growing plant-cells, the percentage of nuclein in the nuclei of embryonic cells (meristem) being at first relatively large and diminishing as the cells increase in size. It agrees further with the fact that of all forms of nuclei those of the spermatozoa, in which growth is suspended, are richest in nucleinic acid, and in this respect stand at the opposite extreme from the nuclei of the rapidly growing egg-cell.

Accurately determined facts in this direction are still too scanty to admit of a safe generalization. They are, however, enough to indicate the probability that chromatin passes through a certain cycle in the lite of the cell, the percentage of albumin or of albumin-radicals increasing during the vegetative activity of the nucleus, decreasing in its reproductive phase./ In other words, a combination of albumin with nuclein or nucleinic acid is an accompaniment of constructive metabolism. As the cell prepares for division, the combination is dissolved and the nuclein-radicle or nucleinic acid is handed on by division to the daughter-cells. A tempting hypothesis, suggested by Mathews on the basis of Kossel's work, is that nuclein, or one of its constituent molecular groups, may in a chemical sense be regarded as the formative centre of the cell which is directly involved in the process by which food-matters are built up into the cell-substance. Could this be established, we should have not only a clear light on the changes of staining-reactions during the cycle of cell-life, but also a clue to the nuclear "control" of the cell through the process of synthetic metabolism. This hypothesis fits well with the conclusions of other physiological chemists that the nucleus is especially concerned in synthetic metabolism. Kossel concludes that the formation of new organic matter is dependent on the nucleus, 1 and that nuclein in some manner plays a leading role in this process ; and he makes

1 Schieffenlecker and Kossel, Geivebelekrg % p. 57.


PHYSIOLOGICAL RELATIONS OF NUCLEUS AND CYTOPLASM 341

some interesting suggestions regarding the synthesis of complex organic matters in the living cell with nuclein as a starting-point. Chittenden, too, in a review of recent chemico-physiological discoveries regarding the cell, concludes : " The cell-nucleus may be looked upon as in some manner standing in close relation to those processes which have to do with the formation of organic substances. Whatever other functions it may possess, it evidently, through the inherent qualities of the bodies entering into its composition, has a controlling power over the metabolic processes in the cell, modifying and regulating the nutritional changes " C94).

These conclusions, in their turn, are in harmony with the hypothesis advanced twenty years ago by Claude Bernard ('78), who maintained that the cytoplasm is the seat of destructive metabolism, the nucleus the organ of constructive metabolism and organic synthesis, and insisted that the rdlc of the nucleus in nutrition gives the key to its significance as the organ of development, regeneration, and inheritance. 1

B. Physiological Relations of Nucleus and Cytoplasm

How nearly the foregoing facts bear on the problem of the morphological formative power of the cell is obvious ; and they have in a measure anticipated certain conclusions regarding the rdle of nucleus and cytoplasm, which we may now examine from a somewhat different point of view.

Briicke long ago drew a clear distinction between the chemical and molecular composition of organic substances, on the one hand, and, on the other hand, their definite grouping in the cell by which arises organization in a morphological sense. Claude Bernard, in like manner, distinguished between chemical synthesis, through which organic matters are formed, and morphological synthesis, by which they are built into a specifically organized fabric ; but he insisted that these two processes are but different phases or degrees of the same phenomenon, and that both are expressions of the nuclear activity. We have now to consider some of the evidence that the power of morphological, as well as of chemical, synthesis centres in the nucleus, and that this is therefore to be regarded as the especial organ of inheritance. This evidence is mainly derived from the comparison of nucleated and non-nucleated masses of protoplasm ; from the form,

1 " II scmble done que la cellule qui a perdu son noyau soit sterilisee au point de vue de la generation, e'est a dire de la synthese morphologique, et qu'elle le soit aussi au point de vue de la synthese chimique, car elle cesse de produire des principes i m media ts, et ne peut gucrc qu'oxyder et detruire ceux qui s'y etaicnt accumules par une elaboration anterieure du noyau. II semble done que le noyau soit le germe de nutrition dela cellule ; il attire autour de lui et llabore les materiaux nutritifs" ('78, p. 523).


34^ CELL-CHEMISTRY AND CELL-PHYSIOLOGY

position, and movements of the nucleus in actively growing or metabolizing cells; and from the history of the nucleus in mitotic celldivision, in fertilization, and in maturation.


I. Experiments on Unicellular Orgt


Brandt ('77) long since observed that enucleated fragments of Aetinosphcerium soon die, while nucleated fragments heal their wounds and continue to live. The first decisive comparison between nucleated and non-nucleated masses of protoplasm was, however, made by Moritz Nussbaumin 1884 in the case of an infusorian, Oxytruha. If one of these animals be cut into two pieces, the subsequent behaviour of the two fragments depends on the presence or absence of the nucleus or a nuclear fragment. The nucleated fragments quickly heal the wound, regenerate the missing portions, and thus produce a perfect animal. On the other hand, enucleated fragments, consisting of cytoplasm only, quickly perish. Nussbaum therefore drew the conclusion that the nucleus is indispensable for the formative energy of the cell. The experiment was soon after repeated by Gruber('S5) in the case of Stenlor, another infusorian, and with the same result (Fig. 159). Fragments possessing a large fragment of the nucleus completely regenerated within twenty-four hours. If the nuclear fragment were smaller, the regeneration proceeded more slowly. If no nuclear substance were present, no regeneration took place, though the wound closed and the fragment lived for a considerable time. The only exception — but it is a very significant one — was the case of individuals in which the process of normal fission had begun ; in these a non-nucleated fragment in which the formation of a new peristome had already been initiated healed the wound and completed the formation of the peri



imal. showing planes o(


The middle piec.




. The enucleated pieces.


the right, swim ab


out for a time, but finally


PHYSIOLOGICAL RELATIONS OF NUCLEUS AND CYTOPLASM 343

stome. Lillie ('96) has recently found that Stentor may by shaking be broken into fragments of all sizes, and that nucleated fragments as small as 5 \ the volume of the entire animal are still capable of complete regeneration. All non-nucleated fragments perish.

These studies of Nussbaum and Gruber formed a prelude to more extended investigations in the same direction by Gruber, Balbiani, Hofer, and especially Verworn Verworn ('88) proved that in Polystomella, one of the Foraminifera, nucleated fragments are able to



Fig. 159. — Regeneration tn the unicellular A. Animal divided into three pieces, each


Stentor, [From GRUBER after Balbiani.]

ining a fragment of the nucleus. B. The three

The three fragments after twentjr-Iour hours, each regenerated


repair the shell, while non-nucleated fragments lack this power. Balbiani ('89) showed that although non-nucleated fragments of Infusoria had no power of regeneration, they might nevertheless continue to live and swim actively about for many days after the operation, the contractile vacuole pulsating as usual. Hofer ('89), experimenting on Amccba, found that non-nucleated fragments might live as long as fourteen days after the operation (Fig. 160). Their movements continued, but were somewhat modified, and little by little ceased, but the pulsations of the contractile vacuole were but slightly affected ; they lost more or less completely the capacity to


344


CELL-CHEMISTRY AND CELL-PHYSIOLOGY


digest food, and the power of adhering to the substratum. Nearly at the same time Vcrworn ('89) published the results of an extended comparative investigation of various Protozoa that placed the whole matter in a very clear light. His experiments, while fully confirming the accounts of his predecessors in regard to regeneration, added many extremely important and significant results. Non-nucleated fragments both of Infusoria {e.g. Lachrymaria) and rhizopods {Poly


[HOFER.]

after the opera


stomella, Thalassicolla) not only live for a considerable period, but perform perfectly normal and characteristic movements, show the same susceptibility to stimulus, and have the same power of ingulfing food, as the nucleated fragments. They lack, however, the power of digestion and secretion. Ingested food-matters may be slightly altered, but are never completely digested. The non-nucleated fragments are unable to secrete the material for a new shell {Polysto


PHYSIOLOGICAL JtELA TtONS OF NUCLEUS AND CYTOPLASM 345

tnella) or the slime by which the animals adhere to the substratum {Amceba, Difflugia, Polystomdla). Beside these results should be placed the well-known fact that dissevered nerve-fibres in the higher animals are only regenerated from that end which remains in connection with the nerve-cell, while the remaining portion invariably degenerates.


Ml


1 1


^




Formation of membranes by protoplasmi of Cucurbits, ihowirif


ts of plasmolyzed cells. [TOWN


Fig. 161 .

A. Plasmol»ied cell, leaf-h; 8. Calyx-hair 'of GailUrdia ; nucleated fra C. Rool-hair of Miinhanlia ; all Ihc fragmei

wiih nucleated fragment of adjoining cell.


These beautiful observations prove that destructive metabolism, as manifested by coordinated forms of protoplasmic contractility, may go on for some time undisturbed in a mass of cytoplasm deprived of a nucleus. On the other hand, the building up of new chemical or morphological products by the cytoplasm is only initiated in the presence of a nucleus and soon ceases in its absence. These facts form a complete demonstration that the nucleus plays an essential


346 CELlr- CHEMISTRY AND CELL- PHYSIOLOGY

part not only in the operations of synthetic metabolism or chemical synthesis, but also in the morphological determination of these operations, i.e. the morphological synthesis of Bernard — a point of capital importance for the theory of inheritance, as will appear beyond.

Convincing experiments of the same character and leading to the same result have been made on the cells of plants. Francis Darwin (J77) observed more than twenty years ago that movements actively continued in protoplasmic filaments, extruded from the leaf -hairs of Dipsacus, that were completely severed from the body of the cell. Conversely, Klebs ('79) soon afterward showed that naked protoplasmic fragments of Vaucheria and other algae were incapable of forming a new cellulose membrane if devoid of a nucleus ; and he afterward showed ('87) that the same is true of Zygnema and CEdogonium. By plasmolysis the cells of these forms may be broken up into fragments, both nucleated and non-nucleated. The former surround themselves with a new wall, grow, and develop into complete plants ; the latter, while able to form starch by means of the chlorophyll they contain, are incapable of utilizing it, and are devoid of the power of forming a new membrane, and of growth and regeneration. A beautiful confirmation of this is given by Townsend ('97), who finds in the case of root-hairs and pollen-tubes, that when the protoplasm is thus broken up, a membrane may be formed by both nucleated and non-nucleated fragments, by the latter however only when they remain connected with the nucleated masses by protoplasmic strands, however fine. If these strands be broken, the membrane-forming power is lost. Of very great interest is the further observation (made on leafhairs in Cucurbita) that the influence of the nucleus may thus extend from cell to cell, an enucleated fragment of one cell having the power to form a membrane if connected by intercellular bridges with a nucleated fragment of an adjoining cell (Fig. 161).

2. Position and Movements of the Nucleus

Many observers have approached the same problem from a different direction by considering the position, movements, and changes of form in the nucleus with regard to the formative activities in the cytoplasm. To review these researches in full would be impossible, and we must be content to consider only the well-known researches of Haberlandt C77) and Korschelt ('89), both of whom have given extensive reviews of the entire subject in this regard. Haberlandt's studies related to the position of the nucleus in plant-cells with especial regard to the growth of the cellulose membrane. He determined the very significant fact that local growth of the cell-wall is always preceded by a movement of the nucleus to the point of growth. Thus, in the formation of epidermal cells, the nucleus lies at first near


PHYSIOLOGICAL RELATIONS OF NUCLEUS AND CYTOPLASM 347

the centre, but as the outer wall thickens, the nucleus moves toward it, and remains closely applied to it throughout its growth, after which the nucleus often moves into another part of the cell (Fig. 162, A, B). That this is not due simply to a movement of the nucleus toward the air and light is beautifully shown in the coats of certain seeds, where the nucleus moves, not to the outer, but to the inner wall of the cell, and here the thickening takes place (Fig. 162, C), The same position





Pig. 162. — Position of the nuclei in growing plant-cells. [Haberlandt.]

A. Young epidermal cell of Luzula with central nucleus, before thickening of the membrane. B. Three epidermal cells of Afonstera, during the thickening of the outer wall. C. Cell from the seed-coat of Scopulina, during the thickening of the inner wall. D. E. Position of the nuclei during the formation of branches in the root-hairs of the pea.


of the nucleus is shown in the thickening of the walls of the guardcells of stomata, in the formation of the peristome of mosses, and in many other cases. In the formation of root-hairs in the pea, the primary outgrowth always takes place from the immediate neighbourhood of the nucleus, which is carried outward and remains near the tip of the growing hair (Fig. 162, D, E). The same is true of the rhizoids of fern-prothallia and liverworts. In the hairs of aerial plants this


348 CELL-CHEMISTRY AND CELL-PHYSIOLOGY

rule is reversed, the nucleus lying near the base of the hair ; but this apparent exception proves the rule, for both Hunter and Haberlandt show that in this case growth of the hair is not apical, but proceeds from the base ! Very interesting is Haberlandt's observation that in the regeneration of fragments of Vaiicheria the growing region, where a new membrane is formed, contains no chlorophyll, but numerous nuclei. The general result, based on the study of a large number of cases, is, in Haberlandt's words, that " the nucleus is in most cases placed in the neighbourhood, more or less immediate, of the points at which growth is most active and continues longest." This fact points to the conclusion that " its function is especially connected with the developmental processes of the cell," l and that "in the growth of the cell, more especially in the growth of the cell-wall, the nucleus plays a definite part."

Korschelt's work deals especially with the correlation between form and position of the nucleus and the nutrition of the cell, and since it bears more directly on chemical than on morphological synthesis, may be only briefly reviewed at this point. His general conclusion is that there is a definite correlation, on the one hand, between the position of the nucleus and the source of food-supply, on the other hand, between the size of the nucleus and the extent of its surface and the elaboration of material by the cell. In support of the latter conclusion many cases are brought forward of secreting cells in which the nucleus is of enormous size and has a complex branching form. Such nuclei occur, for example, in the silk-glands of various lepidopterous larvae (Meckel, Zaddach, etc.), which are characterized by an intense secretory activity concentrated into a very short period. Here the nucleus forms a labyrinthine network (Fig. 14, E\ by which its surface is brought to a maximum, pointing to an active exchange of material between nucleus and cytoplasm. The same type of nucleus occurs in the Malpighian tubules of insects (Leydig, R. Hertwig), in the spinning-glands of amphipods (Mayer), and especially in the nutritive cells of the insect ovary already referred to at page 151. Here the developing ovum is accompanied and surrounded by cells, which there is good reason to believe are concerned with the elaboration of food for the egg-cell. In the earwig Forficula each egg is accompanied by a single large nutritive cell (Fig. 163), which has a very large nucleus rich in chromatin (Korschelt). This cell increases in size as the ovum grows, and its nucleus assumes the complex branching form shown in the figure. In the butterfly Vanessa there is a group of such cells at one pole of the cgg t from which the latter is believed to draw its nutriment (Fig. 77). A very interesting case is that of the annelid Ophryotrocha, referred to at page 151. Here, as described by Korschelt, the egg floats

1 I.e., p. 99.


PHYSIOLOGICAL DELATIONS OF NUCLEUS AND CYTOPLASM



\


V


in the perivisceral fluid, accompanied by a nurse-cell having a very large chromatic nucleus, while that of the egg is smaller and poorer inchromatin. Astheegg com pletes its growth, the nurse-cell dwindles away and finally perishes (Fig, 76). In all these cases it is scarcely possible to doubt that the egg is in a measure relieved of the task of elaborating cytoplasmic products by the nurse-cell, and that the great development of the nucleus in the latter is correlated with this function.

Regarding the position and movements of the nucleus, Korschelt reviews many facts pointing toward the same conclusion. Perhaps the most suggestive of these relate to the nucleus of the egg during its ovarian history. In many of the insects, as in both the juala,

CaSCS referred to above. Below, a portion of ihe nearly ripe egg (*>. showing deuio the e ge .„ud« u , a. first SSSSSSSH^iSiJS^'^m

occupies a central pOSl- cessively younger stages of egg and nurse are shown above.

tion, but as the egg begins to grow, it moves to the periphery on the side turned toward the nutritive cells. The same is true in the ovarian eggs of some other animals, good examples of which are afforded by various ccelcnterates, e.g. in medusae (Claus, Hertwig) and actinians (Korschelt, Hertwig), where the germinal vesicle is always near the point of attachment of the egg. Most suggestive of all is the case of the water-beetle Dytisctis, in which Korschelt was able to observe the movements and changes of form in the living object. The eggs here lie in a single series alternating with chambers of nutritive cells. The latter contain granules which are believed by Korschelt to pass into the egg, perhaps bodily, perhaps by dissolving and entering in a liquid form. At all events,


S V -,


Fig. 163. — Upper port]


350 CELL-CHEMISTRY AND CELL-PHYSIOLOGY

the egg contains accumulations of similar granules, which extend inward in dense masses from the nutritive cells to the germinal vesicle, which they may more or less completely surround. The latter meanwhile becomes amoeboid, sending out long pseudopodia, which are always directed toward the principal mass of granules (Fig. jj\ The granules could not be traced into the nucleus, but the latter grows rapidly during these changes, proving that matter must be absorbed by it, probably in a liquid form. 1

Among other facts pointing in the same direction may be mentioned Miss Huie's ('97) observations on the gland-cells of Drosera, and those of Mathews ('99) on the changes of the pancreas-cell in Necturus. Stimulus of the gland-cells in the leaf of Drosera causes a rapid exhaustion and change of staining-capacity in the cytoplasm. During the ensuing repose the cytoplasm is rebuilt out of material laid down immediately around the nucleus, and agreeing closely in appearance and staining-reaction with the achromatic nuclear constituents. The chromatin increases in bulk during a period preceding the constructive phase, but decreases (while the nucleolar material increases) as the cytoplasm is restored. In the pancreas-cell, as has long been known, the " loaded " cell (before secretion) is filled with metaplasmic zymogen-granules, which disappear during secretion, the cell meanwhile becoming filled with protoplasmic fibrils (Fig. 18). During the ensuing period of " rest " the zymogen-granules are re-formed at the expense of the fibrillar material, which is finally found only at the base of the cell near the nucleus. Upon discharge of the secretion (granule-material) the fibrillae again advance from the nucleus toward the periphery. Mathews shows that many if not all of them may be traced at one end actually into the nuclear wall, and concludes that they are directly formed by the nucleus.

Beside the foregoing facts may be placed the strong evidence reviewed at pages 156-158, indicating the formation of the yolk-nucleus, and indirectly of the yolk-material, by the nucleus. All of these and a large number of other observations in the same direction lead to the conclusion that the cell-nucleus plays an active part in nutrition, and that it is especially active during the constructive phases. On the whole, therefore, the behaviour of the nucleus in this regard is in harmony with the result reached by experiment on the one-celled forms, though it gives in itself a far less certain and convincing result. 2

1 Mention may conveniently here be made of Richard Hertwig's interesting observation that in starved individuals of Actinosplunium the chromatin condenses into a single mass, while in richly fed animals it is divided into fine granules scattered through the nucleus ('98, p. 8).

2 Loeb ('98, '99) makes the interesting suggestion that the nucleus is especially concerned in the oxydative processes of the cell, and that this is the key to its r6U in the synthetic process. It has been shown that oxydations in the living tissues are probably


PHYSIOLOGICAL RELATIONS OF NUCLEUS AND CYTOPLASM 351

We now turn to evidence which, though less direct than the above, is scarcely less convincing. This evidence, which has been exhaustively discussed by Hertwig, Weismann, and Strasburger, is drawn from the history of the nucleus in mitosis, fertilization, and maturation. It calls for only a brief review here, since the facts have been fully described in earlier chapters.

3. The Nucleus in Mitosis

To Wilhelm Roux ('83) we owe the first clear recognition of the fact that the transformation of the chromatic substance during mitotic division is manifestly designed to effect a precise division of all its parts, — i.e. a panmeristic division as opposed to a mere mass-division, — and their definite distribution to the daughter-cells. "The essential operation of nuclear division is the division of the mother-granules " {i.e. the individual chromatin-grains) ; " all the other phenomena are for the purpose of transporting the daughter-granules derived from the division of a mother-granule, one to the centre of one of the daughter-cells, the other to the centre of the other." In this respect the nucleus stands in marked contrast to the cytoplasm, which undergoes on the whole a mass-division, although certain of its elements, such as the plastids and the centrosome, may separately divide, like the elements of the nucleus. From this fact Roux argued, first, that different regions of the nuclear substance must represent different qualities, and second, that the apparatus of mitosis is designed to distribute these qualities, according to a definite law, to the daughtercells. The particular form in which Roux and Weismann developed this conception has now been generally rejected, and in any form it has some serious difficulties in its way. We cannot assume a precise localization of chromatin-elements in all parts of the nucleus ; for on the one hand a large part of the chromatin may degenerate or be cast out (as in the maturation of the egg), and on the other hand in the Protozoa a small fragment of the nucleus is able to regenerate the whole. Nevertheless, the essential fact remains, as Hertwig, Kolliker, Strasburger, De Vries, and many others have insisted, that in mitotic cell-division the chromatin of the mother-cell is distributed with the most scrupulous equality to the nuclei of the daughter-cells, and that in this regard there is a most remarkable contrast between nucleus and cytoplasm. This holds true with such wonderful constancy

dependent upon certain substances (oxydation ferments) that in some manner, not yet clearly understood, facilitate the process; and the work of Spitzer ('97) has shown that these substances (obtained from tissue-extracts) belong to the group of nucleo-proteids, which are characteristic nuclear substances. The view thus suggested opens a further way toward more exact inquiry into the nuclear functions, though it is not to be supposed that the nucleus is the sole oxydative centre of the cell, as is obvious from the prolonged activity of non-nucleated protoplasmic masses.


352


CELL-CHEMISTRY AND CELL-PHYSIOLOGY


throughout the series of living forms, from the lowest to the highest, that it must have a deep significance. And while we are not yet in a position to grasp its full meaning, this contrast points unmistakably to the conclusion that the most essential material handed on by the mother-cell to its progeny is the chromatin, and that this substance therefore has a special significance in inheritance.

4. The Nucleus in Fertilization

The foregoing argument receives an overwhelming reenforcement


from the facts of fertilization.



Although the ovum supplies nearly all the cytoplasm for the embryonic body, and the spermatozoon at most only a trace, the latter is nevertheless as potent in its effect on the offspring as the former. On the other hand, the nuclei contributed by the germ-c^lls, though apparently different, become tn the end exactly equivalent in every visible respect — in structure, in staining-reactions, and in the number and form of the chromosomes to which each gives rise. But furthermore the substance of the two germ-nuclei is distributed with absolute equality, certainly to the first two cells of the embryo, and probably to all later-formed cells. The latter conclusion, which long remained a mere surmise, has been rendered nearly a certainty by the remarkable observations of Riickert, Zoja, and Hacker, described in Chapters IV. and VI. We must therefore accept the high probability of the conclusion that the specific character of the ceil is in the last analysis determined by that of the nucleus, that is by the chromatin, and that in the equal distribution of paternal and maternal chromatin to all the cells of the offspring we find the physiological explanation of the fact that


PHYSIOLOGICAL RELATIONS OF NUCLEUS AND CYTOPLASM 353

every part of the latter may show the characteristics of either or both parents.

Boveri ('89, '95, 1) has attempted to test this conclusion by a most ingenious and beautiful experiment ; and although his conclusions do not rest on absolutely certain ground, they at least open the way to a decisive test. The Hertwig brothers showed that the eggs of seaurchins might be broken into pieces by shaking, and that spermatozoa would enter the enucleated fragments and cause them to segment. Boveri proved that such a fragment, if fertilized by a spermatozoon, would even give rise to a dwarf larva, indistinguishable from the normal in general appearance except in size. The nuclei of such larvae are considerably smaller than those of the normal larvae, and were shown by Morgan ('95, 4) to contain only half the number of chromosomes, thus demonstrating their origin from a single sperm-nucleus. Now, by fertilizing enucleated egg-fragments of one species {Sphcerechinus granulans) with the spermatozoa of .another {Echinus microtuberculatus), Boveri obtained in a few instances dwarf Plutei showing except in size the pure paternal characters {i.e. those of Echinus, Fig. 164). From this he concluded that the maternal cytoplasm has no determining effect on the offspring, but supplies only the material in which the sperm-nucleus operates. Inheritance is, therefore, effected by the nucleus alone.

The later studies of Seeliger ('94), Morgan ('95, 4), and Drisch ('98, 3) showed that this result is not entirely conclusive, since hybrid larvae arising by the fertilization of an entire ovum of one species by a spermatozoon of the other show a very considerable range of variation ; and while most such hybrids are intermediate in character between the two species, some individuals may nearly approximate to the characters of the father or the mother. Despite this fact Boveri ('95, 1) has strongly defended his conclusion, though admitting that only further research can definitely decide the question. It is to be hoped that this highly ingenious experiment may be repeated on other forms which may afford a decisive result.

5. The Nucleus in Maturation

Scarcely less convincing, finally, is the contrast between nucleus and cytoplasm in the maturation of the germ-cells. It is scarcely an exaggeration to say that the whole process of maturation, in its broadest sense, renders the cytoplasm of the germ-cells as unlike, the nuclei as like, as possible. The latter undergo a series of complicated changes which result in a perfect equivalence between them at the time of their union, and, more remotely, a perfect equality of distribution to the embryonic cells. The cytoplasm, on the other

2 A


354 CELL-CHEMISTRY AND CELL-PHYSIOLOGY

hand, undergoes a special differentiation in each to effect a secondary division of labour between the germ-cells. When this is correlated with the fact that the germ-cells, on the whole, have an equal effect on the specific character of the embryo, we are again forced to the conclusion that this effect must primarily be sought in the nucleus, and that the cytoplasm is in a sense only its agent.

C. The Centrosome

Existing views regarding the functions of the centrosome may conveniently be arranged in two general groups, the first including those which regard this structure as a relatively passive body, the second those which assume it to be an active organ. To the first belongs the hypothesis of Heidenhain C94), accepted by Kostanecki ('97, 1) and some others, that the centrosome serves essentially as an insertionpoint for the astral rays ("organic radii"), and plays a relatively passive part in the phenomena of mitosis, the active functions being mainly performed by the surrounding structures. To the same category belongs the view of Miss Foot that the formation of the centrosome is, as it were, incidental to that of the aster — "the expression, rather than the cause, of cell-activity " ('97, p. 810). To the second group belong the views of Van Beneden, Boveri, Butschli, Carnoy, and others who regard the centrosome as playing a more active r6le in the life of the cell. Both of the former authors have assumed the centrosomes to be active centres by the action of which the astral systems arc organized ; and they are thus led to the conclusion that the centrosome is essentially an organ for cell-division and fertilization (Boveri), and in this sense is the "dynamic centre" of the cell. 1 To Carnoy and Butschli is due the interesting suggestion 2 that the centrosomes are to be regarded further as centres of chemical action to which their remarkable effect on the cytoplasm is due. That the centrosome is an active centre, rather than a passive body or one created by the aster-formation, is. strongly indicated by its behaviour both in mitosis and in fertilization. Griffin ('96, '99) points out that at the close of division in Thalassema the daughter-centrosomes migrate away from the old astral centre and incite about themselves in a different region the new astral systems for the ensuing mitosis (Figs. 99, 155); and similar conditions are described by Coe in Ccrcbratulus ('98). In fertilization the aster-formation cannot be regarded as a general action of the cytoplasm, but as a local one due to a local stimulus given by something in the spermatozoon ; for in polyspermy a sperm-aster is formed for every spermatozoon (p. 198). This stimulus is given by something in the middle 1 ( 7- PP- 7 6 » J 2 - 2 0^ P- IIO « 


THE CENTROSOME 355

piece (p. 212), which is itself genetically related to the centrosome of the last cell- generation (p. 1 70). These facts seem explicable only under the assumption that in these cases the centrosome, or a substance which it carries, gives an active stimulus to the cytoplasm which incites the aster-formation about itself, and in the words of Griffin " disengages the forces at work in mitosis " ('96, p. 174). For these reasons I incline to the view that in the artificial aster-formation described by Morgan 1 the centrosomes there observed should not be regarded as the creations of the asters, but rather as local deposits of material which incite the aster-formation around them. That the centrosomes or astral centres are centres of division (whether active or passive) is beautifully shown by Boveri's interesting observations on " partial fertilization " referred to at page 194.

Again, Boveri has observed that the segmenting ovum of Ascaris sometimes contains a supernumerary centrosome that does not enter



Fig. 165. — liggs of Ascata with supernumerary cenlrosome. [BoVI A. First cleavage-spindle above, isolated centrosome below. B. Result of the ensuing division.

into connection with the chromosomes, but lies alone in the cytoplasm (Fig. 165). Such a centrosome forms an independent centre of division, the cell dividing into three parts, two of which are normal blastomeres, while the third contains only the centrosome and attraction-sphere. The fate of such eggs was not determined, but they form a complete demonstration that it is in this case the centrosome and not the nucleus that determines the centres of division in the cell-body. Scarcely less conclusive is the case of dispermic eggs in sea-urchins. In such eggs both sperm-nuclei conjugate with the eggnucleus, and both sperm-centrosomes divide (Fig. t66). The cleavage-nucleus, therefore, arises by the union of three nuclei and four centrosomes. Such eggs divide at the first cleavage into four equal blastomeres, each of which receives one of the centrosomes. 1 Cf. P . 307.


356


CELL-CHEMISTRY AND CELL-PHYSIOLOGY


The latter must, therefore, be the centres of division ; * though it must not be forgotten that, in some cases at any rate, normal division requires the presence of nuclear matter (p. 108).

The centrosome must, however, be something more than a mere division-centre ; for, on the one hand, in leucocytes and pigment-cells the astral system formed about it is devoted, as there is good reason to believe, not to cell-division, but to movements of the cell-body as a whole; and, on the other hand, as we have seen (pp. 165, 172), it is concerned in the formation of the flagella of the spermatozoa and spermatozoids, and probably also in that of cilia in epithelial cells. Strasburger ('97) was thus led to the conclusion that the centrosome is essentially a mass of kinoftlasm, i.e. the active motor plasm, 2 and a nearly similar view has been adopted by several recent zoologists.



A B C

Fig. 166. — Cleavage of dispermic egg of Tqxopncustes.

A. One sperm-nucleus has united with the egg-nucleus, shown at a. b. ; the other lies above. Both sperm-asters have divided to form amphiasters (a. b. and c. d.). B. The cleavage-nucieus, formed by union of the three germ-nuclei, is surrounded by the four asters. C. Result of the first cleavage, the four blastomeres lettered to correspond with the four asters.


Henneguy concludes that the centrosomes are " motor centres of the kinoplasm " both for external and for internal manifestations. 8 Len'hoss£k regards them as " motors " for the control of ciliary action as well as for that of the spermatozoon, 4 and perhaps also for that of muscle-fibriHae. 6 Zimmerman concludes that "the microcentrum is the motor centre of the cell, that is, the * kinocentrum ' opposed to the nucleus as the 'chemocentrum.' " 8 Regarding their control of ciliary action, he makes the same suggestion as that of Henneguy and Lenhoss^k cited above. He adds the further very interesting suggestions that the centrosomes may be concerned with the pseudopodial movements in the epithelial cells of the intestine, and that they may

1 This phenomenon was first observed by Hertwig, and afterward by Driesch. I have repeatedly observed the internal changes in the living eggs of Toxopneustes.

2 Cf. p. 221. * '98, p. 107. • '98, p. 697.


8 '98, p. 495


6 '


99, p. 342.


THE CENTROSOME 357

also be concerned in the protoplasmic contraction of gland-cells by which the excretion is expelled- [This is based on the fact that the centrosomes are found in the free (pseudopodia- forming) ends of the epithelial cells, and on the position of the centrosomes in gobletcells (Fig. 23) and in those of the lachrymal gland.] Peter ('99) has attempted to test these conclusions experimentally by cutting or tearing off cilia from the cell-body (gut-epithelium of Anodonta) and also by isolating the tails of spermatozoa. In groups consisting of only a few cilia, separated from the nucleus, the movements actively continue, while those that are separated from the basal bodies cease to beat. Spermatozoon tails separated from the head also continue to



Pig. 1*7. — Omirosomra


crfly. [HENNEGUV.]


move, but only if they remain connected with the middle-piece. Peter, therefore, supports the above conclusions of Henneguy and Lenhossek. On the other hand, Meves ('99) finds that movements of the undulating membrane in the tails of salamander-spermatozoa continue if the middle-piece be entirely removed ; white a number of earlier observers' have observed in flagellates that a flagellum separated from the body may actively continue its movements for a considerable time.

Further research is therefore required to test these suggestions.

The intimate connection of the centrosomes with the formation, on the

one hand, of the astral rays, on the other of contractile organs, such

> See Klebs, 'S3, Bfitichli, '85, Fischer, '94, 3.


358 CELL-CHEMISTRY AND CELL-PHYSIOLOGY

as cilia, flagella, and pseudopodia, 1 the centrosomes in ciliated cells and spermatozoa, and in the swarm-spores of Noctiluca, is, however, a most striking fact, and is one of the strongest indirect arguments in favour of the general theory of fibrillar contractility in mitosis.

D. Summary and Conclusion

The facts reviewed in the foregoing pages converge to the conclusion that the differentiation of the cell-substance into nucleus and cytoplasm is the expression of a fundamental physiological division of labour in the cell. Experiments upon unicellular forms demonstrate that, in the entire absence of a nucleus, protoplasm is able for a considerable time to liberate energy and to manifest coordinated activities dependent on destructive metabolism. There is here substantial ground for the view that the cytoplasm is the principal seat of these activities in the normal cell. On the other hand, there is strong cumulative evidence that the nucleus is intimately concerned in the constructive or synthetic processes, whether chemical or morphological.

That the nucleus has such a significance in synthetic metabolism is proved by the fact that digestion and absorption of food and growth soon cease with its removal from the cytoplasm, while destructive metabolism may long continue as manifested by the phenomena of irritability and contractility. It is indicated by the position and movements of the nucleus in relation to the food-supply and to the formation of specific cytoplasmic products. It harmonizes with the fact, now universally admitted, that active exchanges of material go on between nucleus and cytoplasm. The periodic changes of staining-capacity undergone by the chromatin during the cycle of celllife, taken in connection with the researches of physiological chemists on the chemical composition and staining-reactions of the nuclein series, indicate that the phosphorus-rich substance known as nuclcinic acid plays a leading part in the constructive process. During the vegetative phases of the cell this substance is combined with a large amount of the albumin radicles histon, protamin, and related substances, and probably in part with albumin itself, to form nuclein. During the mitotic or reproductive processes this combination appears to be dissolved, the albuminous elements being in large part split off, leaving the substance of the chromosomes with a high percentage of nucleinic acid, as is shown by direct analysis of the sperm-nucleus and is indicated by the staining-reactions of the chromosomes. There is, therefore, considerable ground for the hypothesis that in a chemical sense this substance is the most essential nuclear element handed

1 Cf' PP* 9 2 » I02 » on the central granule of the Heliozoa.


LITERATURE 359

on from cell to cell, whether by cell-division or by fertilization ; and that it may be a primary factor in the constructive processes of the nucleus and through these be indirectly concerned with those of the cytoplasm.

The role of the nucleus in constructive metabolism is intimately related with its rdle in morphological synthesis, and thus in inheritance ; for the recurrence of similar morphological characters must in the last analysis be due to the recurrence of corresponding forms of metabolic action of which they are the outward expression. That the nucleus is in fact a primary factor in morphological as well as chemical synthesis is demonstrated by experiments on unicellular plants and animals, which prove that the power of regenerating lost parts disappears with its removal, though the enucleated fragment may continue to live and move for a considerable period. That the nuclear substance, and especially the chromatin, is a leading factor in inheritance is powerfully supported by the facts of maturation, fertilization, and cell-division. In maturation the germ-nuclei are by an elaborate process prepared for the subsequent union of equivalent chromatic elements from the two sexes. By fertilization these elements are brought together, and by mitotic division distributed with exact equality to the embryonic cells. The result, which is especially striking in the case of hybrid-fertilization, proves that the spermatozoon is as potent in inheiitance as the ovum, though the latter contributes an amount of cytoplasm which is but an infinitesimal fraction of that supplied by the ovum.

It remains to be seen whether the chromatin can actually be regarded as the idioplasm or physical basis of inheritance, as maintained by Hertwig and Strasburger. Verworn has justly urged that the nucleus cannot be regarded as the sole vehicle of inheritance, since the cooperation of both nucleus and cytoplasm is essential to complete cell-life ; and, as will be shown in Chapter IX., the cytoplasmic organization plays an important rdle in shaping the course of development. Considered in all their bearings, however, the facts seem to accord best with the hypothesis that the cytoplasmic organization is itself determined, in the last analysis, by the nucleus ; l and the principle for which Hertwig and Strasburger contended is thus sustained.

LITERATURE. VII

Bernard, Claude. — Lemons sur les Phe'nomenes de la Vie: 1st ed. 1878; 2d ed.

1885. Pan's. Chittenden, R. H. — Some Recent Chemico-physiological Discoveries regarding the

Cell: Am. Nat. % XXVIII., Feb., 1894.

1 Cf. p. 43*


360 CELL-CHEMISTRY AND CELL-PHYSIOLOGY

Fischer, A. — See Literature I.

Gruber, A. — Mikroskopische Vivisekton : Ber. d. Naturf. Ges. Freiburg, VII., 1893.

Haberlandt, G. — Uber die Beziehungen zwischen Funktion und Lage des Z ell kerns.

Fischer, 1887. Id. — Physiologische Pflanzenatomie. Leipzig, 1896. Halliburton, W. D. — A Text-book of Chemical Physiology and Pathology. London*

1 891. Id. — The Chemical Physiology of the Cell (Gouldstonian Lectures)-. Brit. Med.

Jour n. 1893. Hammarsten, 0. — Lehrbuch der physiologische Chemie. 3d ed. Wiesbaden* 1895. Hertwig, 0. and R. — Uber den Befruchtungs- und Teilungsvorgang des tierischen

Eies unter dem Einfluss ausserer Agentien. Jena, 1887. Kolliker, A. — Das Karyoplasma und die Vererbung, eine Kritik der WeismaniTschen

Theorie von der Kontinuitat des Keimplasmas: Zeitschr. wiss. ZooL y XLIV.

1886. Korschelt, E. — Beitrage sur Morphologie und Physiologie des Zellkernes : Z00L

Jahrb. Anat. u. Ontog., IV. 1889. Kossel, A. — Uber die chemische Zusammensetzung der Zelle : Arch. Anat. u. Phys.

1 891. Id. — Uber die basischen StofTe des Zellkernes: Zeit. Phys. Chem., XXII., 1896. Lilienfeld, L. — Ober die Wahlverwandtschaft der Zellelemente zu Farbstoffen :

Arch. Anat. u. Phys. 1893. Malfatti, H. — Beitrage zur Kenntniss der Nucleine : Zeitschr. Phys. Chew., XVI.

1 891. Mathews, A. P. — The Metabolism of the Pancreas Cell : Journ. Aforfih., XV. Suppl.

1899. Miescher, F. — Physiologisch-chemische Untersuchungen liber die Lachsmilch : Arch.

Exp. Path. u. Pharm., XXXVII., 1896. Prenant, A. — See Literature VI. Ruckert, J. — Zur Entwicklungsgeschichte des Ovarialeies bei Selachiern : An. Anz. y

VII. 1892. Sachs, J. — Vorlesungen uber Pflanzen-physiologie. Leipzig, 1882. Id. — Stoff und Form der Pflanzen-organe : Gesammelte Abhandlungen* II. 1893. Strasburger. — See footnote, p. 269. Verworn, M. — Die Physiologische Bedeutung des Zellkerns : Arch, /tir die Ges.

Phys., XLI. 1892. Id. — Allgemeine Physiologic Jena, 1895. Whitman, C. 0. — The Seat of Formative and Regenerative Energy : Journ. Aforph.,

II. 1888. Zacharias, E. — Uber des Verhalten des Zellkerns in wachsenden Zellen : Flora, 81.

1895.


CHAPTER VIII

CELL-DIVISION AND DEVELOPMENT

" Wir konnen demnach endlich den Satz aufstellen, dass sammtliche im entwickelten Zustandc vorhandenen Zellen oder Aequivalente von Zellen durch cine fortschreitende Gliedcrung der Eizclle in morphologisch ahnlichc Elemente entstehen, und dass die in einer emhryonischen Organ-Anlage enthaltenden Zellen, so gering auch ihre Zahl sein mag, dennoch die ausschlicssliche ungegliederte Anlage fiir sammtliche Formbestandtheile der spateren Organe enthalten." Remak. 1

Since the early work of Kolliker and Remak it has been recognized that the cleavage or segmentation of the ovum, with which the development of all higher animals begins, is nothing other than a rapid series of mitotic cell-divisions by which the egg splits up into the elements of the tissues. This process is merely a continuation of that by which the germ-cell arose in the parental body. A long pause, however, intervenes during the latter period of its ovarian life, during which no divisions take place. Throughout this period the egg leads, on the whole, a somewhat passive existence, devoting itself especially to the storage of potential energy to be used during the intense activity that is to come. Its power of division remains dormant until the period of full maturity approaches. The entrance of the spermatozoon arouses in the egg a new phase of activity. Its power of division, which may have lain dormant for months or years, is suddenly raised to the highest pitch of intensity, and in a very short time it gives rise by division to a myriad of descendants which are ultimately differentiated into the elements of the tissues.

The divisions of the egg during cleavage are exactly comparable with those of tissue-cells, and all of the essential phenomena of mitosis are of the same general character in both. But for two reasons the cleavage of the egg possesses a higher interest than any other case of cell-division. First, the egg-cell gives rise by division not only to cells like itself, as is the case with most tissue-cells, but also to many other kinds of cells. The operation of cleavage is therefore immediately connected with the process of differentiation, which is the most fundamental phenomenon in development. Second, definite relations may often be traced between the planes of division and the structural axes of the adult body, and these relations are

1 Untersuchungen, 1 855, p. 140.

361


362 CELL-DITZSIQX AXD DEVELOPMENT

sometimes so c'early marked and appear so early that with the very first cleavage the position in which tie embryo will finally appear in the egg may be exactly predicted. Such *• promorphological " relations of the segmenting egg possess a very high interest in their bearing on the theory of germinal localization and on account of the light which they throw on the conditions of the formative process.

The present chapter is in the main a prelude to that which follows, its purpose being to sketch some of the external features of early development regarded as particular expressions of the gen. cral rules of cell-division. For this purpose we may consider the cleavage of the ovum under two heads, namely : —

1. TJu Geometrical Relations of Cleavage-forms, with reference to the general rules of cell-division.

2. The Promorphological Relations of the blastomeres and cleavage-planes to the parts of the adult body to which they give rise.

A. Geometrical Relations of Cleavage-forms

The geometrical relations of the cleavage-planes and the relative size and position of the cells vary endlessly in detail, being modified bv innumerable mechanical and other conditions, such as the amount and distribution of the inert yolk or deutoplasm, the shape of the ovum as a whole, and the like. Yet all the forms of cleavage can be referred to a single type which has been moulded this way or that by special conditions, and which is itself an expression of two general rules of cell-division, first formulated by Sachs in the case of plantcells. These are : —

1. The cell typically tends to divide into equal parts.

2. Each new plane of division tends to intersect the preceding plane at a right angle.

In the simplest and least modified forms the direction of the cleavage-planes, and hence the general configuration of the cellsystem, depends on the general form of the dividing mass; for, as Sachs has shown, the cleavage-planes tend to be either vertical to the surface (anticlines) or parallel to it (periclines). Ideal schemes of division may thus be constructed for various geometrical figures. In a flat circular disc, for example, the anticlinal planes pass through the radii ; the periclines are circles concentric with the periphery. If the disc be elongated to form an ellipse, the periclines also become ellipses, while the anticlines are converted into hyperbolas confocal with the periclines. If it have the form of a parabola, the periclines and anticlines form two systems of confocal parabolas intersecting at right angles. All these schemes are mutatis mutandis, directly convertible into the corresponding solid forms in three dimensions.


GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 363

Sachs has shown in the most beautiful manner that all the above ideal types are closely approximated in nature, and Rauber has applied the same principle to the cleavage of animal-cells. The discoid or spheroid form is more or less nearly realized in the thalloid growths of



Fig. 168. — Geometrical relations of cleavage-planes in gro' after various authors.]

A. Flat ellipsoidal germ-disc of Milottsia (Rosanoff) ; nearly typical relation of elliptic periclines and hyperbolic anticlines. B. C. Apical view of terminal knob on epidermal hair of PiHguitda. B. shows the ellipsoid type. C. the circular (spherical type), somewhat modified [only anticlines present). D. Growing point of Satvinia (Pringsheim), typical ellipsoid type; the single pericline is, however, incomplete. E. Growing point of Atolla (Strasburger) ; circular or spheroidal type transitional 10 ellipsoidal. F. Root-cap of Eguisetum (Nageli and Lritgeb) ; modified circular type. G. Cross-section ol leaf-vein. Triekemami (Prantl) : ellipsoidal type with incomplete periclines. H. Embryo of Aliima; typical ellipsoid type, pericline incomplete only at lower side. /. Growing point ol bud of the pine (Aiits) ; typical paraboloid type, both anticlines and periclines having the form of parabolas (Sachs).


various lower plants, in the embryos of flowering plants, and elsewhere (Fig. 168). The paraboloid form is according to Sachs characteristic of the growing points of many higher plants ; and here, too, the actual form is remarkably similar to the ideal scheme (Fig. 168, 1).


364 CELL-DIVISION AND DEVELOPMENT

For our purpose the most important form is the sphere, which is the typical shape of the egg-cell; and all forms of cleavage may be related to the typical division of a sphere in accordance with Sachs's rules. The ideal form of cleavage would here be a succession of rectangular cleavages in the three dimensions of space, the anticlines passing through the centre so as to split the egg in the initial stages successively into halves, quadrants, and octants, the periclines being parallel to the surface so as to separate the inner ends of these cells from the outer. No case is known in which this order is accurately followed throughout, and the periclinal cleavages are of comparatively rare occurrence, being found as a regular feature of the early cleavage only in those cases where the primary germ-layers are separated by delamination. The simplest and clearest form of eggcleavage occurs in eggs like those of echinoderms, which are of spherical form, and in which the deutoplasm is small in amount and equally distributed through its substance. Such a cleavage is beautifully displayed in the egg of the holothurian Synapta, as shown in the diagrams, Fig. 169, constructed from Selenka's drawings. The first cleavage is vertical, or meridional, passing through the egg-axis and dividing the egg into equal halves. The second, which is also meridional, cuts the first plane at right angles and divides the egg into quadrants. The third is horizontal, or equatorial, dividing the egg into equal octants. The order of division is thus far exactly that demanded by Sachs's rule and agrees precisely with the cleavage of various kinds of spherical plant-cells. The later cleavages depart from the ideal type in the absence of periclinal divisions, the embryo becoming hollow, and its walls consisting of a single layer of cells in which anticlinal cleavages occur in regular rectangular succession. The fourth cleavage is again meridional, giving two tiers of eight cells each ; the fifth is horizontal, dividing each tier into an upper and a lower layer. The regular alternation is continued up to the ninth division (giving 512 cells), when the divisions pause while the gastrulation begins. In later stages the regularity is lost.

Hertwigs Development of Sachs's Rules. — Beside Sachs's rules may be placed two others formulated by Oscar Hertwig in 1884, which bear directly on the facts just outlined and which lie behind Sachs's principle of the rectangular intersection of successive divisionplanes. These are : —

1 . The nucleus tends to take up a position at the centre of its sphere of influence y i.e. of the protoplasmic mass in zuhich it lies.

2. The axis of the mitotic fgu res typically lies in the longest axis of the protoplasmic mass, and division therefore tends to cut this axis at a right angle.

The second rule explains the normal succession of the division


GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 365

planes according to Sachs's second rule. The first division of a homogeneous spherical egg, for example, is followed by a second division at right angles to it, since each hemisphere is twice as long in the plane of division as in any plane vertical to it. The mitotic figure of the second division lies therefore parallel to the first plane, which forms the base of the hemisphere, and the ensuing division is vertical to it. The same applies to the third division, since each quadrant is as long as the entire egg while at most only half its diameter. Division is therefore transverse to the long axis and vertical to the first two planes.

Taken together the rules of Sachs and Hertwig, applied to the egg, give us a kind of ideal type or model, well illustrated by the



Pig. 169. — Cleavage of Ibe ovum in Ihe hololhurian Synapta (slightly tchematized). [After Selenka.]

A-E. Successive cleavages (o the 33-cell stage. F. Blastula of laS cells.

cleavage of Synapta, described above, to which all the forms of cleavage may conveniently be referred as a basis of comparison. Numerous exceptions to all four of these rules are, however, known, and they are of little value save as a starting-point for a closer study of the facts. Cleavage of such schematic regularity as that of Synapta is extremely rare, both the form and the order of division being endlessly varied and in extreme cases showing scarcely a discoverable connection with the "type." We may conveniently consider these modifications under the following three heads: —


366 CEU^DIVISION AXD DEVELOPMENT

1. Variation in the rhythm of division.

2. Displacement of the cells (including variations in the direction of cleavage).

3. Unequal division of the cells.

Nothing is more common than a departure from the regular rhythm of division. The variations are sometimes quite irregular, sometimes follow a definite rule, as, for instance, in the annelid Xercis (Fig. 171), where the typical succession in the number of cells is with great constancy 2, 4, 8, 16, 20, 23, 29, 32, 37, 38, 41, 42, after which the order is more or less variable. The factors that determine such variations in the rhythm of division are very little understood. Balfour, one of the first to consider the subject, sought an explanation in the varying distribution of metaplasmic substances, maintaining ('75,

  • 8o) that the rapidity of division in any part of the ovum is in general

inversely proportional to the amount of deutoplasm that it contains. The entire inadequacy of this view has been demonstrated by a long series of precise studies on cell-lineage, which show that while the large deutoplasm-bearing cells often do divide more slowly than the smaller protoplasmic ones the reverse is often the case, while remarkable differences in the rhythm of division are often observed in cells which do not perceptibly differ in metaplasmic content. 1 All the evidence indicates that the rhythm of division is at bottom determined by factors of a very complex character which cannot be disentangled from those which control growth in general. Lillie ('95, '99) points out the very interesting fact, determined through an analysis of the cell-lineage of mollusks and annelids, that the rate of cleavage shows a direct relation to the period at which the products become functional. Thus in Unio the more rapid cleavage of a certain large cell ("d. 2 M ), formed at the fourth cleavage, is obviously correlated with the early formation of the shell-gland to which it gives rise, while the relatively slow rate of division in the first ectomerequartet is correlated with reduction of the pne-trochal region. The prospective character shown here will be found to apply also to other characters of cleavage, as described beyond.

When we turn to the factors that determine the direction of cleavage or the displacement of cells subsequent to division, we find, as in the case of the division-rhythm, obvious mechanical factors combined with others far more complex. The arrangement of tissue-cells usually tends toward that of least resistance or greatest economy of space ; and in this regard they have been shown to conform, broadly speaking, with the behaviour of elastic spheres, such as soap-bubbles when massed together and free to move. Such bodies, as Plateau

1 Cf. Wilson, '92, Kofoid, '94, Lillie, '95, Zur Strassen, '95, Ziegler, '95, and especially Jennings, '97.


GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS


367


and Lamarle have shown, assume a polyhedral form and tend toward such an arrangement that the area of surface-contact between them is a minimum. Spheres in a mass thus tend to assume the form of interlocking polyhedrons so arranged that three planes intersect in a line, while four lines and six planes meet at a point. If arranged in a single layer on an extended surface, they assume the form of



C D

Fig. 170. — Cleavage of Polygordius, from life.

A. Four-cell stage, from above. B. Corresponding view of eight-cell stage, the same (contrast Fig. 169, C) . D. Sixteen-cell stage from the side.


C. Side view of


hexagonal prisms, three planes meeting along a line as before. Both these forms are commonly shown in the arrangement of the cells of plant and animal tissues; and Berthold ('86) and Errera ('86, '87), carefully analyzing the phenomena, have endeavoured to show that not only the form and relative position of cells, but also the direction of cell-division, is, partially at least, thus determined.

It is through displacements of the cells of this type that many of


368 CELL-DIVISION AND DEVELOPMENT

the most frequent modifications of cleavage arise. Sometimes, as in Synapta, the alternation of the cells is effected through displacement of the blastomeres after their formation. More commonly it arises during the division of the cells, and may even be predetermined by the position of the mitotic figures before the slightest external sign of division. Thus arises that form of cleavage known as the spiral, oblique, or alternating type, where the blastomeres interlock during their formation and lie in the position of least resistance from the beginning. This form of cleavage, especially characteristic of many worms and mollusks, is typically shown by the egg of Polygordins (Fig. 170). The four-celled stage is nearly like that of Synapta, though even here the cells slightly interlock. The third division is, however, oblique, the four upper cells being virtually rotated to the right (with the hands of a watch) so as to alternate with the four lower ones. The fourth cleavage is likewise oblique, but at right angles to the third, so that all of the cells interlock as shown in Fig. 170, D. This alternation regularly recurs for a considerable period.

In many worms and mollusks the obliquity of cleavage appears still earlier, at the second cleavage, the four cells being so arranged that two of them meet along a " cross-furrow " at the lower pole of the egg, while the other two meet at the upper pole along a similar, though often shorter, cross-furrow at right angles to the lower {e.g. in Nereis, Fig. 171). It is a curious fact that the direction of the displacement is quite constant, the first or upper quartet in the eightcell stage being rotated to the right, or with the hands of a watch, the second quartet to the left, the third to the right, and so on. Crampton ('94) has discovered the remarkable fact that in Pfiysa, a gasteropod having a reversed or sinistral shell, the whole order of displacement is likewise reversed, and the same has recently been shown by Holmes ('99) to be true of Ancylus.

The spiral or alternating type of cleavage beautifully illustrates Sachs's second rule as affected by modifying conditions ; for, as may be seen by an inspection of Figs. 170, 171, each division-plane is approximately at right angles to the preceding and succeeding (whence the " alternation of the spirals " described by students of cell-lineage), while they are so directed that each cell as it is formed is placed at once in the position of least resistance in the mass, i.e. in the position of minimal surface-contact. It is impossible to resist the conclusion that one of the factors by which the position of the cells (and hence the direction of cell-division) is determined is a purely mechanical one, identical with that which determines the arrangement of soap-bubbles and the like.

Very little acquaintance with the facts of development is however


GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 369

required to show- that this purely mechanical factor, though doubtless real, must be subordinate to some other. This is strikingly shown, for example, in the development of annelids and mollusks, where the spiral cleavage, strictly maintained during the earlier stages, finally gives way more or less completely to a bilateral type of division in which the rule of minimal surface-contact is often violated. We see here a tendency operating directly against, and finally overcoming,



Pig. 171. — Cleavage of Ntr. nd of a marked determinate ch


£ F

An example of a spiral cleavage, unequal from the beginning

A. Two-cell stage (the circles are oil-drops). B, Four-cell stage; the second cleavage-plane passes through the future median plane. C. The same from the right side. D. Eight-cell stage. E, Sixteen cells ; from the cells marked / arises the prototroch or larval ciliated belt, from X the ventral nerve-cord and other structures, from D the mcsoblast-bnnds, the germ-cells, and a part ol the alimentary canal. F. Twenty-nine-cell stage, from the right side ; f. girdle of protolrochal cells which give rise to the ciliated belt

the mechanical factor which predominates in the earlier stages; and in some cases, e.g. in the egg of Claveliita (Fig. 177) and other tunicates, this tendency predominates from the beginning. In both these cases this " tendency " is obviously related to the growth-process to which the future bilateral embryo will owe its form ; x and every attempt to explain the position of the cells and the direction of cleavage must reckon with the morphogenic process taken as a whole. The blastomere is not merely a cell dividing under the stress of rude 1 Cf. WiUon ('9* p. 444)


370


CELL-DIVISION AND DEVELOPMENT


mechanical conditions; it is beyond this "a builder which lays one stone here, another there, each of which is placed with reference to future development." 1

The third class of modifications, due to unequal division of the cells, not only leads to the most extreme types of cleavage but also to its



C D

Fig. 172. — The eight-cell stage of four different animals showing gradations in the inequality of the third cleavage.

A. The leech Clepune (Whitman). B. The chsctopod Rhynckelmis (Vejdovsky). C. The lamellibranch Unio (Lillie). D. Amphioxus,


most difficult problems. Unequal divisions appear sooner or later in all forms of cleavage, the perfect equality so long maintained in Synapta being a rare phenomenon. The period at which the inequality first appears varies greatly in different forms. In Polygordius (Fig. 170) the first marked inequality appears at the fifth cleavage;

1 Lillie, '95, p. 46.


GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 371

in sea-urchins it appears at the fourth (Fig. 3); in Amphioxus at the third (Fig. 172); in the tunicate Clavelina at the second (Fig. 177); in Nereis at the first division (Figs. 60, 171). The extent of the inequality varies in like manner. Taking the third cleavage as a type, we may trace every transition from an equal division (echinoderms, Polygordius), through forms in which it is but slightly marked {Amphioxus, frog), those in which it is conspicuous {Nereis, Lymnaa, polyclades, Pctromyzon, etc.), to forms such as Clepsine, where the cells of the upper quartet are so minute as to appear like mere buds from the four large lower cells (Fig. 172). At the extreme of the series we reach the partial or meroblastic cleavage, such as occurs in the cephalopods, in many fishes, and in birds and reptiles. Here the lower hemisphere of the egg does not divide at all, or only at a late period, segmentation being confined to a disc-like region or blastoderm at one pole of the egg (Fig. 173).

Very interesting is the case of the tcloblasts or pole-cells characteristic of the development of many annelids and mollusks and found in some arthropods. These remarkable cells are large blastomeres, set aside early in the development, which bud forth smaller cells in regular succession at a fixed point, thus giving rise to long cords of cells (Fig. 175). The teloblasts are especially characteristic of apical growth, such as occurs in the elongation of the body in annelids, and they are closely analogous to the apical cells situated at the growing point in many plants, such as the ferns and stoneworts.

Still more suggestive is the formation of rudimentary cells, arising as minute buds from the larger blastomeres, and, in some cases, apparently taking no part in the formation of the embryo (Fig. 174). 1

We are as far removed from an explanation of unequal division as from that of the rhythm and direction of division. Inequality of division, like difference of rhythm, is often correlated with inequalities in the distribution of metaplasmic substances — a fact generalized by Balfour in the statement ('8o) that the size of the cells formed in cleavage varies inversely to the relative amount of protoplasm in the % region of the egg from which they arise. Thus, in all telolecithal ova, where the deutoplasm is mainly stored in the lower or vegetative hemisphere, as in many worms, mollusks, and vertebrates, the cells of the upper or protoplasmic hemisphere are smaller than those of the lower, and may be distinguished as micromeres from the larger macromeres of the lower hemisphere. The size-ratio between micromeres and macromeres is on the whole directly proportional to the ratio between protoplasm and deutoplasm. Partial or discoidal cleavage occurs when the mass of deutoplasm is so great as entirely to prevent cleavage in the lower hemisphere. This has been beautifully con 1 Sec Wilson, '98, '99, 2.


372


CEtl-DIF/SION AND DEVELOPMENT


firmed by O. Hertwig ('98), who, by placing frogs' eggs in a centrifugal machine, has caused them to undergo a meroblastic cleavage through the artificial accumulation of yolk at the lower pole, due to the centrifugal force.

While doubtless containing an element of truth, this explanation is, however, no more adequate than Balfour's rule regarding the relation between deutoplasm and rhythm (p. 366); for innumerable cases are known in which no correlation can be made out between the distribution of inert substance and the inequality of division. This is the case, for example, with the teloblasts mentioned above, which contain no deutoplasm, yet regularly divide unequally. It seems to be inap


Fig. 173.


the squid Loligo. [Watase.)


plicable to the inequalities of the first two divisions in annelids and gastcropods. It is conspicuously inadequate in the history of individual blastomeres, where the history of division has been accurately determined. In Nereis, for example, a large cell known as the first somatoblast, formed at the fourth cleavage (A', Fig. 171, E\ undergoes an invariable order of division, three unequal divisions being followed by an equal one, then by three other unequal divisions, and again by an equal. This cell contains little or no deutoplasm and undergoes no perceptible changes of substance.

The collapse of the rule is most complete in case of the rudimentary cells referred to above. In some of the annelids, eg. in Aricia, where they were first observed,' these cells are derived from the very large primary mcsoblast-cell, which first divides into equal halves. Each of these then buds forth a cell so small as to be no larger than a polar body, and then immediately proceeds to give rise

1 Cf. Wilson, 'ga, 'oS.


GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS


373


to the mesoblast-bands by continued divisions, always in the same plane at right angles to that in which the rudimentary cells are formed (Fig. 174)- The cause of the definite succession of equal and unequal divisions is here wholly unexplained. No less difficult is the extreme inequality of division involved in the formation of the polar bodies. We cannot explain this through the fact that deutoplasm is collected in the lower hemisphere ; for, on the one hand, the succeeding divisions (first cleavages) are often equal, while, on the other hand, the inequality is no less pronounced in eggs having equally



Pig. 174-— Rudimentary blastomcres in the embryo of an annelid, Aritia. A. From lower pole; rudimentary cells Mr.*; the heavy outline is the lip of ihe blastopore. B. The same in sagittal optical section, showing rudimentary cell (f). primary mesoblasl f,W), and mesoblast-band («).

distributed deutoplasm, or in those, like cchinoderm-eggs, which are " alecithal."

Such cases prove that Balfour's law is only a partial explanation, being probably the expression of a more deeply lying cause, and there is reason to believe that this cause lies outside the immediate mechanism of mitosis. Conklin ('94) has called attention to the fact 1 that the immediate cause of the inequality probably does not lie either in the nucleus or in the amphiaster ; for not only the chromatin-halvcs, but also the asters, are exactly equal in the early prophases, and the inequality of the asters only appears as the division proceeds. Probably, therefore, the cause lies in some relation between the mitotic figure and the cell-body in which it lies. 1 In the cleavage of gasteropod eggs.


CELL-DIVISION AA'D DEVELOPMENT


I believe there is reason to accept the conclusion that this relation is one of position, however caused. A central position of the mitotic



figure results in an equal division ; an eccentric position caused by a radial movement of the mitotic figure, in the direction of its axis toward the periphery, leads to unequal division, and the greater the


GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 375

eccentricity, the greater the inequality, an extreme form being beautifully shown in the formation of the polar bodies. Here the original amphiaster is perfectly symmetrical, with the asters of equal size (Fig. 97, A). As the spindle rotates into its radial position and approaches the periphery, the development of the outer aster becomes, as it were, suppressed, while the central aster becomes enormously large. The size of the aster, in other words, depends upon the extent of the cytoplasmic area that falls within the sphere of influence of the ccntrosome ; and this area depends upon the position of the centrosome. If, therefore, the polar amphiaster could be artificially prevented from moving to its peripheral position, the egg would probably divide equally. 1

This leads us to a further consideration of the attempts that have been made to explain the movements of the mitotic figure through mechanical or other causes. 2 Highly interesting experiments have been made by Pfliiger ('84), Roux ('85), Dricsch ('92), and a number of later investigators which show that the direction of cleavage may be determined, or at least modified, by such a purely mechanical cause as pressure, through which the form of the dividing mass is changed.

Thus, Driesch has shown that when the eggs of sea-urchins are flattened by pressure, the amphiasters all assume the position of least resistance, i.e. parallel to the flattened sides, so that the cleavages are all vertical, and the egg segments as a flat plate of eight, sixteen, or thirty-two cells (Fig. 186). This is totally different from the normal form of cleavage; yet such eggs, when released from pressure, are capable of development and give rise to normal embryos. This interesting experiment makes it highly probable that the disc-like cleavage of meroblastic eggs, like that of the squid or bird, is in some degree a mechanical result of the accumulation of yolk by which the formative protoplasmic region of the ovum is reduced to a thin layer at the upper pole ; and it indicates, further, that the unequal cleavage of less modified telolecithal eggs, like those of the frog or snail, are in like manner due to the displacement of the mitotic figures toward the upper pole.

The results of Pfliiger's and Driesch's pressure experiments obviously harmonize with Hertwig's second rule, for the position of least resistance for the spindle is obviously in the long axis of the protoplasmic mass which is here artificially modified ; and it harmonizes further with Driiner's hypothesis of the active elongation of the spindle in mitosis (p. 105). There are, however, a large number of facts which show that neither the form of the protoplasmic mass nor

1 Cf. Francotte on the polar bodies of Turbellaria, p. 235.

2 For a good review and critique, see Jennings, '97.


376 CELL-DIVISIOX AXD DEVELOPMENT

the distribution of metaplasmic materials is sufficient to explain the position of the spindle, whether with reference to the direction or the inequality of the cleavage.

As regards the direction of the spindle, Berthold ('86) long since clearly pointed out that prismatic or cylindrical vegetable cells, for instance, those of the cambium, often divide lengthwise ; and numerous contradictions of Hertwig's " law " have since been observed by students of cell-lineage with such accuracy that all attempts to explain them away have failed. 1 In some of these cases the position of the spindle is not that of least but of greatest resistance,* the spindle ac


[KOSTANECKI and SlEDLECK].]


tually pushing away the adjoining cell to make way for itself. Similar difficulties, some of which have been already considered (p. 372), stand in the way of the attempt to explain the eccentricity of the spindle in unequal division. All these considerations drive us to the view that the simpler mechanical factors, such as pressure, form, and the like, are subordinate to far more subtle and complex operations involved in the genera! development of the organism, a conclusion strikingly illustrated by the phenomena of teloblastic division (p. 371 ), where the constant succession of unequal divisions, always in the

1 Cf. Watase ('91), Mead ('94, '97, 2), Heidenhain ("95), Wheeler ('95), Castle ('96), Jennings C'J7'h

1 See especially the case observed by Mead ("94, '97, a), in the egg of Amphitriie.


GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 377

same plane, is correlated with a deeply lying law of growth affecting the entire formation of the body. We cannot comprehend the forms of cleavage without reference to the end-result ; and thus these phenomena acquire a certain teleological character so happily expressed by Lillie (p. 370). This has been clearly recognized in various ways by a number of recent writers. Roux ('94), while seeking to explain many of the operations of mitosis on a mechanical basis, holds that the position of the spindle is partly determined by "immanent" nuclear tendencies. Braem ('94) recognizes that the position of the spindle is determined not merely as that of least resistance for the mitotic figure, but also for that of the resulting products. I pointed out ('92) that the bilateral form of cleavage in annelids must be regarded as a " forerunner " of the adult bilaterality. Jennings ('97) concludes that the form and direction of cleavage are related to the later morphogenetic processes ; and many similar expressions occur in the works of recent students of cell-lineage. 1

The clearest and best expression of this view is, however, given by Lillie ('95, '99), who not only correlates the direction and rate of cleavage, but also the size-relations of the cleavage-cells with the arrangement of the adult parts, pointing out that in general the size, as well as the position, of the blastomeres is directly correlated with that of the parts to which they give rise, and showing that on this basis "one can thus go over every detail of the cleavage, and knowing the fate of the cells, can explain all the irregularities and peculiarities exhibited." 2 Of the justice of this conclusion I think any one must be thoroughly convinced who carefully examines the recent literature of cell-lineage. It gives no real explanation of the phenomena, and is hardly more than a restatement of fact. Neither does it in any way lessen the importance of studying fully the mechanical conditions of cell-division. It does, however, show how inadequate have been most of the attempts thus far to formulate the " laws " of cell-division, and how superficially the subject has been considered by some of those who have sought for such "laws."

We now pass naturally to the second or promorphological aspect of cleavage, to a study of which we are driven by the foregoing considerations.

1 Conklin ('99) believes that many of the peculiarities of cleavage may be explained by the assumption of protoplasmic currents which " carry the centrosomes where they will, and control the direction of division and the relative size and quality of the daughter-cells,' , I.e., p. 90. He suggests that such currents are of a chemotropic character, but recognizes that their causation and direction remain unexplained.

1 Cf. ('95), P- 39


378 CELL-DIVISION AND DEVELOPMENT


B. Promorphological Relations of Cleavage

The cleavage of the ovum has thus far been considered in the main as a problem of cell-division. We have now to regard it in an even more interesting and suggestive aspect ; namely, in its morphological relations to the body to which it gives rise. From what has been said above it is evident that cleavage is not merely a process by which the egg simply splits up into indifferent cells which, to use the phrase of Pfliiger, have no more definite relation to the structure of the adult body than have snowflakes to the avalanche to which they contribute. 1 It is a remarkable fact that in a very large number of cases a precise relation exists between the cleavage-products and the adult parts to which they give rise ; and this relation may often be traced back to the beginning of development, so that from the first division onward we are able to predict the exact future of every individual cell. In this regard the cleavage of the ovum often goes forward with a wonderful clocklike precision, giving the impression of a strictly ordered scries in which every division plays a definite role and has a fixed relation to all that precedes and follows it.

But more than this, the apparent predetermination of the embryo may often be traced still farther back to the regions of the undivided and even unfertilized ovum. The egg, therefore, may exhibit a distinct promorphology ; and the morphological aspect of cleavage must be considered in relation to the promorphology of the ovum of which it is an expression. ..

I . Promorphology of the Ovum

(a) Polarity and the Egg-axis. — It was long ago recognized by von Baer ('34) tnat * ne unsegmented egg of the frog has a definite egg-axis connecting two differentiated poles, and that the position of the embryo is definitely related to it. The great embryologist pointed out, further, that the early cleavage-planes also are definitely related to it, the first two passing through it in two meridians intersecting each other at a right angle, while the third is transverse to it, and is hence equatorial. 2 Remak afterward recognized the fact 3 that the larger cells of the lower hemisphere represent, broadly speaking, the "vegetative layer" of von Baer, i.e. the inner germ-layer or entoblast, from which the alimentary organs arise ; while the smaller cells

i C8 3 \ p. 64.

2 The third plane is in this case not precisely at the equator, but considerably above it, forming a " parallel " cleavage.

8 '55» P* I 3°* Among others who early laid stress on the importance of the egg-polarity maybe mentioned Auerbach ('74), Hatschek C77), Whitman C78), and Van Beneden ('83).


PROMORPHOLOGICAL RELATIONS OF CLEAVAGE 379

of the upper hemisphere represent the " animal layer," outer germlayer or ectoblast from which arise the epidermis, the nervous system, and the sense-organs. This fact, afterward confirmed in a very large number of animals, led to the designation of the two poles as animal and vegetative 1 formative and nutritive, or protoplasmic and deiitoplasmic, the latter terms referring to the fact that the nutritive deutoplasm is mainly stored in the lower hemisphere, and that development is therefore more active in the upper. The polarity of the ovum is accentuated by other correlated phenomena. In every case where an egg-axis can be determined by the accumulation of deutoplasm in the lower hemisphere the egg-nucleus sooner or later lies eccentrically in the upper hemisphere, and the polar bodies are formed at the upper pole. Even in cases where the deutoplasm is equally distributed or is wanting — if there really be such cases — an egg-axis is still determined by the eccentricity of the nucleus and the corresponding point at which the polar bodies are formed.

In vastly the greater number of cases the polarity of the ovum has a definite promorphological significance ; for the egg-axis shows a definite and constant relation to the axes of the adult body. It is a very general rule that the upper or ectodermic pole, as marked by the position of the polar bodies, lies in the median plane at a point which is afterward found to lie at or near the anterior end. Throughout the annelids and mollusks, for example, the upper pole is the point at which the cerebral ganglia are afterward formed ; and these organs lie in the adult on the dorsal side near the anterior extremity. This relation holds true for many of the Bilateralia, though the primitive relation is often disguised by asymmetrical growth in the later stages, such as occur in echinoderms. There is, however, some reason to believe that it is not a universal rule. The recent observations of Castle C96), which are in accordance with the earlier work of Seeliger, show that in the tunicate Ciona the usual relation is reversed, the polar bodies being formed at the vegetative {i.e. deutoplasmic or entodermic) pole, which afterward becomes the dorsal side of the larva. My own observations O95) on the echinoderm-egg indicate that here the primitive egg-axis has an entirely inconstant and casual relation to the gastrula-axis. It may, however, still be possible to show that these exceptions are only apparent, and the principle involved is too important to be accepted without further proof.

(6) Axial Relations of the Primary Cleavage-planes. — Since the egg-axis is definitely related to the embryonic axes, and since the first two cleavage-planes pass through it, we may naturally look for a definite relation between these planes and the embryonic axes ; and if such a relation exists, then the first two or four blastomeres must likewise have a definite prospective value in the development. Such


380 CELL-DIVISIQX AXD DEVELOPMENT

relations have, in fact, been accurately determined in a large number of cases. The first to call attention to such a relation seems to have been Newport ('541, who discovered the remarkable fact that the first cleavage-plane in the frog s egg coincides with Ike median plane of the adult body; that, in other words, one of the first two blastomeres gives rise to the left side of the body, the other to the right. This discover)', though long overlooked and, indeed, forgotten, was coofirmed more than thirty years later by Pfliigcr and Rou.x ('S/L It



tagc of the tunicate egg. .ed from the ventral side. B. Siiteen-cell stage |Va: 1 thiotgh Ihe gastrula stage (Castle) ; a. antetiot irmaiion according to Castle.]


was placed beyond all question by a remarkable experiment by Roux ('88), who succeeded in killing one of the blastomeres by puncture with a heated needle, whereupon the uninjured cell gave rise to a half-body as if the embryo had been bisected down the middle line (Fig. 182).

A similar result has been reached in a number of other animals by following out the cell-lineage; e.g. by Van Beneden and Julin ('84)


PROMORPHOLOGLCAL RELATIONS OF CLEAVAGE 38I

in the egg of the tunicate Clavelina (Fig. 177), and by Watase ('91) in the eggs of cephalopods (Fig. 178). In both these cases all the early stages of cleavage show a beautiful bilateral symmetry, and not only can the right and left halves of the segmenting egg be distinguished with the greatest clearness, but also the anterior and posterior regions, and the dorsal and ventral aspects. These discoveries seemed, at first, to justify the hope that a fundamental law of development had been discovered, and Van" Beneden was thus led, as early as 18S3, to express the view that the development of all bilateral animals would probably be found to agree with the frog and ascidian in respect to the relations of the first cleavage.

This cleavage was soon proved to have been premature. In one series of forms, not the first but the second cleavage-plane was found



Fig. 178. — Bilateral cleavage of the squill's e A. Eight-cell stage. B. The fifth cleavage in progress. The first cleavage {a-f) coincide) with the future median plane ; the second (l-r) is transverse.

to coincide with the future long axis {Nereis, and some other annelids ; Crepidula, Umbrella, and other gasteropods). In another series of forms neither of the first cleavages passes through the median plane, but both form an angle of about 45 to it {Clcpsine and other leeches ; Rhynclulmis and other annelids ; Planorbis, Nassa, Unio, and other mollusks; Dtscoecelis and other platodes). In a few cases the first cleavage departs entirely from the rule, and is equatorial, as in Ascaris and some other nematodes. The whole subject was finally thrown into apparent confusion, first by the discovery of Clapp ('91 ), Jordan, and Eycleshymer ('94) that in some cases there seems to be no constant relation whatever between the early cleavage-planes and the adult axes, even in the same species (teleosts, urodeles) ; and even in


382


CELL-DIVISION AND DEVELOPMENT


the frog Hertwig showed that the relation described by Newport and Roux is not invariable. Driesch finally demonstrated that the direction of the early cleavage-planes might be artificially modified by pressure without perceptibly affecting the end-result (cf. p. 375).

These facts prove that the promorphology of the early cleavageforms can have no fundamental significance. Nevertheless, they are of the highest interest and importance ; for the fact that the formative forces by which development is determined may or may not coincide with those controlling the cleavage, gives us some hope of



v v

Fig- 1 79- ~ Outline of unsegmented squid's egg, to show bilaterality. [Watas#.] A. From right side. B. From posterior aspect. a-p. antero-posterior axis ; d-v. dorso-ventral axis ; /. left side ; r. right side.


disentangling the complicated factors of development through a comparative study of the different forms.

(c) Other P romorphological Characters of the Ovum. — Besides the polarity of the ovum, which is the most constant and clearly marked of its promorphological features, we are often able to discover other characters that more or less clearly foreshadow the later development. One of the most interesting and clearly marked of these is the bilateral symmetry of the ovum in bilateral animals, which is sometimes so clearly marked that the exact position of the embryo may be predicted in the unfertilized egg, sometimes even before it is laid. This is the case, for example, in the cephalopod egg, as shown by Watase (Fig. 179). Here the form of the new-laid egg, before cleavage begins, distinctly foreshadows that of the embryonic bodv, and forms as it were a mould in which the whole development is cast. Its general shape is that of a hen's egg slightly flattened on one side,


PRQMORPHQLQGICAL RELATIONS Of CLEAVAGE 383

the narrow end, according to Watase, representing the dorsal aspect, the broad end the ventral aspect, the flattened side the posterior region, and the more convex side the anterior region. All the early cleavage-furrows are bilaterally arranged with respect to the plane of



Fig. 180. — Eggs of the it

A. Earl]' stage before formation of the ei

plane of symmetry. C. The embryo in its fir

a. anterior end ; p. posterior ; /. left side,

refer to xhejSital position of the embryo, whi

first develops) ; m.micropyle. near p is the


31 Cerira, [MF.TSCHNtKOFF.]


re side. 13. The

il, d. dorsal aspect. (These


viewed in the


symmetry in the undivided egg; and the same is true of the later development of all the bilateral parts.

Scarcely less striking is the case of the insect egg, as has been pointed out especially by Hallez, Blochmann, and Wheeler (Figs. 62, 180). In a large number of cases the egg is elongated and


384 CELL-DJ VISION AND DEVELOPMENT

bilaterally symmetrical, and, according to Blochmann and Wheeler, may even show a bilateral distribution of the yolk corresponding with the bilaterality of the ovum. Hallez asserts as the results of a study of the cockroach (Periplaneta), the water-beetle {Hydrophilus\ and the locust (Locusta) that "the egg-cell possesses the same orientation as the maternal organism that produces it ; it has a cephalic pole and a caudal pole, a light side and a left, a dorsal aspect and a ventral ; and these different aspects of the egg-cell coincide with the corresponding aspects of the embryo." 1 Wheeler ('93), after examining some thirty different species of insects, reached the same result, and concluded that even when the egg approaches the spherical form the symmetry still exists, though obscured. Moreover, according to Hallez ('86) and later writers, the egg always lies in the same position in the oviduct, its cephalic end being turned forwards toward the upper end of the oviduct, and hence toward the head-end of the mother. 2

2. Meaning of the Promorphology of the Ovum

The interpretation of the promorphology of the ovum cannot be adequately treated apart from the general discussion of development given in the following chapter; nevertheless it may briefly be considered at this point. Two widely different interpretations of the facts have been given. On the one hand, it has been suggested by Flemming and Van Beneden, 8 and urged especially by Whitman, 4 that the cytoplasm of the ovum possesses a definite primordial organization which exists from the beginning of its existence even though invisible, and is revealed to observation thiough polar differentiation, bilateral symmetry, and other obvious characters in the unsegmented egg. On the other hand, it has been maintained by Pfluger, Mark, Oscar Hertwig, Driesch, Watas£, and the writer that all the promorphological features of the ovum are of secondary origin; that the egg-cytoplasm is at the beginning isotropous — i.e. indifferent or homaxial — and gradually acquires its promorphological features during its preembryonic history. Thus the egg of a bilateral animal is at the beginning not actually, but only potentially, bilateral. Bilaterality once established, however, it forms as it were the mould in which the cleavage and other operations of development are cast

I believe that the evidence at our command weighs heavily on the side of the second view, and that the first hypothesis fails to

1 See Wheeler, '93, p. 67.

2 The micropyle usually lies at or near the anterior end, but may be at the posterior. It is a very important fact that the position of the polar bodies varies, being sometimes at the anterior end, sometimes on the side, either dorsal or lateral (Heider, Blochmann).

8 See p. 298. 4 Cf. pp. 299, 300.


PROMORPHOLOGICAL RELATIONS OF CLEAVAGE 385

take sufficient account of the fact that development does not necessarily begin with fertilization or cleavage, but may begin at a far earlier period during ovarian life. As far as the visible promorphological features of the ovum are concerned, this conclusion is beyond question. The only question that has any meaning is whether these visible characters are merely the expression of a more subtle pre


Plg. 181. — Variations 111 the axial re] as they lie in the oviduct. [Hacker.]

A. Group of eggs showing variations in relative position of the polar spindles and (he spermnucleus (the latter black) ; in a the sperm-nucleus is opposite lo the polar spindle, in b, near il or at the side. B. Group showing variations in the axis of first cleavage with reference to the polar bodies (the latter black ) ; a, b, and c show three different positions.


existing invisible organization of the same kind. I do not believe that this question can be answered in the affirmative save by the trite and, from this point of view, barren statement that every effect must have its preexisting cause. That the egg possesses no fixed and predetermined cytoplasmic localization with reference to the adult parts, has, I think, been demonstrated through the remarkable


386 CELL-DIVISION AND DEVELOPMENT

m

experiments of Driesch, Roux, and Boveri, which show that a fragment of the egg may give rise to a complete larva (p. 353). There is strong evidence, moreover, that the egg-axis is not primordial but is established at a particular period ; and even after its establishment it may be entirely altered by new conditions. This is proved, for example, by the case of the frog's egg, in which, as Pfluger ('84), Born ('85), and Schultze ('94) have shown, the cytoplasmic materials may be entirely rearranged under the influence of gravity, and a new axis established. In sea-urchins, my own observations C95) render it probable that the egg-axis is not finally established until after fertilization. These and other facts, to be more fully considered in the following chapter, give strong ground for the conclusion that the promorphological features of the egg are as truly a result of development as the characters coming into view at later stages. They are gradually established during the preembryonic stages, and the egg, when ready for fertilization, has already accomplished part of its task by laying the basis for what is to come.

Mark, who was one of the first to examine this subject carefully, concluded that the ovum is at first an indifferent or homaxial cell (i.e. isotropic), which afterward acquires polarity and other promorphological features. 1 The same* view was very precisely formulated by Watas6 in 1891, in the following statement, which I believe to express accurately the truth : *' It appears to me admissible to say at present that the ovum, which may start out without any definite axis at first, may acquire it later, and at the moment ready for its cleavage the distribution of its protoplasmic substances may be such as to exhibit a perfect symmetry, and the furrows of cleavage may have a certain definite relation to the inherent arrangement of the protoplasmic substances which constitute the ovum. Hence, in a certain case, the plane of the first cleavage-furrow may coincide with the plane of the median axis of the embryo, and the sundering of the protoplasmic material may take place into right and left, according to the preexisting organization of the c^g at the time of cleavage ; and in another case the first cleavage may roughly correspond to the differentiation of the ectoderm and the entoderm, also according to the preorganized constitution of the protoplasmic materials of the ovum.

" It does not appear strange, therefore, that we may detect a certain structural differentiation in the unsegmented ovum, with all the axes foreshadowed in it, and the axial symmetry of the embryonic organism identical with that of the adult." 2

This passage contains, I believe, the gist of the whole matter, as far as the promorphological relations of the ovum and of cleavage lf 8i, p. 512. a '9i,p. 280.


PROMORPHOLOGICAL RELATIONS OF CLEAVAGE 387

forms are concerned, though Watas6 does not enter into the question as to how the arrangement of protoplasmic materials is effected. In considering this question, we must hold fast to the fundamental fact that the egg is a cell, like other cells, and that from an a priori point of view there is every reason to believe that the cytoplasmic differentiations that it undergoes must arise in essentially the same way as in other cells. We know that such differentiations, whether in form or in internal structure, show a definite relation to the environment of the cell — to its fellows, to the source of food, and the like. We know further, as Korschelt especially has pointed out, that the eggaxis, as expressed by the eccentricity of the germinal vesicle, often shows a definite relation to the ovarian tissues, the germinal vesicle lying near the point of attachment or of food-supply. Mark made the pregnant suggestion, in 1881, that the primary polarity of the egg might be determined by " the topographical relation of the egg (when still in an indifferent state) to the remaining cells of the maternal tissue from which it is differentiated" and added that this relation might operate through the nutrition of the ovum. " It would certainly be interesting to know if that phase of polar differentiation which is manifest in the position of the nutritive substance and of the germinal vesicle bears a constant relation to the free surface of the epithelium from which the egg takes its origin. If, in cases where the egg is directly developed from epithelial cells, this relationship were demonstrable, it would be fair to infer the existence of corresponding, though obscured, relations in those cases where (as, for example, in mammals) the origin of the ovum is less directly traceable to an epithelial surface." 1 The polarity of the egg would therefore be comparable to the polarity of epithelial or gland-cells, where, as pointed out at page 57, the nucleus usually lies toward the base of the cell, near the source of food, while the centrosomes, and often also characteristic cytoplasmic products, such as zymogen granules and other secretions, appear in the outer portion. 2 The exact conditions under which the ovarian egg develops are still too little known to allow of a positive conclusion regarding Mark's suggestion. Moreover, the force of Korschelt's observation is weakened by the fact that in many eggs of the extreme telolecithal type, where the polarity is very marked, the germinal vesicle occupies a central or sub-central position during the period of yolk-formation and only moves toward the periphery near the time of maturation.

Indeed, in mollusks, annelids, and many other cases, the germinal vesicle remains in a central position, surrounded by yolk on all sides, until the spermatozoon enters. Only then does the egg-nucleus move

^Si.p. 515.

2 Hatschek has suggested the same comparison (ZoMogit, p. 112).


388 CELL-DIVISION AND DEVELOPMENT

to the periphery, the deutoplasm become massed at one pole, and the polarity of the egg come into view {Nereis, Figs. 60 and 97). 1 In such cases the axis of the egg may perhaps be predetermined by the position of the centrosome, and we have still to seek the causes by which the position is established in the ovarian' history of the egg. These considerations show that this problem is a complex one, involving, as it does, the whole question of cell-polarity ; and I know of no more promising field of investigation than the ovarian history of the ovum with reference to this question. That Mark's view is correct in principle is indicated by a great array of general evidence considered in the following chapter, where its bearing on the general theory of development is more fully dealt with.

C. Cell-division and Growth

The general relations between cell-division and growth, which have already been briefly considered at page 58 and in the course of this chapter, may now be more critically examined, together with some account of the causes that incite or inhibit division. It has been shown above that every precise inquiry into the rate form, or direction of cell-division, inevitably merges into the larger problem of the general determination of growth. We may conveniently approach this subject by considering first the energy of division and the limitation of growth.

All animals and plants have a limit of growth, which is, however, much more definite in some forms than in others, and differs in different tissues. During the individual development the energy of cell-division is most intense in the early stages (cleavage) and diminishes more and more as the limit of growth is approached. When the limit is attained a more or less definite equilibrium is established, some of the cells ceasing to divide and perhaps losing this power altogether (nerve-cells), others dividing only under special conditions (connective tissue-cells, gland-cells, muscle-cells), while others continue to divide throughout life, and thus replace the worn-out cells of the same tissue (Malpighian layer of the epidermis, etc.). The limit of size at which this state of equilibrium is attained is an hereditary character, which in many cases shows an obvious relation to the environment, and has therefore probably been determined and is maintained by natural selection. From the cytological point of view the limit of body-size appears to be correlated with the total number of cells formed rather than with their individual size. This relation has been carefully studied by Conklin C96) in the case of the gastero 1 The immature egg of Nereis show's, however, a distinct polarity in the arrangement of the fat-drops, which form a ring in the equatorial regions.


CELL-DIVISION AND GROWTH 389

pod Crepidula, an animal which varies greatly in size in the mature condition, the dwarfs having in some cases not more than ^ the volume of the giants. The eggs are, however, of the same size in all, and their number is proportional to the size of the adult. The same is true of the tissue-cells. Measurements of cells from the epidermis, the kidney, the liver, the alimentary epithelium, and other tissues show that they are on the whole as large in the dwarfs as in the giants. The body-size therefore depends on the total number of cells rather than on their size individually considered, and the same appears to be the case in plants. 1

A result which, broadly speaking, agrees with the foregoing, is given through the interesting experimental studies of Morgan ('95, 1, '96), supplemented by those of Driesch ('98), in which the number of cells in normal larvae of echinoderms, ascidians, and Amphioxus is compared with those in dwarf larvae of the same species developed from egg-fragments (Morgan) and isolated blastomeres (Driesch). Unless otherwise specified, the following data are cited from Driesch.

The normal blastula of Spharechinus possesses about 500 cells (Morgan), of which from 75 to 90 invaginate to form the archenteron (Driesch). In half-gastrulas the number varies from 35^045, occasionally reaching 50. In the same species, the normal number of mesenchyme-cells is 54 to 60, in the half-larvae 25 to 30. In Echinus the corresponding numbers are 30± and 13 to 15. In the ascidian larvae — a particularly favourable object — there are 29 to 35 (exceptionally as high as 40) chorda-cells ; in the hapf-larvae, 1 3 to 17. While these comparisons are not mathematically prfecise, owing to the difficulty of selecting exactly equivalent stages, they nevertheless show that, on the whole, the size of the organ, as of the entire organism, is directly proportional to the number and not to the size of the cells, just as in the mature individuals of Crepidula. The available data are, however, too scanty to justify any very positive conclusions, and it is probable that further experiment will disclose factors at present unknown. It would be highly interesting to determine whether such dwarf embryos could in the end restore the normal number of cells, and, hence, the normal size of the body. In all the cases thus far determined the dwarf gastrulas give rise to larvae (Plutci, etc.) correspondingly dwarfed ; but their later history has not yet been sufficiently followed out.

The gradual diminution of the energy of division during development by no means proceeds at a uniform pace in all of the cells, and, during the cleavage, the individual blastomeres are often found to exhibit entirely different rhythms of division, periods of active division being succeeded by long pauses, and sometimes by an entire cessa 1 See Amelung ('93) and Strasburger ('93).


390 CELL-DIVISION AND DEVELOPMENT

tion of division even at a very early period. In the echinoderms, for example, it is well established that division suddenly pauses, or changes its rhythm, just before the gastrulation (in Synapta at the 512-cell stage, according to Selenka), and the same is said to be the case in Amphioxus (Hatschek, Lwoff ). In Nereis, one of the blastomeres on each side of the body in the forty-two-cell stage suddenly ceases to divide, migrates into the interior of the body, and is converted into a unicellular glandular organ. 1 In the same animal, the four lower cells (macromeres) of the eight-cell stage divide in nearly regular succession up to the thirty-eight-cell stage* when a long pause takes place, and when the divisions are resumed they are of a character totally different from those of the earlier period. The cells of the ciliated belt or prototroch in this and other annelids likewise cease to divide at a certain period, their number remaining fixed thereafter. 2 Again, the number of cells produced for the foundation of particular structures is often definitely fixed, even when their number is afterward increased by division. In annelids and gasteropods, for example, the entire ectoblast arises from twelve micromeres segmented off in three successive quartets of micromeres from the blastomeres of the fourcell stage. Perhaps the most interesting numerical relations of this kind are those recently discovered in the division of teloblasts, where the number of divisions is directly correlated with the number of segments or somites. It is well known that this is the case in certain plants (Characece\ where the. alternating nodes and internodes of the stem are derived from corirMxmding single cells successively segmented off from the apical dSif Vejdovsky's observations on the annelid Dcndrobana give strong ground to believe that the number of metamerically repeated parts of this animal, and probably of other annelids, corresponds in like manner with that of the number of cells segmented off from the teloblasts. The most remarkable and accurately determined case of this kind is that of the isopod Crustacea, where the number of somites is limited and perfectly constant. In the embryos of these animals there are two groups of teloblasts near the hinder end of the embryo, viz. an inner group of mesoblasts, from which arise the mesoblast-bands, and an outer group of ectoblasts, from which arise the neural plates and the ventral ectoblast. McMurrich ('95) has recently demonstrated that the mesoblasts always divide exactly sixteen times, the ectoblasts thirty-two (or thirty-three) times, before relinquishing their teleoblastic mode of division and breaking up into smaller cells. Now the sixteen groups of cells thus formed give rise to the sixteen respective somites of the post-naupliar region of the embryo {i.e. from the second maxilla backward). In other

1 This organ, doubtfully identified by me as the head-kidney, is probably a mucus-gland (Mead). 2 Cf. Fig. 171.


CELLr-DI VISION AND GROWTH 39 1

words, each single division of the mesoblasts and each double division of the ectoblasts splits off the material for a single somite ! The number of these divisions, and hence of the corresponding somites, is a fixed inheritance of the species.

The causes that determine the rhythm of division, and thus finally establish the adult equilibrium, are but vaguely comprehended. The ultimate causes must of course lie in the inherited constitution of the organism, and are referable in the last analysis to the structure -of the germ-cells. Every division must, however, be the response of the cell to a particular set of conditions or stimuli ; and it is through the investigation of these stimuli that we may hope to penetrate farther into the nature of development. The immediate, specific causes of cell-division are still imperfectly known. In the adult, cells may be stimulated to divide by the utmost variety of agencies — by chemical stimulus, as in the formation of galls, or in hyperplasia induced by the injection of foreign substances into the blood ; by mechanical pressure, as in the formation of calluses ; by injury, as in the healing of wounds and in the regeneration of lost parts ; and by a multitude of more complex physiological and pathological conditions, — by any agency, in short, that disturbs the normal equilibrium of the body. In all these cases, however, it is difficult to determine the immediate stimulus to division ; for a long chain of causes and effects may intervene between the primary disturbance and the ultimate reaction of the dividing cells. Thus there is reason to believe that the formation of a callus is not directly caused by pressure or friction, but through the determination of an increased blood-supply to the part affected and a heightened nutrition of the cells. Cell-division is here probably incited by local chemical changes ; and the opinion is gaining ground that the immediate causes of division, whatever their antecedents, are to be sought in this direction. That such is the case is indicated by nothing more clearly than the recent experiments on the egg by R. Hertwig, Mead, Morgan, and Loeb already referred to in part at pages 1 1 1 and 215. The egg-cell is, in most cases, stimulated to divide by the entrance of the spermatozoon, but in parthenogenesis exactly the same result is produced by an apparently quite different cause. The experiments in question give, however, ground for the conclusion that the common element in the two cases is a chemical stimulus. In the eggs of Chatopterus under normal conditions the first polar mitosis pauses at the anaphase until the entrance of the spermatozoon, when the mitotic activity is resumed and both polar bodies are formed. Mead C98) shows, however, that the same effect may be produced without fertilization by placing the eggs for a few minutes in a weak solution of potassium chloride. In like manner R. Hertwig ('96) and Morgan ('99) show that unfertilized


392 CELL-DIVISION AND DEVELOPMENT

echinoderm-eggs may be stimulated to division by treatment with weak solution of strychnine, sodium-chloride, and other reagents, the result being here more striking than in the case of Chcetopterus, since the entire mitotic system is formed anew under the chemical stimulus. The climax of these experiments is reached in Loeb's artificial production of parthenogenesis in sea-urchin eggs by treatment with dilute magnesium chloride. Beside these interesting results may be placed the remarkable facts of gall-formation in plants, which seem to leave no doubt that extremely complex and characteristic abnormal growths may result from specific chemical stimuli, and many pathologists have held that tumours and other pathological growths in the animal body may be incited through disturbances of circulation or other causes resulting in abnormal local chemical conditions. 1

But while we have gained some light on the immediate causes of division, we have still to inquire how those causes are set in operation and are coordinated toward a typical end ; and we are thus brought again to the general problem of growth. A very interesting suggestion is the resistance-theory of Thiersch and Boll, according to which each tissue continues to grow up to the limit afforded by the resistance of neighbouring tissues or organs. The removal or lessening of this resistance through injury or disease causes a resumption of growth and division, leading either to the regeneration of the lost parts or to the formation of abnormal growths. Thus the removal of a salamander's limb would seem to remove a barrier to the proliferation and growth of the remaining cells. These processes are therefore resumed, and continue until the normal barrier is reestablished by the regeneration. To speak of such a "barrier" or "resistance" is, however, to use a highly figurative phrase which is not to be construed in a rude mechanical sense. There is no doubt that hypertrophy, atrophy, or displacement of particular parts often leads to compensatory changes in the neighbouring parts ; but it is equally certain that such changes are not a direct mechanical effect of the disturbance, but a highly complex physiological response to it. How complex the problem is, is shown by the fact that even closely related animals may differ widely in this respect. Thus Fraisse has shown that the salamander may completely regenerate an amputated limb, while the frog only heals the wound without further regeneration. 2 Again, in the case of coelenterates, Loeb and Bickford have shown that the tubularian hydroids are able to regenerate the tentacles at both ends of a segment of the stem, while the polyp Cerianthus can regenerate them only at the distal end of a section (Fig. 194).

1 Cf. p. 97. For a good discussion of this subject, see E. Ziegler, '89.

2 In salamanders regeneration only takes place when the bone is cut across, and does not occur if the limb be exarticulated and removed at the joint.


CELL-DJ VISION AND GROWTH 393

In the latter case, therefore, the body possesses an inherent polarity which cannot be overturned by external conditions. A very curious case is that of the earthworm, which has long been known to possess a high regenerative capacity. If the posterior region of the worm be cut off, a new tail is usually regenerated. If the same operation be performed far forward in the anterior region, a new head is often formed at the front end of the posterior piece. If, however, the section be in the middle region the posterior piece sometimes regenerates a head, but more usually a tail, as was long since shown by Spallanzani and recently by Morgan ('99). Why such a blunder should be committed remains for the present quite unexplained.

It remains to inquire more critically into the nature of the correlation between growth and cell-division. In the growing tissues the direction of the division-planes in the individual cells evidently stands in a definite relation with the axes of growth in the body, as is especially clear in the case of rapidly elongating structures (apical buds, teloblasts, and the like), where the division-planes are predominantly transverse to the axis of elongation. Which of these is the primary factor, the direction of general growth or the direction of the divisionplanes ? This question is a difficult one to answer, for the two phenomena are often too closely related to be disentangled. As far as the plants are concerned, however, it has been conclusively shown by Hofmeister, De Bary, and Sachs that the growth of the mass is the primary factor ; for the characteristic mode of growth is often shown by the growing mass before it splits up into cells, and the form of cell-division adapts itself to that of the mass : " Die Pflanze bildet Zellen, nicht die Zelle bildet Pflanzen " (De Bary).

Much of the recent work in normal and experimental embryology, as well as that on regeneration, indicates that the same is true in principle of animal growth. Among recent writers who have urged this view should be mentioned Rauber, Hertwig, Adam Sedgwick, and especially Whitman, whose fine essay on the Inadequacy of the Celltheory of Development C93) marks a distinct advance in our point of view. Still more recently this view has been almost demonstrated through some remarkable experiments on regeneration, which show that definitely formed material, in some cases even the adult tissues, may be directly moulded into new structures. Driesch has shown (95» 2 » 99) that if gastrulas of Sphcerechinus be bisected through the equator so that each half contains both ectoderm and entoderm, the wounds heal, each half forming a typical gastrula, in which the enteron differentiates itself into the three typical regions (fore, middle, and hind gut) correctly proportioned, though the whole structure is but half the normal size. Here, therefore, the formative process is in the main independent of cell-division or increase in size. Miss Bickf ord


394 CELL-DIVISION AND DEVELOPMENT

('94) found that in the regeneration of decapitated hydranths of tubalarians the.new hydranth is primarily formed, not by new cell-formation and growth from the cut end, but by direct- transformation of the distal portion of the stem. 1 Morgan's remarkable observations on Planaria, finally, show that here also, when the animal is cut into pieces, complete animals are produced from these pieces, but only in small degree through the formation of new tissue, and mainly by direct remoulding of the old material into a new body having the correct proportions of the species. As Driesch has well said, it is as if a plan or mould of the new little worm were first prepared and then the old material were poured into it. 2

Facts of this kind, of which a considerable store has been accumulated, give strong ground for the view that cell-formation is subordinate to growth, or rather to the general formative process of which growth is an expression ; and they furnish a powerful argument against Schwann's conception of the organism as a cell-composite (p. 58). That conception is, however, not to be rejected in Mo, but contains a large element of truth ; for there are many cases in which cells possess so high a degree of independence that profound modifications may occur in special regions through injury or disease, without affecting the general equilibrium of the body. The most striking proof of this lies in the fact that grafts or transplanted structures may perfectly retain their specific character, though transferred to a different region of the body, or even to another species. Nevertheless the facts of regeneration prove that even in the adult the formative processes in special parts are in many cases definitely correlated with the organization of the entire mass ; and there is reason to conclude that such a correlation is a survival, in the adult, of a condition characteristic of the embryonic stages, and that the independence of special parts in the adult is a secondary result of development. The study of celldivision thus brings us finally to a general consideration of development which forms the subject of the following chapter.

LITERATURE. VIII

Bert hold, G. — Studien uber Protoplasma-mechanik. IMpzig, 1886.

Boll, Fr. — Das Frincip des Wachsthums. Berlin, 1876.

Bourne, G. C. — A Criticism of the Cell-theory ; being an answer to Mr. Sedgwick's

article on the Inadequacy of the Cellular Theory of Development : Quart. Journ.

Mic.Sci., XXXVIII. 1. 1895.

1 Driesch suggests for such a process the term reparation in contradistinction to true regeneration.

- '99, p. 55. It is mainly on these considerations that Driesch ('99) has built his recent theory of vitalism (ef. p. 417), the nature of the formative power being regarded as a problem sui generis, and one which the " machine-theory of life " is powerless to solve. Cf. also the views of Whitman, p. 416.


LITERA TURE 395

Castle, W. E.— The early Embryology of Ciona. Bull. Mus. Comp. Zoo/., XXVII.

1896. Conklin, E. G. — The Embryology of Crepidula : Journ. Morph., XIII. 1897. Driesch, H. — (See Literature, IX.) Errera, L. — Zellformen und Seifenblasen : Tagebl. der 60 Versammlung deutscher

Naturforscher und Aerste zu Wiesbaden. 1887. Hertwig, 0. — Das Problem der Befruchtung und der Isotropic des Eies, eine Theo rie der Vererbung. Jena* 1884. Hofmeister. — Die Lehre von der Pflanzenzelle. Leipzig, 1867. Jennings, H. S. — The Early Development of Asplanchna : Bull. Mus. Comp. Zodl.*

XXX. 1. Cambridge, 1896. Kofoid, C. A. — On the Early Development of Limax : Bull. Mus. Comp. Zool.,

XXVII. 1895. Lillie, F. R. — The Embryology of the Unionidae : Journ. Morph.* X. 1895. Id. — Adaptation in Cleavage : H'ood^s Holl Biol. lectures. 1899. McMurrich, J. P. — Embryology of the Isopod Crustacea: Journ. Morph., XI. 1.

1895. Mark, E. L. — Limax. (See list IV.) Morgan, T. H. — (See Literature, IX.) fiauber, A. — Neue Grundlegungen zur Kenntniss derZelle: Morph. Jahrb., VIII.

1883. Rhumbler, L. — Allgemeine Zellmechanik : Merkel u. Bonnet, Ergeb., VIII. 1898. Sachs, J. — Pflanzenphysiologie. (See list VII.) Sedgwick, H. — On the Inadequacy of the Cellular Theory of Development, etc. :

Quart. Journ . Mic. Sci. , XXX V 1 1 . 1 . 1 894. Strasburger, E. — Uber die Wirkungssphare der Kerne und die Zellgrbsse : Histo logische Beitrdge, V. 1893. Zur Strassen. 0. — Embryonalentwickelung der Ascaris : Arch. Entom.* III. 1896. Watasl, S. — Studies on Cephalopods ; I., Cleavage of the Ovum : Journ. Morph.,

IV. 3. 1891. Whitman, C. 0. — The Inadequacy of the Cell-theory of Development : Wood^s Holl

Biol. Lectures. 1893. Wilson, Edm. B. — The Cell-lineage of Nereis: Journ. Morph., VI. 3. 1892. Id. — Amphioxus and the Mosaic Theory of Development : Journ. Morph.* VIII. 3.

. 1893. Id. — Considerations on Cell-lineage and Ancestral Reminiscence: Ann. N. Y.

Acad. , XI. 1898: also Wood \r Holl Biol. Lectures* 1899.


CHAPTER IX

THEORIES OF INHERITANCE AND DEVELOPMENT

" It is certain that the germ is not merely a body in which life Is dormant or potential, but that it is itself simply a detached portion of the substance of a preexisting living body.' 1

Huxley. 1

" Inheritance must be looked at as merely a form of growth." Darwin. 5

" Ich mochte daher wohl den Versuch wagen, durch eine Darstellung des Beobachtetei Sie zu einer tiefern Einsicht in die Zeugungs- und Entwickelungsgeschichte der organischen Korpcr zu fuhren und zu zeigen, wie dieselben weder vorgebildet sind, noch auch, wie mu sich gewohnlich denkt, aus ungeformter Masse in einem bestimmten Momente plotzlkh ausschiessen." Von Baejl*

Every discussion of inheritance and development must take as its point of departure the fact that the germ is a single cell similar in its essential nature to any one of the tissue-cells of which the body is composed. That a cell can carry with it the sum total of the heritage of the species, that it can in the course of a few days or weeks give rise to a mollusk or a man, is the greatest marvel of biological science. In attempting to analyze the problems that it involves, we must from the outset hold fast to the fact, on which Huxley insisted, that the wonderful formative energy of the germ is not impressed upon it from without, but is inherent in the egg as a heritage from the parental life of which it was originally a part. The development of the embryo is nothing new. It involves no breach of continuity, and is but a continuation of the vital processes going on in the parental body. What gives development its marvellous character is the rapidity with which it proceeds and the diversity of the results attained in a span so brief.

But when we have grasped this cardinal fact, we have but focussed our instruments for a study of the real problem. How do the adult characteristics lie latent in the germ-cell ; and how do they become patent as development proceeds ? This is the final question that looms in the background of every investigation of the cell. In approaching it we may well make a frank confession of ignorance ; for in spite of all that the microscope has revealed, we have not yet penetrated the mystery, and inheritance and development still remain in their fun 1 Evolution, Science and Culture, p. 291.

2 Variation of Animals and Plants* II., p. 398. 8 Entwick. der Thierc, II., 1837, P- &

396


THE THEORY OF GERMINAL LOCALIZATION 397

damental aspects as great a riddle as they were to the Greeks. What we have gained is a tolerably precise acquaintance with the external aspects of development. The gross errors of the early preformationists have been dispelled. 1 We know that the germ-cell contains no predelineated embryo ; that development is manifested, on the one hand, by the cleavage of the egg, on the other hand, by a process of differentiation, through which the products of cleavage gradually assume diverse forms and functions, and so accomplish a physiological division of labour. We can clearly recognize the fact that these processes fall in the same category as those that take place in the tissuecells ; for the cleavage of the ovum is a form of mitotic cell-division, while, as many eminent naturalists have perceived, differentiation is nearly related to growth and has its root in the phenomena of nutrition and metabolism. The real problem of development is the orderly sequence and correlation of these phenontetia toward a typical result. We cannot escape the conclusion that this is the outcome of the organization of the germ-cells ; but the nature of that which, for lack of a better term, we call " organization/' is and doubtless long will remain almost wholly in the dark.

In the following discussion, which is necessarily compressed within narrow limits, we shall disregard the earlier baseless speculations, such as those of the seventeenth and eighteenth centuries, which attempted a merely formal solution of the problem, confining ourselves to more recent discussions that have grown directly out of modern research. An introduction to the general subject may be given by a preliminary examination of two central hypotheses about which most recent discussions have revolved. These are, first, the theory of Germinal Localisation 2 of Wilhelm His C74), and, second, the Idioplasm Hypothesis of Nageli ('84). The relation between these two conceptions, close as it is, is not at first sight very apparent ; and for the purpose of a preliminary sketch they may best be considered separately.

A. The Theory of Germinal Localization

Although the naive early theory of preformation and evolution was long since abandoned, yet we find an after-image of it in the theory of germinal localization which in one form or another has been advocated by some of the foremost students of development. It is maintained that, although the embryo is not preformed in the germ, it must nevertheless be predetermined in the sense that the egg contains

1 Cf. Introduction, p. 8.

3 I venture to suggest this term as an English equivalent for the awkward expression " Organbildende Keimbezirke " of His.


398 INHERITANCE AND DEVELOPMENT

definite areas or definite substances predestined for the formation of corresponding parts of the embryonic body. The first clear statement of this conception is found in the interesting and suggestive work of Wilhelm His ('74) entitled Unsere Korperform. Considering the development of the chick, he says : " It is clear, on the one hand, that every point in the embryonic region of the blastoderm must represent a later organ or part of an organ, and, on the other hand, that every organ developed from the blastoderm has its preformed germ (vorgebildete Anlage) in a definitely located region of the flat germdisc. . . . The material of the germ is already present in the flat germ-disc, but is not yet morphologically marked off and hence not directly recognizable. But by following the development backwards we may determine the location of every such germ, even at a period when the morphological differentiation is incomplete or before it occurs ; logically, indeed, we must extend this process back to the fertilized or even the unfertilized egg. According to this principle, the germ-disc contains the organ-germs spread out in a flat plate, and, conversely, every point of the germ-disc reappears in a later organ ; I call this the principle of organ- forming germ-regions." 1 His thus conceived the embryo, not as preformed, but as having all of its parts prelocalized in the egg-protoplasm (cytoplasm).

A great impulse to this conception was given during the following decade by discoveries relating, on the one hand, to protoplasmic structure, on the other hand, to the promorphological relations of the ovum. Ray Lankester writes, in 1877: "Though the substance of a cell 2 may appear homogeneous under the most powerful microscope, it is quite possible, indeed certain, that it may contain, already formed and individualized, various kinds of physiological molecules. The visible process of segregation is only the sequel of a differentiation already established, and not visible." 8 The egg-cytoplasm has a definite molecular organization directly handed down from the parent; cleavage sunders the various " physiological molecules " and isolates them in particular cells. Whitman expresses a similar thought in the following year : " While we cannot say that the embryo is predelineated, we can say that it is predetermined. The ' histogenetic sundering ' of embryonic elements begins with the cleavage, and every step in the process bears a definite and invariable relation to antecedent and subsequent steps. ... It is, therefore, not surprising to find certain important histological differentiations and fundamental structural relations anticipated in the early phases of cleavage, and foreshadowed even before cleavage begins." 4 It was, however, Flem 1 /. c. % p. 19.

2 It is clear from the context that by " substance " lankester had in mind the cytoplasm, though this is not specifically stated. 3 '77, p. 14. * '78, p. 49.


THE THEORY OF GERMINAL LOCALIZATION 399

ming who gave the first specific statement of the matter from the cytological point of view : " But if the substance of the egg-cell has a definite structure (Bau), and if this structure and the nature of the network varies in different regions of the cell-body, we may seek in it a basis for the predetermination of development wherein one egg differs from another, and it will be possible to look for it with the microscope. How far this search can be carried no one can say, but its ultimate aim is nothing less than a true morphology of inheritance l In the following year Van Beneden pointed out how nearly this conception approaches to a theory of preformation : " If this were the case (i.e. if the egg-axis coincided with the principal axis of the adult body), the old theory of evolution would not be as baseless as we think to-day. The fact that in the ascidians, and probably in other bilateral animals, the median plane of the body of the future animal is marked out from the beginning of cleavage, fully justifies the hypothesis that the materials destined to form the right side of the body are situated in one of the lateral hemispheres of the egg, while the left hemisphere gives rise to all of the organs of the left half." 2 The hypothesis thus suggested seemed, for a time, to be placed on a secure basis of fact through a remarkable experiment subsequently performed by Roux ('88) on the frog's egg. On killing one of the blastomeres of the two-cell stage by means of a heated needle the uninjured half developed in some cases into a well-formed half-larva (Fig. 182), representing approximately the right or left half of the body, containing one medulUry fold, one auditory pit, etc. 8 Analogous, though less complete, results were obtained by operating with the four-cell stage. Roux was thus led to the declaration (made with certain subsequent reservations) that " the development of the froggastrula and of the embryo formed from it is from the second cleavage onward a mosaic-work, consisting of at least four vertical independently developing pieces." 4 This conclusion seemed to form a very strong support to His's theory of germinal localization, though, as will appear beyond, Roux transferred this theory to the nucleus, and thus developed it in a very different direction from Lankester or Van Beneden. His's theory also received very strong apparent support through investigations on cell-lineage by Whitman, Rabl, and

1 Zellsuhstanz, '82, p. 70 : the italics are in the original.

2 '^ P- 571.

8 The accuracy of this result was disputed by Oscar Hertwig ('93, 1), who found that the

uninjured blastomere gave rise to a defective larva, in which certain parts were missing, but not to a true half-body. I^ater observers, especially Schultze, Kndrcs, and Morgan, have, however, shown that both Hertwig and Roux were right, proving that the uninjured blastomere may give rise to a true half-larva, to a larva with irregular defects, or to a whole larva of half-size, according to circumstances (p. 422). 4 /.r., p. 30.


1


400


INHERITANCE AND DEVELOPMENT


many later observers, which have shown that in the cleavage of annelids, mollusks, platodes, tunicates, and many other animals, every cell has a definite origin and fate, and plays a definite part in the building of the body. 1



Pig. 1B1. — Half-embryos of the frog (in transverse section) arising from a blastomere o two-cell stage after killing the other blastomere. [Roux.]

A. Hall-blastula (dead blastomere on the left). B. Later siage. C. Half-tadpole with one medullary fold and one mesoblast plate ; regeneration of the missing (right) half in process.

or. archentcric cavity; ex. cleavage-cavity; ch. notochord; m.f. medullary fold; nij. mesoblast-plate.


In an able series of later works Whitman has followed out the suggestion made in his paper of 1878, cited above, pointing out how essential a part is played in development by the cytoplasm and insisting that cytoplasmic preorganization must be regarded as a leading factor in the ontogeny. Whitman's interesting and suggestive views are expressed with great caution and with a full recognition of the " Cf. p. 378


THE IDIOPLASM THEORY 401

difficulty and complexity of the problem. From his latest essay, indeed ('94), it is not easy to gather his precise position regarding the theory of cytoplasmic localization. Through all his writings, nevertheless, runs the leading idea that the germ is definitely organized before development begins, and that cleavage only reveals an organization that exists from the beginning. " That organization precedes cell-formation and regulates it, rather than the reverse, is a conclusion that forces itself upon us from many sides." l " The organism exists before cleavage sets in, and persists throughout every stage of cell-multiplication." 2

All of these views, excepting those of Roux, lean more or less distinctly toward the conclusion that the cytoplasm of the egg-cell is from the first mapped out, as it were, into regions which correspond with the parts of the future embryonic body. The cleavage of the ovum does not create these regions, but only reveals them to view by marking off their boundaries. Their topographical arrangement in the egg does not necessarily coincide with that of the adult parts, but only involves the latter as a necessary consequence — somewhat as a picture in the kaleidoscope gives rise to a succeeding picture composed of the same parts in a different arrangement. The germinal localization may, however, in a greater or less degree, foreshadow the arrangement of adult parts — for instance, in the egg of the tunicate or cephalopod, where the bilateral symmetry and anteroposterior differentiation of the adult is foreshadowed not only in the cleavage stages, but even in the unsegmented egg.

By another set of writers, such as Roux, De Vries, Hertwig, and Weismann, germinal localization is primarily sought not in the cytoplasm, but in the nucleus ; but these views can be best considered after a review of the idioplasm hypothesis, to which we now proceed.


B. The Idioplasm Theory

We owe to Nageli the first systematic attempt to discuss heredity regarded as inherent in a definite physical basis; 3 but it is hardly necessary to point out his great debt to earlier writers, foremost among them Darwin, Herbert Spencer, and Hackel. The essence of Nageli's hypothesis was the assumption that inheritance is effected by the transmission not of a cell, considered as a whole, but of a particular substance, the idioplastn, contained within a cell, and forming the physical basis of heredity. The idioplasm is to be sharply distinguished from the other constituents of the cell, which play no direct part in inheritance and form a "nutritive plasma" or tropho 1 '93, p. 115. 3 /,c, p. 112. 8 Theorie der Abstammungslehre, 1884.

2D


402 INHERITANCE AND DEVELOPMENT

plasm. Hereditary traits are the outcome of a definite molecular organization of the idioplasm. The hen's egg differs from the frog's because it contains a different idioplasm. The species is as completely contained in the one as in the other, and the hen's egg differs from a frog's egg as widely as a hen from a frog.

The idioplasm was conceived as an extremely complex substance, consisting of elementary complexes of molecules known as micella. These are variously grouped to form units of higher orders, which, as development proceeds, determine the development of the adult cells, tissues, and organs. The specific peculiarities of the idioplasm are therefore due to the arrangement of the micellae ; and this, in its turn, is owing to dynamic properties of the micellae themselves. During development the idioplasm undergoes a progressive transformation of its substance, not through any material change, but through dynamic alterations of the conditions of tension and movement of the micellae. These changes in the idioplasm cause reactions on the part of surrounding structures leading to definite chemical and plastic changes, i.e. to differentiation and development.

Nageli made no attempt to locate the idioplasm precisely or to identify it with any of the known morphological constituents of the cell. It was somewhat vaguely conceived as a network extending through both nucleus and cytoplasm, and from cell to cell throughout the entire organism. Almost immediately after the publication of his theory, however, several of the foremost leaders of biological investigation were led to locate the idioplasm in the nucleus, and concluded that it is to be identified with chromatin. The grounds for this conclusion, which have already been stated in Chapter VII., may be here again briefly reviewed. The beautiful experiments of Nussbaum, Gruber, and Verworn proved that the regeneration of differentiated cytoplasmic structures in the Protozoa can only take place when nuclear matter is present (cf. p. 342). The study of fertilization by Hertwig, Strasburger, and Van Beneden proved that in the sexual reproduction of both plants and animals the nucleus of the germ is equally derived from both sexes, while the cytoplasm is derived almost entirely from the female. The two germ-nuclei, which by their union give rise to that of the germ, were shown by Van Beneden to be of exactly the same morphological nature, since each gives rise to chromosomes of the same number, form, and size. Van Beneden and Boveri proved (p. 182) that the paternal and maternal nuclear substances are equally distributed to each of the first two cells, and the more recent work of Hacker, Riickert, Herla, and Zoja establishes a strong probability that this equal distribution continues in the later divisions. Roux pointed out the telling fact that the entire complicated mechanism of mitosis seems designed to affect


UNION OF THE TWO THEORIES 403

the most accurate division of the entire nuclear substance in all of its parts, while fission of the cytoplasmic cell-body is in the main a mass-division, and not a meristic division of the individual parts. Again, the complicated processes of maturation show the significant fact that while the greatest pains is taken to prepare the germ-nuclei for their coming union, by rendering them exactly equivalent, the cytoplasm becomes widely different in the two germ-cells and is devoted to entirely different functions.

It was in the main these considerations that led Hertwig, Strasburger, Kolliker, and Weismann independently and almost simultaneously to the conclusion that the mule us contains the physical basis of inhetitance, and that chromatin^ its essential constituent, is the idioplasm postulated in NagclVs theory. This conclusion is now widely accepted and rests upon a basis so firm that it must be regarded as a working hypothesis of high value. To accept it is, however, to reject the theory of germinal localization in so far as it assumes a prelocalization of the egg-cytoplasm as a fundamental character of the egg. For if the specific character of the organism be determined by an idioplasm contained in the chromatin, then every characteristic of the cytoplasm must in the long run be determined from the same source. A striking illustration of this point is given by the phenomena of colour-inheritance in plant-hybrids, as De Vries has pointed out. Pigment is developed in the embryonic cytoplasm, which is derived from the mother-cell ; yet in hybrids it may be inherited from the male through the nucleus of the germ-cell. The specific form of cytoplasmic metabolism by which the pigment is formed must therefore be determined by the paternal chromatin in the germ-nucleus, and not by a predetermination of the egg-cytoplasm.

C. Union of the Two Theories

We have now to consider the attempts that have been made to transfer the localization-theory from the cytoplasm to the nucleus, and thus to bring it into harmony with the theory of nuclear idioplasm. These attempts are especially associated with the names of Roux, De Vries, Weismann, and Hertwig ; but all of them may be traced back to Darwin's celebrated hypothesis of pangenesis as a prototype. This hypothesis is so well known as to require but a brief review. Its fundamental postulate assumes that the germ-cells contain innumerable ultra-microscopic organized bodies or gennnules y each of which is the germ of a cell and determines the development of a similar cell during the ontogeny. The germ-cell is, therefore, in Darwin's words, a microcosm formed of a host of inconceivably minute self-propagating organisms, every one of which predetermines


404 INHERITANCE AND DEVELOPMENT

the formation of one of the adult cells. De Vries ('89) brought this conception into relation with the theory of nuclear idioplasm by assuming that the gemmules of Darwin, which he called pangens, are contained in the nucleus, migrating thence into the cytoplasm step by step during ontogeny, and thus determining the successive stages of development. The hypothesis is further modified by the assumption that the pangefis are not cell-germs, as Darwin assumed, but ultimate protoplasmic units of which cells are built, and which are the bearers of particular hereditary qualities. The same view was afterward accepted by Hertwig and Weismann. 2

The theory of germinal localization is thus transferred from the cytoplasm to the nucleus. It is not denied that the egg-cytoplasm may be more or less distinctly differentiated into regions that have a constant relation to the parts of the embryo. This differentiation is, however, conceived, not as a primordial characteristic of the egg, but as one secondarily determined through the influence of the nucleus. Both De Vries and Weismann assume, in fact, that the entire cytoplasm is a product of the nucleus, being composed of pangens that migrate out from the latter, and by their active growth and multiplication build up the cytoplasmic substance. 8


D. The Roux-Weismann Theory of Development

We now proceed to an examination of two sharply opposing hypotheses of development based on the theory of nuclear idioplasm. One of these originated with Roux ('83) and has been elaborated especially by Weismann. The other was clearly outlined by De Vries ('89), and has been developed in various directions by Oscar Hertwig, Driesch, and other writers. In discussing them, it should be borne in mind that, although both have been especially developed by the advocates of the pangen-hypothesis, neither necessarily involves that hypothesis in its strict form, i.e. the postulate of discrete self-propagating units in the idioplasm. This hypothesis may therefore be laid

1 Cf. p. 290.

2 The neo-pangenesis of De Vries differs from Darwin's hypothesis in one very important respect. Darwin assumed that the gemmules arose in the body, being thrown off as germs by the individual tissue-cells, transported to the germ-cells, and there accumulated as in a reservoir; and he thus endeavoured to explain the transmission of acquired characters. De Vries, on the other hand, denies such a transportal from cell to cell, maintaining that the pangens arise or preexist in the germ-cell, and those of the tissue-cells are derived from this source by cell-division.

8 This conception obviously harmonizes with the role of the nucleus in the synthetic process. In accepting the view that the nuclear control of the cell is effected by an emanation of specific substances from the nucleus, \vc need not, however, necessarily adopt the pangen-hypothesis.


THE ROUX-WEISMANN THEORY OF DEVELOPMENT 405

aside as an open question, 1 and will be considered only in so far as it is necessary to a presentation of the views of individual writers.

The Roux-Weismann hypothesis has already been touched on at page 245. Roux conceived the idioplasm {i.e. the chromatin) not as a single chemical compound or a homogeneous mass of molecules, but as a highly complex mixture of different substances, representing different qualities, and having their seat in the individual chromatingranules. In mitosis these become arranged in a linear series to form the spireme-thread, and hence may be precisely divided by the splitting of the thread. Roux assumes, as a fundamental postulate, that division of the granules may be either quantitative or qualitative. In the first mode the group of qualities represented in the mothergranule is first doubled and then split into equivalent daughter-groups, the daughter-cells therefore receiving the same qualities and remaining of the same nature. In "qualitative division," on the other hand, the mother-group of qualities is split into dissimilar groups, which, passing into the respective daughter-nuclei, lead to a corresponding differentiation in the daughter-cells. By qualitative divisions, occurring in a fixed and predetermined order, the idioplasm is thus split up during ontogeny into its constituent qualities, which are, as it were, sifted apart and distributed to the various nuclei of the embryo. Every cell-nucleus, therefore, receives a specific form of chromatin which determines the nature of the cell at a given period and its later history. Every cell is thus endowed with a power of selfdcter?nination, which lies in the specific structure of its nucleus, and its course of development is only in a minor degree capable of modification through the relation of the cell to its fellows (" correlative differentiation ").

Roux's hypothesis, be it observed, does not commit him to the theory of pangenesis. It was reserved for Weismann to develop the hypothesis of qualitative division in terms of the pangen-hypothesis, and to elaborate it as a complete theory of development. In his first essay ('85), published before De Vries's paper, he went no farther than Roux. " I believe that we must accept the hypothesis that in indirect nuclear division, the formation of non-equivalent halves may take place quite as readily as the formation of equivalent halves, and that the equivalence or non-equivalence of the subsequently produced daughter-cells must depend upon that of the nuclei. Thus, during ontogeny a gradual transformation of the nuclear substance takes place, necessarily imposed upon it, according to certain laws, by its own nature, and such transformation is accompanied by a gradual change in the character of the cell-bodies." 2 In later writings Weismann advanced far beyond this, building up an elaborate artificial system, which appears in its final form in the remarkable

1 Cf. Chapter VI. 2 Essay IV., p. 193, 1885.


406 INHERITANCE AND DEVELOPMENT

book on the germ-plasm ('92 ). Accepting De Vries's conception of the pangens, he assumes a definite grouping of these bodies in the germ-plasm or idioplasm (chromatin), somewhat as in Nagelfs conception. The pangens or biophores are conceived to be successively aggregated in larger and larger groups; namely, (1) determinants, which are still beyond the limits of microscopical vision; (2) ids, which are identified with the visible chromatin-granules ; and (3) idants, or chromosomes. The chromatin has, therefore, a highly complex fixed architecture, which is transmitted from generation to generation, and determines the development of the embryo in a definite and specific manner. Mitotic division is conceived as an apparatus which may distribute the elements of the chromatin to the daughter-nuclei either equally or unequally. In the former case (" homoeokinesis," integral or quantitative division), the resulting nuclei remain precisely equivalent. In the second case (" heterokinesis" qualitative ox differential division), the daughter-cells receive different groups of chromatinelements, and hence become differently modified. During ontogeny, through successive qualitative divisions, the elements of the idioplasm or germ-plasm (chromatin) are gradually sifted apart, and distributed in a definite and predetermined manner to the various parts of the body. " Ontogeny depends on a gradual process of disintegration of the id of germ-plasm, which splits into smaller and smaller groups of determinants in the development of each individual. . . . Finally, if we neglect possible complications, only one kind of determinant remains in each cell, viz. that which has to control that particular cell or group of cells. ... In this cell it breaks up into its constituent biophores, and gives the cell its inherited specific character." 1 Development is, therefore, essentially evolutionary and not epigenetic ; 2 its point of departure is a substance in which all of the adult characters are represented by preformed, prearranged germs ; its course is the result of a predetermined harmony in the succession of the qualitative divisions by which the hereditary substance is progressively disintegrated. In order to account for heredity through successive generations, Weismann is obliged to assume that, by means of quantitative or integral division, a certain part of the original germ-plasm is carried on unchanged, and is finally delivered, with its original architecture unaltered, to the germ-nuclei. The power of regeneration is explained, in like manner, as the result of a transmission of unmodified or slightly modified germ-plasm to those parts capable of regeneration.

1 Germ-plasm y pp. 76, 77. 2 /.<-., p. 15.


CRITIQUE OF THE ROUX-WEISMANN THEORY


E. Critique of the Roux-Weismann Theory

It is impossible not to admire the thoroughness, candour, and logical skill with which Weismann has developed his theory, or to deny that, in its 6nal form, it does afford up to a certain point a formal solution of the problems with which it deals. Its fundamental weakness is its ^«<«;-metaphysical character, which, indeed, almost places it outside



C

Fig. 183. — Hair and who

A. Normal sixtecn-cell stage, showing th B. Half sixteen-ccll stage developed from 01 by shaking (Driesch). C. Half blaslula r IX Half-sized six!een-cell Mage of Toiopnti iown>. This embryo, developed from


like at




cleavage in the eggs of sea-urchins.

four micromeres above (from Driesch, after Selenka).

blaslomere of Ihe two-cell stage after killing the oilier

<ulling. [he dead blaslomere at the right (I)riesch). Its, viewed from Ihe micromere-potc (the eighl lower n isolated blaslomere of the two-cell stage, segmented


the sphere of legitimate scientific hypothesis. Save in the maturation of the germ-cells ("reducing divisions"), none of the visible phenomena of cell-division give even a remote suggestion of qualitative division. All the facts of ordinary mitosis, on the contrary, indicate that the division of the chromatin is carried out with the most exact equality,


408


INHERITANCE AND DEVELOPMENT


The hypothesis mainly rests upon a quite different order of phenomena, namely, on facts indicating that isolated blastomeres, or other cells, have a certain power of self-determination, or " self-differentiation" (Roux), peculiar to themselves, and which is assumed to be primarily due to the specific quality of the nuclei. This assumption, which may or may not be true, 1 is itself based upon the further assumption of qualitative nuclear division of which we actually know nothing whatever. The fundamental hypothesis is thus of purely a priori character; and every fact opposed to it has been met by subsidi


Fig. 184. — Normal and dwarf gastrulas of Amphioxus.

A. Normal gastrula. B. Half-sized dwarf, from an isolated blastomere of the two-cell stage. C. Quarter-sized dwarf, from an isolated blastomere of the four-cell stage.


ary hypotheses, which, like their principal, relate to matters beyond the reach of observation.

Such an hypothesis cannot be actually overturned by a direct appeal to fact. We can, however, make an indirect appeal, the results of which show that the hypothesis of qualitative division is not only so improbable as to lose all semblance of reality, but is in fact quite superfluous. It is rather remarkable that Roux himself led the way in this direction. In the course of his observations on the development of a half-embryo from one of the blastomeres of the two-cell stage of the frog's egg f he determined the significant fact that the half-embryo in the end restores more or less completely

1 Cf.p. 426.


CRITIQUE OF THE ROUX-WEISMANN THEORY


409


the missing half by a peculiar process, related to regeneration, which Roux designated as post-generation. Later studies showed that an isolated blastomere is able to give rise to a complete embryo in many other animals, sometimes developing in its earlier stages as though



Fig. 185,.- Dwarf and


ouble embryos of


A. Isolated blaslomere of [he two-cell si


ige segmenting like


B. Twin gaBtrulas from a single egg. O'Do



by shaking, of the blastomeres of the two-eel


stage. D.EJ-'. D


forms as the last



still forming part of a complete embryo ("partial development"), but in other cases developing directly into a complete dwarf embryo, as if it were an egg of diminished size. In 1891 Driesch was able to follow out the development of isolated blastomeres of sea-urchin


4IO INHERITANCE AND DEVELOPMENT

eggs separated by shaking to pieces the two-cell and four-cell stages. Blastomeres thus isolated segment as if still forming part of an entire larva, and give rise to a half- (or quarter-) blastula (Fig. 183). The opening soon closes, however, to form a small complete blastula, and the resulting gastrula and Pluteus larva is a perfectly formed dwarf of only half (or quarter) the normal size. Incompletely separated blastomeres give rise to double embryos like the Siamese twins. Shortly afterward the writer obtained similar results in the case of Amphioxus, but here the isolated bias tome re behaves from the beginning like a complete ovum of half the usual size, and gives rise to a complete blastula, gastrula, and larva. Complete embryos have also been obtained from a single blastomere in the teleost Fundulus (Morgan, '95, 2), in Triton (Herlitzka, '95), and in a number of hydromedusae (Zoja, '95, Bunting, '99); and nearly complete embryos in the tunicates Ascidiella (Chabry, '87), Phallusia (Driesch, '94), and Molgula (Crampton, '98). 1 Perhaps the most striking of these cases is that of the hydroid Clytia y in which Zoja was able to obtain perfect embryos, not only from the blastomeres of the twoceil and four-cell stages, but from eight-cell and even from sixteenceli stages, the dwarfs in the last case being but one-sixteenth the normal size.

These experiments render highly improbable the hypothesis of qualitative division in its strict form, for they demonstrate that the earlier cleavages, at least, do not in these cases sunder fundamentally different materials, either nuclear or cytoplasmic, but only split the egg up into a number of parts, each of which is capable of producing an entire body of diminished size, and hence must contain all of the material essential to complete development. Both Roux and Weismann endeavour to meet this adverse evidence with the assumption of a " reserve idioplasm," containing all of the elements of the germplasm which is in these cases distributed equally to all the cells in addition to the specific chromatin conveyed to them by qualitative division. This subsidiary hypothesis renders the principal one {i.e. that of qualitative division) superfluous, and brings us back to the same problems that arise when the assumption of qualitative division is discarded.

The theory of qualitative nuclear division has been practically disproved in another way by Driesch, through the pressure-experiments already mentioned at page 375. Following the earlier experiments of Pfliiger ('84) and Roux ('85) on the frog's cgg f Driesch subjected segmenting eggs of the sea-urchin to pressure, and thus obtained flat plates of cells in which the arrangement of the nuclei differed totally

1 The " partial " development in the earlier stages of some of these forms is considered at page 419.


CRITIQUE OF THE ROUX-WEISMANN THEORY


411


from the normal (Fig. 186); yet such eggs when released from pressure continue to segment, without rearrangement of the nuclei, and give rise to perfectly normal larvae. I have repeated these experiments not only with sea-urchin eggs, but also with those of an annelid {Nereis), which yield a very convincing result, since in this case the histological differentiation of the cells appears very early. In the normal development of this animal the archenteron arises from four large cells or macromeres (entomeres), which remain after the successive formation of three quartets of micromeres (ectomeres) and the parent-cell of the mesoblast. After the primary differentiation of the germ-layers the four entomeres do not divide again until a very late period (free-swimming trochophore), and their substance always retains a characteristic appearance, differing from that of the other



Pig. 186. — Modification of cleavage in sea-urchin eggs by pressure.

A. Normal eight-cell stage of Toxopruustes. B. Eight-cell stage of Echinus segmenting under pressure. Both forms produce normal Flutei.


blastomeres in its pale non-granular character and in the presence of large oil-drops. If unsegmented eggs be subjected to pressure, as in Driesch's echinoderm experiments, they segment in a flat plate, all of the cleavages being vertical. In this way are formed eight-celled plates in which all of the cells contain oil-drops (Fig. 187, D). If they are now released from the pressure, each of the cells divides in a plane approximately horizontal, a smaller granular micromere being formed above, leaving below a larger clear macromere in which the oil-drops remain. The sixteen-cell stage, therefore, consists of eight deutoplasm-laden macromeres and eight protoplasmic micromeres (instead of four macromeres and twelve micromeres, as in the usual development). These embryos developed into free-swimming trochophores containing eight instead of four macromeres, which have the typical clear protoplasm containing oil-drops. In this case there can


412


INHERITANCE AND DEVELOPMENT


be no doubt whatever that four of the entoblastic nuclei were normally destined for the first quartet of micromeres (Fig. 187, B), from which arise the apical ganglia and the prototroch. Under the conditions of the experiment, however, they have given rise to the nuclei of cells which differ in no wise from the other entoderm-cells. Even




Fig. 187. — Modifications of cleavage by pressure in Nereis.

A. B. Normal four- and eight-cell stages. C. Normal trochophore larva resulting, with four entoderm-cells. D. Eight-cell stage arising from an egg flattened by pressure ; such eggs give rise to trochophores with eight instead of four entoderm-cells. Numerals designate the successive cleavages.


in a highly differentiated type of cleavage, therefore, the nuclei of the segmenting egg are not specifically different, as the Roux-Weismann hypothesis demands, but contain the same materials even in the cells that undergo the most diverse subsequent fate. But there is, furthermore, very strong reason for believing that this may be true in later


NATURE AND CAUSES OF DIFFERENTIATION 413

stages as well, as Kolliker insisted in opposition to Weismann as early as 1886, and as has been urged by many subsequent writers. The strongest evidence in this direction is afforded by the facts of regeneration ; and many cases are known — for instance, among the hydroids and the plants — in which even a small fragment of the body is able to reproduce the whole. It is true that the power of regeneration is always limited to a greater or less extent according to the species. But there is no evidence whatever that such limitation arises through specification of the nuclei by qualitative division, and, as will appear beyond, its explanation is probably to be sought in a very different direction.

F. On the Nature and Causes of Differentiation

We have now cleared the ground for a restatement of the problem of development and an examination of the views opposed to the Roux-Weismann theory. After discarding the hypothesis of qualitative division the problem confronts us in the following form. If chromatin be the idioplasm in which inheres the sum total of hereditary forces, and if it be equally distributed at every cell-division, how can its mode of action so vary in different cells as to cause diversity of structure, i.e. differentiation ? It is perfectly certain that differentiation is an actual progressive transformation of the egg-substance involving both physical and chemical changes, occurring in a definite order, and showing a definite distribution in the regions of the egg. These changes are sooner or later accompanied by the cleavage of the egg into cells whose boundaries may sharply mark the areas of differentiation. What gives these cells their specific character? Why, in the four-cell stage of an annelid egg, should the four cells contribute equally to the formation of the alimentary canal and the cephalic nervous system, while only one of them (the lefthand posterior) gives rise to the nervous system of the trunk-region and to the muscles, connective tissues, and the germ-cells? (Figs. 171, 188, £.) There cannot be a fixed relation between the various regions of the egg which these blastomeres represent and the adult parts arising from them ; for in some eggs these relations may be artificially changed. A portion of the egg which under normal conditions would give rise to only a fragment of the body will, if split off from the rest, give rise to an entire body of diminished size. What then determines the history of such a portion? What influence moulds it now into an entire body, now into a part of a body ?

De Vries, in his remarkable essay on Intracellular Pangenesis ('89), endeavoured to cut this Gordian knot by assuming that the character of each cell is determined by pangens that migrate from


414


INHERITANCE AND DEVELOPMENT


the nucleus into the cytoplasm, and, there becoming active, set up specific changes and determine the character of the cell, this way or that, according to their nature. But what influence guides the migrations of the pangens, and so correlates the operations of development? Both Driesch and Oscar Hertwig have attempted to



Fig. i88. - Diagrams il


lustratingthe value of the quanclE in a polyclade {I*ptoplana\ . n [amel

[[branch (Unto), and a gs


slrronod {Crepidula). A. Lrfhflama, showing mesoblast-formatioii


in the second qu;ir!ct. //.



lom«5oh].is[ ifmm tjuiidrjiH


1 11). (.'. Uhio, eclomesoblasl formed only from j".


Inallthi' figures Ihesucc


essIyb quarleis are numbered with Arabic figures ; ecloblasl unshaded.


mesoblest dolled, enloblasr


vertically lined.


answer this question, though the first-named author does not commit himself to the pangen-hypothesis. These writers have maintained that the particular mode of development in a given region or blastomere of the egg is a result of its relation to the remainder of the mass, i.e. a product of what may be called the intra-em'bryonic environ


NATURE AND CAUSES OF DIFFERENTIATION 41 5

ment. Hertwig insisted that the organism develops as a whole as the result of a physiological interaction of equivalent blastomeres, the transformation of each being due not to an inherent specific power of self-differentiation, as Roux's mosaic-theory assumed, but to the action upon it of the whole system of which it is a part. "According to my conception," said Hertwig, "each of the first two blastomeres contains the formative and differentiating forces not simply for the production of a half-body, but for the entire organism ; the left blastomere develops into the left half of the body only because it is placed in relation to a right blastomere." l Again, in a later paper : " The egg is a specifically organized elementary organism that develops epigenetically by breaking up into cells and their subsequent differentiation. Since every elementary part (i.e. cell) arises through the division of the germ, or fertilized Qgg f it contains also the germ of the whole, but during the process of development it becomes ever more precisely differentiated and determined by the formation of cytoplasmic products according to its position with reference to the entire organism (blastula, gastrula, etc.)." 2

An essentially similar view was advocated by the writer ('93, '94) nearly at the same time, and the same general conception was expressed with great clearness and precision by Driesch shortly after Hertwig: "The fragments (i.e. cells) produced by cleavage are completely equivalent or indifferent." "The blastomeres of the seaurchin are to be regarded as forming a uniform material, and they may be thrown about, like balls in a pile, without in the least degree impairing thereby the normal power of development." 3 " The relative position of a blastomere in the whole determines in general what develops from it ; if its position be changed, it gives rise to something different ; in other words, its prospective value is a f miction of its position. " 4

In this last aphorism the whole problem of development is brought to a focus. It is clearly not a solution of the problem, but only a highly suggestive restatement of it ; for everything turns upon how the relation of the part to the whole is conceived. Very little consideration is required to show that this relation cannot be a merely geometrical or rudely mechanical one, for in the eggs of different

1 '92, 1, p. 481.

2 '93» P- 793- It should be pointed out that Roux himself in several papers expressly recognizes the fact that development cannot be regarded as a pure mosaic-work, and that besides the power of self-differentiation postulated by his hypothesis we must assume a " correlative differentiation " or differentiating interaction of parts in the embryo. Cf. Roux, '92, '93, 1.

8 Studien IV., p. 25.

  • Studien IV., p. 39. Cf. His, " Es muss die Wachsthumserregbarkeit des Eies eine

Function des Raumes sein." ('74, p. 153.)


416 INHERITANCE AND DEVELOPMENT

animals blastomeres may almost exactly correspond in origin and relative position, yet differ widely in their relation to the resulting embryo. Thus we find that the cleavage of polyclades, annelids, and gasteropods (Fig. 188) shows a really wonderful agreement in form, yet the individual cells differ markedly in prospective value. In all of these forms three quartets of micromeres are successively formed according to exactly the same remarkable law of the alternation of the spirals ; l and, in all, the posterior cell of a fourth quartet lies at the hinder end of the embryo in precisely the same geometrical relation to the remainder of the embryo ; yet in the gasteropods and annelids this cell gives rise to the mesoblast-bands and their products, in the polyclade to a part of the archenteron, while important differences also exist in the value of the other quartets. The relation of the part to the whole is therefore of a highly subtle character, the prospective value of a blastomere depending not merely upon its geometrical position, but upon its relation to the whole complex inherited organization of which it forms a part. The apparently simple conclusion stated in Driesch's clever aphorism thus leads to further problems of the highest complexity. It should be here pointed out that Driesch does not accept Hertwig's theory of the interaction of blastomeres as such, but, like Whitman, Morgan, and others, has brought forward effective arguments against that too simple and mechanical conception. That theory is, in fact, merely Schwann's cell-composite theory of the organism applied to the developing embryo, and the general arguments against that theory find some of their strongest support in the facts of growth and development. 2 This has been forcibly urged by Whitman ('93), who almost simultaneously with the statements of Driesch and Hertwig, cited above, expressed the conviction that the morphogenic process cannot be conceived as merely the sum total or resultant of the individual cell-activities, but operates as a unit without respect to cell-boundaries, precisely as De Bary concludes in the case of growing plant-tissues (p. 393), and the nature of that process is due to the organization of the egg as a whole.

While recognizing fully the great value of the results attained during the past few years in the field of experimental and speculative embryology, we are constrained to admit that as far as the essence of the problem is concerned we have not gone very far beyond the conclusions stated above ; for beyond the fact that the inherited organization is involved in that of the germ-cells we remain quite ignorant of its essential nature. This has been recognized by no one more clearly than by Driesch himself, to whose critical researches we owe so much in this field. At the climax of a recent elaborate analysis, the high interest of which is somewhat obscured by

1 Cf. p. 368. * Cf. pp. 38S-394.


NATURE AND CAUSES OF DIFFERENTIATION 417

its too abstruse form, Driesch can only reiterate his former aphorism, 1 finally taking refuge in an avowed theory of vitalism which assumes the localization of morphogenic phenomena to be determined by "a wholly unknown principle of correlation/' 2 and forms a problem sui generis? This conclusion recognizes the fact that the fundamental problem of development remains wholly unsolved, thus confirming from a new point of view a conclusion which it is only fair to point out has been reached by many others.

But while the fundamental nature of the morphogenic process thus remains unknown, we have learned some very interesting facts regarding the conditions under which it takes place, and which show that Driesch's aphorism loses its meaning unless carefully qualified. The experiments referred to at pages 353, 410, show that up to a certain stage of development the blastomeres of the early echinoderm, Amp/iioxus or medusa-embryo, are " totipotent " (Roux), or "equipotential" (Driesch), i.e. capable of producing any or all parts of the body. Even in these cases, however, we cannot accept the early conclusion of Pfliiger ('83), applied by him to the frog's egg, and afterward accepted by Hertwig, that the material of the egg f or of the blastomeres into which it splits up, is absolutely "isotropic," i.e. consists of quite uniform indifferent material, devoid of preestablished axes. Whitman and Morgan, and Driesch himself, showed that this cannot be the case in the echinoderm Qgg\ for the ovum possesses a polarity predetermined before cleavage begins, as proved by the fact that at the fourth cleavage a group of small cells or micromeres always arises at a certain point, which may be precisely located before cleavage by reference to the eccentricity of the first cleavage-nucleus, 4 and which, as Morgan showed, 6 is indicated before the third, and sometimes before the second cleavage, by a migration of pigment away from the micromere-pole. These observers are thus led to the assumption of a primary polarity of the egg-protoplasm, to which Driesch, in the course of further analysis of the phenomena, is compelled to add the assumption of a secondary polarity at right angles to the first. 6 These polarities, inherent not only in the entire egg, but also in each of the blastomeres into which it divides, form the primary conditions under which the bilaterally symmetrical organism develops by epigenesis. To this extent, therefore, the material of the blastomeres, though " totipotent,' ' shows a certain predetermination with respect to the adult body.

1 '99, pp. 86-87.

2 This phrase is cited by Driesch from an earlier work ('92, p. 596) as giving a correct though " unanalytical " statement of his view. It may be questioned whether many readers will regard as an improvement the " analytical " form it assumes in his last work.

  • I.e., p. 90. 4 Cf. Fig. 103. 6 '94, p. 142.

6 See Driesch, '93, pp. 229, 241 ; '96, and '99, p. 44. 2 £


4l8 INHERITANCE AND DEVELOPMENT

We now proceed to the consideration of experiments which show that in some animal eggs such predetermination may go much farther, so that the development does, in fact, show many of the features of a mosaic-work, as maintained by Roux. The best-determined of these cases is that of the ctenophore-egg, as shown by the work of Chun,



ihore Brroi. [DmEscH and MORGAN.] n isolated blaslomere. B. Resuming larva, with (our i

ows of pl.iles and two gastric pouches. E. '. trie pouches, from a nucleated fragment of a


Driesch, and Morgan ('95), and Fischel ('98). These observers have demonstrated that isolated blastomeres of the two-, four-, or eight-cell stage undergo a cleavage which, through the earliest stages, is exactly like that which it would have undergone if forming part of a com


NATURE AND CAUSES OF DIFFERENTIATION


419


plete embryo, and gives rise to a defective larva, having only four, two, or one row of swimming-plates (Fig. 189); and Fischer s observations give strong reason to believe that each of the eight micromeres of the sixteen-cell stage is definitely specified for the formation of one of the rows of plates. In like manner Crampton ('96) found that in case of the marine gasteropod Ilyanassa isolated blastomeres of twocell or four-cell stages segmented exactly as if forming part of an entire embryo, and gave rise to fragments of a larva, not to complete dwarfs, as in the echinoderm (Fig. 190). Further, in embryos from which the " yolk-lobe w (a region of that macromere from which the primary mesoblast normally arises) had been removed, no mesoblastbands were formed. Most interesting of all, Driesch and Morgan discovered that if a part of the cytoplasm of an unscgmented ctenophore-egg were removed, the remainder gave rise to an incomplete larva, showing definite defects (Fig. 189, E, F).

In none of these cases is the embryo able to complete itself, though it should be remarked that neither in the ctenophore nor in the snail is the partial embryo identical with a fragment of a whole embryo, since the micromeres finally enclose the macromeres, leaving no surface of fracture. This extreme is, however, connected by a series of forms with such cases as those of Amphioxus or the medusa, where the fragment develops nearly or quite as if it were a whole. In the tunicates the researches of Chabry ('87), Driesch ('94), and Crampton C97) show that an isolated blastomere of the two-cell stage undergoes a typical half-cleavage (Crampton), but finally gives rise to a nearly perfect tadpole larva lacking only one of the asymmetrically placed sense-organs (Driesch). Next in the series may be placed the frog, where, as Roux, Endres, and Walter have shown, a blastomere of the two-cell stage may give rise to a typical half-morula, half-gastrula, and half-embryo 1 (Fig. 182), yet finally produces a perfect larva. A further stage is given by the echinoderm-egg, which, as Driesch showed, undergoes a half-cleavage and produces a haif-blastula, which, however, closes to form a whole before the gastrula-stage (Fig. 183). Perfectly formed though dwarf larvae result. Finally, we reach Amphioxus and the hydromasae in which a perfect " whole development " usually takes place from the beginning, though it is a very interesting fact that the isolated blastomeres of Amphioxus sometimes show, in the early stages of cleavage, peculiarities of development that recall their behaviour when forming part of an entire embryo. 2

We see throughout this series an effort, as it were, on the part of the isolated blastomere to assume the mode of development characteristic of a complete egg, but one that is striving against conditions that

1 This is not invariably the case, as described beyond.

2 Cf. Wilson, '93, pp. 590, 608.


420


INHERITANCE AND DEVELOPMENT


tend to confine its operations to the rdle it would have played if still forming part of an entire developing egg. In Amphioxus or Ctytia this tendency is successful almost from the beginning. In other forms the limiting conditions are only overcome at a later period, while in the ctenophore or snail they seem to afford an insurmount


A. Normal eight-cell stage. B. Normal sixteen sola ted hi asto mere of Ihe mo-cell stage. D. Halfr n the cleavage of an isolated hlastomere of the four-cell stage jelow a one-fourth sixteen-cell stage.


c. C. Half eight-cell stage, from stace succeeding. E. Two stains ahove a one-fourth eight-cell stage.


able barrier to complete development. What determines the limitations of development in these various cases ? They cannot be due to nuclear specification ; for in the ctenophore the fragment of an Hi/segmented egg, containing the normal egg-nucleus, gives rise to a defective larva; and my experiments on Nereis show that even in a highly


NATURE AND CAUSES OF DIFFERENTIATION


421


determinate cleavage, essentially like that of the snail, the nuclei may be shifted about by pressure without altering the end-result. Neither can they lie in the form of the dividing mass as some authors have assumed ; for in Crampton's experiments the half or quarter blastomere does not retain the form of a half or quarter sphere, but rounds



Pig. 191. — Double embryos of frog developed from eggs in' JO. Schui.tzeJ

A. Twins with heads turned in opposite directions. B. Twi united by their ventral sides. D. Double-headed tadpole.


rted when in the Iwo-cell stage, < united back to back. C. Twin*


off to a spheroid like the egg. But if the limiting conditions lie neither in the nucleus nor in the form of the mass, we must seek them in the cytoplasm ; and if we find here factors by which the tendency of the part to develop into a whole may be, as it were, hemmed in, we shall reach a proximate explanation of the mosaic-like character of cleavage shown in the forms under consideration, and the mosaic


422 INHERITANCE AND DEVELOPMENT

theory of cytoplasmic localization will find a substantial if somewhat restricted basis.

That we are here approaching the true explanation is indicated by certain very remarkable and interesting experiments on the frog's egg t which prove that each of the first two blastomeres may give rise either to a half-embryo or to a whole embryo of half size, according to circumstances, and which indicate, furthermore, that these circumstances lie in a measure in the arrangement of the cytoplasmic materials. This most important result, which we owe especially to Morgan, 1 was reached in the following manner. Born had shown, in 1885, that if frogs' eggs be fastened in an abnormal position, — e.g. upside down, or on the side, — a rearrangement of the egg-material takes place, the heavier deutoplasm sinking toward the lower side, while the nucleus and protoplasm rise. A new axis is this established in the egg-, which has the same relation to the body-axes as in the ordinary development (though the pigment retains its original arrangement). This proves that in eggs of this character (telolecithal) the distribution of deutoplasm, or conversely of protoplasm, is one of the primary formative conditions of the cytoplasm ; and the significant fact is that by artificially changing this distribution the axis of the embryo is shifted. Oscar Schultze ('94) discovered that if the egg be turned upside down when in the two-cell stage, a whole embryo (or half of a double embryo) may arise from each blastomere instead of a half-embryo as in the normal development, and that the axes of these embryos show no constant relation to one another (Fig. 191). Morgan ('95, 3) added the important discovery that either a half-embryo or a whole half-sized dwarf might be formed, according to the position of the blastomere. If, after destruction of one blastomere, the other be allowed to remain in its normal position, a half -embryo always results, 2 precisely as described by Roux. If, on the other hand, the blastomere be inverted, it may give rise either to a half-embryo 3 or to a whole dwarf. 4 Morgan therefore concluded that the production of whole embryos by the inverted blastomeres was, in part at least, due to a rearrangement or rotation of the egg-materials under the influence of gravity, the blastomere thus returning, as it were, to a state of equilibrium like that of an entire ovum.

This beautiful experiment gives most conclusive evidence that each of the two blastomeres contains all the materials, nuclear and cytoplasmic, necessary for the formation of a whole body ; and that these materials may be used to build a whole body or half-body, according to the grouping that they assume. After the first cleavage takes

1 Anat. Anz., X. 19, 1895. 8 Three cases.

a Eleven cases observed. * Nine cases observed.


NATURE AND CAUSES OF DIFFERENTIATION 423

place, each blastomere is set, as it were, for a half-development, but not so firmly that a rearrangement is excluded.

I have reached a nearly related result in the case of both Amp hioxus and the echinoderms. In Amphioxus the isolated blastomere usually segments like an entire ovum of diminished size. This is, however, not invariable, for a certain number of such blastomeres show a more or less marked tendency to divide as if still forming part of an entire embryo. The sea-urchin Toxopnetistes reverses this rule, for the isolated blastomere of the two-cell stage usually shows a perfectly typical half-cleavage, as described by Driesch, but in rare cases it may segment like an entire ovum of half-size(Fig. 183, Z?)and give rise to an entire blastula. We may interpret this to mean that in Amphioxus the differentiation of the cytoplasmic substance is at first very slight, or readily alterable, so that the isolated blastomere, as a rule, reverts at once to the condition of the entire ovum. In the seaurchin, the initial differentiations are more extensive or more firmly established, so that only exceptionally can they be altered. In the snail and ctenophore we have the opposite extreme to Amphioxus, the cytoplasmic conditions having been so firmly established that they cannot be readjusted, and the development must, from the outset, proceed within the limits thus set up.

Through this conclusion we reconcile, as I believe, the theories of cytoplasmic localization and mosaic development with the hypothesis of cytoplasmic totipotence. Primarily the egg-cytoplasm is totipotent in the sense that its various regions stand in no fixed relation with the parts to which they respectively give rise, and the substance of each of the blastomeres into which it splits up contains all of the materials necessary to the formation of a complete body. Secondarily, however, development may assume more or less of a mosaic-like character through differentiations of the cytoplasmic substance involving local chemical and physical changes, deposits of metaplasmic material, and doubtless many other unknown subtler processes. Both the extent and the rate of such differentiations seem to vary in different cases; and here probably lies the explanation of the fact that the isolated blastomeres of different eggs vary so widely in their mode of development. When the initial differentiation is of small extent or is of such a kind as to be readily modified, cleavage is indeterminate in character and may easily be remodelled (as in Amphioxus). When they are more extensive or more rigid, cleavage assumes a mosaic-like or determinate character, 1 and qualitative division, in a certain sense, becomes a fact. Conklin's ('99) interesting observations on the highly determinate cleavage of gasteropods {Crepiduld)

1 The convenient terms indeterminate and determinate cleavage were suggested by Conklin ('98).


424


INHERITANCE AND DEVELOPMENT


show that here the substance of the attraction-spheres is unequally distributed, in a quite definite way, among the cleavage-ceils, each sphere of a daughter-cell being carried over bodily into one of the granddaughter-cells (Fig. 192). We have here a substantial basis for the conclusion that in cleavage of this type qualitative division of the cytoplasm may occur.

It is important not to lose sight of the fact that development and differentiation do not in any proper sense first begin with the cleavage of the ovum, but long before this, during its ovarian history. 1 The primary differentiations thus established in the cytoplasm form the immediate conditions to which the later development must conform; and the difference between Amphioxus on the one hand, and the



A B

Fig. 191. — Two successive stages in the third cleavage of the egg of CrtpiduL upper pole. [CoNKLIN.]

In both figures the old spheres (dotted) lie at the upper pole of the embryo, i cleavage they pass into the four respective cells of the first quartet of micromeri somes are seen in the neu spheres.


ind at the third


snail or ctenophore on the other, simply means, I think, that the initial differentiation is less extensive or less firmly established in the one than in the other.

The origin of the cytoplasmic differentiations existing at the beginning of cleavage has already been considered (p. 386). If the conclusions there reached be placed beside the above, we reach the following conception. The primary determining cause of development lies in the nucleus, which operates by setting up a continuous series of specific metabolic changes in the cytoplasm. This process begins during ovarian growth, establishing the external form of the egg, its primary polarity, and the distribution of substances within it. The cytoplasmic differentiations thus set up form as it were a frame1 See Wilson ('96), Driesch ('98, 1).


THE NUCLEUS IN LATER DEVELOPMENT 425

work within which the subsequent operations take place in a course which is more or less firmly fixed in different cases. If the cytoplasmic conditions be artificially altered by isolation or other disturbance of the blastomeres, a readjustment may take place and development may be correspondingly altered. Whether such a readjustment is possible depends on secondary factors — the extent of the primary differentiations, the physical consistency of the eggsubstance, the susceptibility of the protoplasm to injury, and doubtless a multitude of others. The same doubtless applies to the later stages of development ; and we must here seek for some of the factors by which the power of regeneration in the adult is determined and limited. It is, however, not improbable, as pointed out below, that in the later stages differentiation may occur in the nuclear as well as in the cytoplasmic substance.

G. The Nucleus in Later Development

The foregoing conception, as far as it goes, gives at least an intelligible view of the more general features of early development and in a measure harmonizes the apparently conflicting results of experiment on various forms. But there are a very large number of facts relating especially to the later stages of differentiation, which it seems to leave unexplained* and which indicate that the nucleus as well as the cytoplasm may undergo progressive changes of its substance. It has been assumed by most critics of the Roux- Weismann theory that all of the nuclei of the body contain the same idioplasm, and that each therefore, in Hertwig's words, contains the germ of the whole. It is, however, doubtful whether this assumption is well founded. The power of a single cell to produce the entire body is in general limited to the earliest stages of cleavage, rapidly diminishes, and as a rule soon disappears entirely. When once the germ-layers have been definitely separated, they lose entirely the power to regenerate one another save in a few exceptional cases. In asexual reproduction, in the regeneration of lost parts, in the formation of morbid growths, each tissue is in general able to reproduce only a tissue of its own or a nearly related kind. Transplanted or transposed groups of cells (grafts and the like) retain more or less. completely their autonomy and vary only within certain well-defined limits, despite their change of environment. All of these statements are, it is true, subject to exception ; yet the facts afford an overwhelming demonstration that differentiated cells possess a specific character, that their power of development and adaptability to changed conditions becomes in a greater or less degree limited with the progress of development. As indicated above, this progressive specification of the tissue-cells


426 INHERITANCE AND DEVELOPMENT

is no doubt due in part to differentiation of the cytoplasm. There is, however, reason to suspect that, beyond this, differentiation may sooner or later involve a specification of the nuclear substance. When we reflect on the general role of the nucleus in metabolism and its significance as the especial seat of the formative power, we may well hesitate to deny that this part of Roux's conception may be better founded than his critics have admitted. Nageli insisted that the idioplasm must undergo a progressive transformation during development, and many subsequent writers, including such acute thinkers as Boveri and Nussbaum, and many pathologists, have recognized the necessity for such an assumption. Boveri's remarkable observations on the nuclei of the primordial germ-cells in Ascaris demonstrate the truth of this view in a particular case ; for here all oft/ie somatic nuclei lose a portion of their chromatin, and only the progenitors of the germ-neclei retain the entire ancestral heritage. Boveri himself has in a measure pointed out the significance of his discovery, insisting that the specific development of the tissue-cells is conditioned by specific changes in the chromatin that they receive, 1 though he is careful not to commit himself to any definite theory. It hardly seems possible to doubt that in Ascaris the limitation of the somatic cells in respect to the power of development arises through a loss of particular portions of the chromatin. One cannot avoid the thought that further and more specific limitations in the various forms of somatic cells may arise through an analogous process, and that we have here a key to the origin of nuclear specification without recourse to the theory of qualitative division. We do not need to assume that the unused chromatin is cast out bodily ; for it may degenerate and dissolve, or may be transformed into linin-substance or into nucleoli.

This suggestion is made only as a tentative hypothesis, but the phenomena of mitosis seem well worthy of consideration from this point of view. Its application to the facts of development becomes clearer when we consider the nature of the nuclear "control" of the cell, i.e. the action of the nucleus upon the cytoplasm. Strasburger, following in a measure the lines laid down by Nageli, regards the action as essentially dynamic, i.e. as a propagation of molecular movements from nucleus to cytoplasm in a manner which might be compared to the transmission of a nervous impulse. When, however, we consider the role of the nucleus in synthetic metabolism, and the relation between this process and that of morphological synthesis, we must regard the question in another light ; and opinion has of late strongly tended to the conclusion that nuclear "control" can only be explained as the result of active exchanges of material between nucleus and cytoplasm. De Vries, followed by Hertwig,

1 '9>» p. 433


THE NUCLEUS IN LATER DEVELOPMENT 427

assumes a migration of pangens from nucleus to cytoplasm, the character of the cell being determined by the nature of the migrating pangens, and these being, as it were, selected by circumstances (position of the cell, etc.). But, as already pointed out, the pangenhypothesis should be held quite distinct from the purely physiological aspect of the question, and may be temporarily set aside; for specific nuclear substances may pass from the nucleus into the cytoplasm in an unorganized form. Sachs, followed by Loeb, has advanced the hypothesis that the development of particular organs is determined by specific " formative substances " which incite corresponding forms of metabolic activity, growth, and differentiation. It is but a step from this to the very interesting suggestion of Driesch that the nucleus is a storehouse of ferments which pass out into the cytoplasm and there set up specific activities. Under the influence of these ferments the cytoplasmic organization is determined at every step of the development, and new conditions are established for the ensuing change. This view is put forward only tentatively as a " fiction " or working hypothesis ; but it is certainly full of suggestion. Could we establish the fact that the number of ferments or formative substances in the nucleus diminishes with the progress of differentiation, we should have a comparatively simple and intelligible explanation of the specification of nuclei and the limitation of development. The power of regeneration might then be conceived, somewhat as in the Roux-Weismann theory, as due to a retention of idioplasm or germ-plasm — i.e. chromatin — in a less highly modified condition, and the differences between the various tissues in this regard, or between related organisms, would find a natural explanation.

Development may thus be conceived as a progressive transformation of the egg-substance primarily incited by the nucleus, first manifesting itself by specific changes in the cytoplasm, but sooner or later involving in some measure the nuclear substance itself. This process, which one is tempted to compare to a complicated and progressive form of crystallization, begins with the youngest ovarian egg and proceeds continuously until the cycle of individual life has run its course. Cell-division is an accompaniment but not a direct cause of differentiation. The cell is no more than a particular area of the germinal substance comprising a certain quantity of cytoplasm and a mass of idioplasm in its nucleus. Its character is primarily a manifestation of the general formative energy acting at a particular point under given conditions. When once such a circumscribed area has been established, it may, however, emancipate itself in a greater or less degree from the remainder of the mass, and acquire a specific character so fixed as to be incapable of further change save within the limits imposed by its acquired character.


INHERITANCE AND DEVELOPMENT


H. The External Conditions of Development

We have thus far considered only the internal conditions of development which are progressively created by the germ-cell itself. We must now briefly glance at the external conditions afforded by the environment of the embryo. That development is conditioned by the external environment is obvious. But we have only recently

to realize how intimate the relation is; and it has been especially the

service of Loeb, Herbst, and Driesch to show how essential a part is played by the environment in the development of specific organic forms. The limits of this work will not admit of any adequate review of the vast array of known facts in this field, for which the reader is referred to the works especially of Herbst. I shall only consider one or two cases which may serve to bring out the general principle that they involve. Every living organism at every stage of its existence reacts to its environment by physiological and morphological changes. The developing embryo, like the adult, is a moving equilibrium — a product of the response of the inherited organization to the external stimuli working upon it. If these stimuli be altered, development is altered. This is beautifully shown by the experiments of Herbst and others on the development of sea-urchins. Pouchet and Chabry showed that if the embryos of these animals be made to develop in sea-water containing no lime-salts, the larva fails to develop not only its calcareous skeleton, but also its ciliated arms, and a larva thus results that resembles in some particulars an entirely different specific form ; namely, the Tornaria larva of Balanoglossus. This result is not due simply to the lack of necessary material : for Herbst showed that the same result is attained if a slight excess of potassium chloride be added to sea-water containing the normal amount of lime (Fig. 193). In the latter case the specific metabolism of the protoplasm is altered by a particular chemical stimulus, and a new form results.



Fig. 193-



[HERHST.J


  • slight ;


THE EXTERNAL CONDITIONS OF DEVELOPMENT 429

The changes thus caused by slight chemical alterations in the water may be still more profound. Herbst ('92) observed, for example, that when the water contains a very small percentage of lithium chloride, the blastula of sea-urchins fails to invaginate to form a typical gastrula, but evaginates to form an hour-glass-shaped



A. Polyp (CtriaMfiHi), prodi B. Hydroid ( TWn/ar/j), general water. C. D. Similar generalion of heai

larva, one half of which represents the archenteron, the other half the ectoblast. Moreover, a much larger number of the blastula-cells undergo the differentiation into entoblast than in the normal development, the ectoblast sometimes becoming greatly reduced and occasionally disappearing altogether, so that the entire blastula is


430 INHERITANCE AND DEVELOPMENT

differentiated into cells having the histological character of the normal entoblast ! One of the most fundamental of embryonic differentiations is thus shown to be intimately conditioned by the chemical environment.

The observations of botanists on the production of roots and other structures as the result of local stimuli are familiar to all. Loeb's interesting experiments on hydroids give a similar result ('91). It has long been known that Tubularia, like many other hydroids, has the power to regenerate its " head " — i.e. hypostome, mouth, and tentacles — after decapitation. Loeb proved that in this case the power to form a new head is conditioned by the environment. For if a Tnbnlaria stem be cut off at both ends and inserted in the sand upside down, i.e. with the oral end buried, a new head is regenerated at the free (formerly aboral) end. Moreover, if such a piece be suspended in the water by its middle point, a new head is produced at each end (Fig. 194); while if both ends be buried in the sand, neither end regenerates. This proves in the clearest manner that in this case the power to form a definite complicated structure is called forth by the stimulus of the external environment.

These cases must suffice for our purpose. They prove incontestably that normal development is in a greater or less degree the response of the developing organism to normal conditions ; and they show that we cannot hope to solve the problems of development without reckoning with these conditions. But neither can we regard specific forms of development as directly caused by the external conditions ; for the egg of a fish and that of a polyp develop, side by side, in the same drop of water, under identical conditions, each into its predestined form. Every step of development is a physiological reaction, involving a long and complex chain of cause and effect between the stimulus and the response. The character of the response is determined, not by the stimulus, but by the inherited organization. While, therefore, the study of the external conditions is essential to the analysis of embryological phenomena, it serves only to reveal the mode of action of the germ and gives but a dim insight into its ultimate nature.

I. Development, Inheritance, and Metabolism

In bringing the foregoing discussion into more direct relation with the general theory of cell-action, we may recall that the cell-nucleus appears to us in two apparently different roles. On the one hand, it is a primary factor in morphological synthesis and hence in inheritance, on the other hand an organ of metabolism especially concerned with the constructive process. These two functions we may with


DEVELOPMENT, INHERITANCE, AND METABOLISM 431

Claude Bernard regard as but different phases of one process. The building of a definite cell-product, such as a muscle-fibre, a nerveprocess, a cilium, a pigment-granule, a zymogen-granule, is in the last analysis the result of a specific form of metabolic activity, as we may conclude from the fact that such products have not only a definite physical and morphological character, but also a definite chemical character. In its physiological aspect, therefore, inheritance is the recurrence, in successive generations, of like forms of metabolism ; and this is effected through the transmission from generation to generation of a specific substance or idioplasm which we have seen reason to identify with chromatin. The validity of this conception is not affected by the form in which we conceive the morphological nature of the idioplasm — whether as simply a mixture of chemical substances, as a microcosm of invisible germs or pangens, as assumed by De Vries, Weismann, and Hertwig, as a storehouse of specific ferments as Driesch suggests, or as a complex molecular substance grouped in micellae as in Nageli's hypothesis. It is true, as Verworn insists, that the cytoplasm is essential to inheritance ; for without a specifically organized cytoplasm the nucleus is unable to set up specific forms of synthesis. This objection, which has already been considered from different points of view, by both De Vries and Driesch, disappears as soon as we regard the egg-cytoplasm as itself a product of the nuclear activity ; and it is just here that the general rdle of the nucleus in metabolism is of such vital importance to the theory of inheritance. If the nucleus be the formative centre of the cell, if nutritive substances be elaborated by or under the influence of the nucleus while they are built into the living fabric, then the specific character of the cytoplasm is determined by that of the nucleus, and the contradiction vanishes. In accepting this view we admit that the cytoplasm of the egg is, in a measure, the substratum of inheritance, but it is so only by virtue of its relation to the nucleus, which is, so to speak, the ultimate court of appeal. The nucleus cannot operate without a cytoplasmic field in which its peculiar powers may come into play ; but this field is created and moulded by itself.

J. Preformation and Epigenesis. The Unknown Factor in

Development

We have now arrived at the farthest outposts of cell-research, and here we find ourselves confronted with the same unsolved problems before which the investigators of evolution have made a halt. For we must now inquire what is the guiding principle of embryological development that correlates its complex phenomena and directs them


432 INHERITANCE AND DEVELOPMENT

to a definite end. However we conceive the special mechanism of development, we cannot escape the conclusion that the power behind it is involved in the structure of the germ-plasm inherited from foregoing generations. What is the nature of this structure and how has it been acquired ? To the first of these questions we have as yet no certain answer. The second question is merely the general problem of evolution stated from the standpoint of the cell-theory. The first question raises once more the old puzzle of preformation or epigenesis. The pangen-hypothesis of De Vries and Weismann recognizes the fact that development is epigenetic in its external features ; but like Darwin's hypothesis of pangenesis, it is at bottom a theory of preformation, and Weismann expresses the conviction that an epigenetic development is an impossibility. 1 He thus explicitly adopts the view, long since suggested by Huxley, that "the process which in its superficial aspect is epigenesis appears in essence to be evolution in the modified sense adopted in Bonnet's later writings ; and development is merely the expansion of a potential organism or 'original preformation ' according to fixed laws." 2 Hertwig ('92, 2), while accepting the pangen-hypothesis, endeavours to take a middle ground between preformation and epigenesis, by assuming that the pangens (idioblasts) represent only cell-characters, the traits of the multicellular body arising epigenetically by permutations and combinations of these characters. This conception certainly tends to simplify our ideas of development in its outward features, but it does not explain why cells of different characters should be combined in a definite manner, and hence does not reach the ultimate problem of inheritance.

What lies beyond our reach at present, as Driesch has very ably urged, is to explain the orderly rhythm of development — the coordinating power that guides development to its predestined end. We are logically compelled to refer this power to the inherent organization of the germ, but we neither know nor can we even conceive what that organization is. The theory of Roux and Weismann demands for the orderly distribution of the elements of the germ-plasm a prearranged system of forces of absolutely inconceivable complexity. Hertwig's and De Vries's theory, though apparently simpler, makes no less a demand ; for how are we to conceive the power which guides the countless hosts of migrating pangens throughout all the long and complex events of development? The same difficulty confronts us under any theory we can frame. If with Herbert Spencer we assume the germ-plasm to be an aggregation of like units, molecular or supra-molecular, endowed with predetermined polarities which lead to their grouping in specific forms,

1 Germ-plasm, p. 14. 2 Evolution, Science, and Culture* p. 296.


PREFORMATION AND EPI GENESIS 433

we but throw the problem one stage farther back, and, as Weismann himself has pointed out, 1 substitute for one difficulty another of exactly the same kind.

The truth is that an explanation of development is at present beyond our reach. The controversy between preformation and epigenesis has now arrived at a stage where it has little meaning apart from the general problem of physical causality. What we know is that a specific kind of living substance, derived from the parent, tends to run through a specific cycle of changes during which it transforms itself into a body like that of which it formed a part ; and we are able to study with greater or less precision the mechanism by which that transformation is effected and the conditions under which it takes place. But despite all our theories we no more know how the organization of the germ-cell involves the properties of the adult body than we know how the properties of hydrogen and oxygen involve those of water. So long as the chemist and physicist are unable to solve so simple a problem of physical causality as this, the embryologist may well be content to reserve his judgment on a problem a hundred-fold more complex.

The second question, regarding the historical origin of the idioplasm, brings us to the side of the evolutionists. The idioplasm of every species has been derived, as we must believe, by the modification of a preexisting idioplasm through variation, and the survival of the fittest. Whether these variations first arise in the idioplasm of the germ-cells, as Weismann maintains, or whether they may arise in the body-cells and then be reflected back upon the idioplasm, is a question to which the study of the cell has thus far given no certain answer. Whatever position we take on this question, the same difficulty is encountered ; namely, the origin of that coordinated fitness, that power of active adjustment between internal and external relations, which, as so many eminent biological thinkers have insisted, overshadows every manifestation of life. The nature and origin of this power is the fundamental problem of biology. When, after removing the lens of the eye in the larval salamander, we see it restored in perfect and typical form by regeneration from the posterior layer of the iris, 2 we behold an adaptive response to changed conditions of which the organism can have had no antecedent experience either ontogenetic or phylogenetic, and one of so marvellous a character that we are made to realize, as by a flash of light, how far we still are from a solution of this problem. It may be true, as Schwann himself urged, that the adaptive power of living beings differs in degree only, not in kind, from that of unor 1 Germinal Selection, January, 1896, p. 284. 8 See Wolff, '95, and Miiller, '96.

2F


434 INHERITANCE AND DEVELOPMENT

ganized bodies. It is true that we may trace in organic nature long and finely graduated series leading upward from the lower to the higher forms, and we must believe that the wonderful adaptive manifestations of the more complex forms have been derived from simpler conditions through the progressive operation of natural causes. But when all these admissions are made, and when the conserving action of natural selection is in the fullest degree recognized, we cannot close our eyes to two facts : first, that we are utterly ignorant of the manner in which the idioplasm of the germ-cell can so respond to the influence of the environment as to call forth an adaptive variation ; and second, that the study of the cell has on the whole seemed to widen rather than to narrow the enormous gap that separates even the lowest forms of life from the inorganic world.

I am well aware that to many such a conclusion may appear reactionary or even to involve a renunciation of what has been regarded as the ultimate aim of biology. In reply to such a criticism I can only express my conviction that the magnitude of the problem of development, whether ontogenetic or phylogenetic, has been underestimated ; and that the progress of science is retarded rather than advanced by a premature attack upon its ultimate problems. Yet the splendid achievements of cell-research in the past twenty years stand as the promise of its possibilities for the future, and we need set no limit to its advance. To Schleiden and Schwann the present standpoint of the cell-theory might well have seemed unattainable We cannot foretell its future triumphs, nor can we doubt that the way has already been opened to better understanding of inheritance and development.


LITERATURE. IX

Barfurth, D. — Regeneration und Involution: Merkel u. Bonnet, Ergeb. y I .-VI 1 1.

1891-99. Boveri, Th. — Ein geschlechtlich erzeugter Organismus ohne mutterliche Eigen schaften: Sit z.- Iter. d. Ges.f. Morph. und Phys. in Munchen, V. 1889. Sec

also Arch. f. Entw. 1895. Brooks, W. K. — The Law of Heredity. Baltimore, 1883. Id. — The Foundations of Zoology. New York, 1899.

Davenport, C. B. — Experimental Morphology: I., II. New York, 1897, 1899. Driesch, H. — Analvtische Theorie der organischen Entwicklung. Leipzig* 1894. Id. — Die Localisation morphogenetischer Vorgange: Arch. Entw., VII. 1. 1899. Id. — Resultate und Probleme der Entwickelungs-physiologie der Tiere : Merkel u.

Bonnet, Frgeb., VIII., 1898. (Full literature.) Herbst, C. — Uber die Bedeutung der Reizphysiologie fur die kausale Auffassung

von Vorgangen in der tierischen Ontogenese: Biol. Centralb., XIV., XV.

1894-95. Eertwig, 0. — Altere und neuere Entwicklungs-theorien. Berlin, 1892.


LITERATURE 435

Hertwig, 0. — Urmund und Spina Bifida: Arch. mik. Anat., XXXIX. 1892. Id. — Uber den Werth der Ersten Furchungszellen fiir die Organbildung des Embryo: Arch. mik. Anat., XLII. 1893. Id. — Zeit und Streitfragen der Biologic I. Berlin, 1894. II. Jena, 1897. Id. — Die Zelle und die Gewebe, II. Jena, 1898. His, W. — Unsere Korperform und das physiologische Problem ihrer Entstehung.

Leipzig* 1874. Loeb» J. — Untersuchungen zur physiologischen Morphologie : I. Heteromorphosis.

IViirzburg, 1891. II. Organbildung und Wachsthum. WUrzburg, 1892. Id. — Some Facts and Principles of Physiological Morphology: Wood's Holl Biol.

Lectures. 1893. Morgan, T. H. — Experimental Studies of the Regeneration of Phanaria Maculata :

Arch. Entw., VII. 2, 3. 1898. Id. — The Development of the Frog's Egg. New York, 1897. Nageli, C. — Mechahisch-physiologische Theorie der Abstammungslehre. Afiin chen u. Leipzig, 1884. Rous, W. — Uber die Bedeutung der Kernteilungsfiguren. Leipzig, 1883. Id. — Ober das kunstliche Hervorbringen halber Embryonen durch Zerstorung einer

der beiden ersten Furchungskugeln, etc. : Virchovfs Archiv, 114. 1888. Id. — Fiir unsere Programme und seine Verwirklichung : Arch. Entw., V. 2. 1897.

(See also Gesammelte Abhandlungen liber Entwicklungsmechanik der Organ ismen, 1895.) Sachs, J. — Stoflf und Form der Pflanzenorgane : Ges. Abhandlungen, II. 1893. Weismann, A. — Essays upon Heredity, First Series. Oxford, 1891. Id. — Essays upon Heredity, Second Series. Oxford, 1892. Id. — Aussere Einfllisse als Entwicklungsreize. Jena, 1894. Id. — The Germ-plasm. New York* 1893.

Whitman, C. 0. — Evolution and Epigenesis : Wood's Moll Biol. Lectures. 1894. Wilson, Edm. B. — On Cleavage and Mosaic-work: Arch, fur Entwicklungsm. y

III. 1. 1896. See also Literature, VIII., p. 394.)


•I


1


\'


GLOSSARY

[Obsolete terms are enclosed in brackets. The name and date refer to the first use of the word ;

subsequent changes of meaning are indicated in the definition.]

Achro'matin (see Chromatin), the non-staining substance of the nucleus, as opposed to chromatin ; comprising the ground-substance and the linin-network. (Flemming, 1879.)

A'croaome ( axpov, apex, oxS/jo, body), the apical body situated at the anterior end of head of spermatozoon. (Lenhossek, 1897.)

[Akaryo'ta] (see Karyota), non-nucleated cells. (Flemming, 1882.)

Ale'cithal (d-priv. ; Ac'kl0o?, the yolk of an egg), having little or no yolk (applied to eggs). (Balfour, 1880.)

AUoplasma'tic (aAAo?, different). Applied to active substances formed by differentiation from the protoplasm proper, e.g. the substance of cilia, of nerve-fibrillae, and of muscle-fibrillae. Alloplasmatic organs are opposed to " protoplasmatic," which arise only by division of preexisting bodies of the same kind. (A. Meyer, 1896.)

Amito'sis (see Mitosis), direct or amitotic nuclear division; mass-division of the nuclear substance without the formation of chromosomes and amphiaster. (Flemming, 1882.)

Amphiaster (dfi<^t, on both sides ; don/p, a star), the achromatic figure formed in mitotic cell-division, consisting of two asters connected by a spindle. (Fol,

1877) Amphipy'renin (see Py renin), the substance of the nuclear membrane.

(SCHWARZ, 1887.)

Amy'loplasts (dfivXov, starch ; irAaoro?, irAao-o-eLv, form), the colourless starchforming plastids of plant-cells. (Errera, 1882.)

An'aphase (dva, back or again), the later period of mitosis during the divergence of the daughter-chromosomes. (Strasburger, 1884.)

Aniso'tropy (see Isotropy), having a predetermined axis or axes (as applied to the egg) . ( Pfl'uger, 1883.)

Antherozo'id, the same as Spermatosoid.

Anti'podal cone, the cone of astral rays opposite to the spindle-fibres. (Van Beneden, 1883.)

Archiam'phiaster (dp\t = first, + amphiaster), the amphiaster by which the first or second polar body is formed. (Whitman, 1878.)

Ar'choplasma or Archoplasm (ap\u)v, a ruler) (sometimes written archt'plasm), the substance from which the attraction-sphere, the astral rays, and the spindlefibres are developed, and of which they consist. (Boveri, 1888.)

Arrhe'noid (apprjv, male). The sperm-aster or attraction-sphere formed during the fertilization of the ovum. (Henking, 1890.)

As'ter (aoTiJp, a star). 1. The star-shaped structure surrounding the centrosome. (Fol, 1877.) [2. The star-shaped group of chromosomes during mitosis (see Karyaster). (Flemming, 1892.)]

[As'troccele] (dori;p, a star ; koIXos, hollow), a term somewhat vaguely applied to the space in which the centrosome lies. (Fol, 1891.)

437


438 GLOSSARY

As'trosphere (see Centrocphere). i. The central mass of the aster, exclusive of the rays, in which the centrosome lies. Equivalent to the -attraction-sphere" of Van Beneden. (Fou 1891 : Strasbirger. 1892.) 2. The entire aster exclusive of the centrosome. Equivalent to the - astral sphere ~ of Mark. (Boveri. 1895.)

Attraction-sphere (see Centrosphere), the central mass of the aster from which the rays proceed. Also the mass of - archoplasm." derived from the aster, by which the centrosome is surrounded in the resting cell. (Van Bexeden, 1885.)

[Au'toblast] (auroc. self), applied by Altmann to bacteria and other minute organisms, conceived as independent solitary "bioplasts." (1890.)

Axial filament, the central filament probably contractile, of the spermatozoonnagellum. (Eimer, 1874.)

Baaichro' matin (see Chromatin), the same as chromatin in the usual sense. That portion of the nuclear network stained by basic tar-colours. (Hexdexhain. 1894.)

Bi'oblast (fiios. life : /Jaooto?. a germ), a term applied by Altmann to the hypothetical ultimate vital unit (equivalent to piasome), and identified by him as the u granulum. r '

Bi'ogen (/ftb?, life ; -ycvijs, producing), equivalent to plasonu, etc ( Verworx, 1895.)

Bi'ophores (/Kbc, life ; -<t>6ptK. bearing), the ultimate supra-molecular vital units. Equivalent to the pangens of De Vries, the plasomes of Wiesner, etc (Weismaxn. 1893.)

Bi'oplasm (/ftb?, xAa?/ia). The active "living, forming germinal material,"* as opposed to 4i formed material.** Nearly equivalent to protoplasm in the wider sense. (Beale. 1870.)

Bi'oplast, equivalent to cell. (Beale, 1870.)

Bivalent, applied to chromatin-rods representing two chromosomes joined end to end. (Hacker, 1892.)

Ble pharoplast (#Ac<£apts, eye-lash or cilium). The centrosome-like bodies in plant-spermatids in connection with which the cilia of the spermatozoids are formed. (Webber, 1897.)

Cell-plate (see Mid -body), the equatorial thickening of the spindle-fibres from which the partition-wall arises during the division of plant-cells. (Strasburger, 1875.)

Cell-sap. the more liquid ground-substance of the nucleus. [Kolliker, 1865: more precisely denned by R. Hertwig, 1876.]

Central spindle, the primary spindle by which the centrosomes are connected, as opposed to the contractile mantle-fibres surrounding it. (Hermann, 1891.)

Cen'triole, a term applied by Boveri to a minute body or bodies (•' Central -korn") within the centrosome. In some cases not to be distinguished from the centrosome. (Boveri, 1895.)

Centrodes miis (jccVrpov. centre; 8e?/io?, a band), the primary connection between the centrosomes, formed by a substance from which arises the central spindle. (Heidenmain, 1894.)

Centrodeu'toplasm, the granular material of the testis-cells which may contribute to the formation of the Nebenkern or to that of the idiozome. (Erlanger, 1897.)

Centrole cithal (KcVrpov, centre ; Ac'ki0os. yolk), that type of ovum in which the deutoplasm is mainly accumulated in the centre. (Balfour, 1880.)

Cen troplasm (xcVrpov, centre ; -rrkdafjua). the protoplasm forming the attractionsphere or central region of the aster ; the substance of the centrosphere. (Erlanger, 1895.)


GLOSSARY 439

Cen'troaome (Kcvrpov, centre ; crco/ua, body), a body found at the centre of the aster or attraction-sphere, regarded by some observers as the active centre of celldivision and in this sense as the dynamic centre of the cell. Under its influence arise the asters and spindle (amphiaster) of the mitotic figure. (Boveri, 1888.)

Cen'troaphere, used in this work as equivalent to the " astrosphere " of Strasburger ; the central mass of the aster from which the rays proceed and within which lies the centrosome. The attraction-sphere. [Strasburger, 1892; applied by him to the " astrosphere " and centrosome taken together.]

Chloroplaa'tids (\\<M>p6s, green ; irAaoros, form), the green plastids or chlorophyllbodies of plant and animal cells. (Schimper, 1883.)

Chromatin (xp<o/ia, colour), the deeply staining substance of the nuclear network and of the chromosomes, consisting of nuclein. (Flemmixg, 1879.)

Chro'matophore (xpcS/ja, colour ; -<f>6p<y;, bearing), a general term applied to the coloured plastids of plant and animal cells, including chloroplastids and chromo plastids. (SCHAARSCHMIDT, l88o; SCHMITZ, 1882.)

Chro'matoplaam (xpwpa, colour ; irAaa/ia, anything formed or moulded), the substance of the chromoplastids and other plastids. (Strasburger, 1882.)

Chro'miole, the smallest chromatin-granules which by their aggregation form the larger chromomeres of which the chromosomes are composed. (Eisen, 1899.)

Chro'momere (xpw/ja, colour; fiipos, a part), one of the chromatin-granules of which the chromosomes are made up. Identified by Weismann as the *'id." See Chromiole. (Fol, 1891.)

Chromoplas'tidB (xpwpa, colour ; 7rAaoro5, form), the coloured plastids or pigmentbodies other than the chloroplasts, in plant-cells. (Schimper, 1883.)

Chro'moplaats, net-knots or chromatin-nucleoli; also used by some authors as equivalent to Chromoplaatid. (Eisen, 1899.)

Chromosomes (xpu/ia, colour; oxo/m, body), the deeply staining bodies into which the chromatic nuclear network resolves itself during mitotic cell-division. (Waldeyer, 1888.)

Cleavage-nucleus, the nucleus of the fertilized egg, resulting from the union of egg-nucleus and sperm-nucleus. (O. Hertwig, 1875.)

Cortical zone, the outer zone of the centrosphere. (Van Beneden, 1887.)

Cyano'philous (kvolvos, blue ; <^iA.ctv, to love), having an especial affinity for blue or green dyes. (Auerbach.)

Cy 'taster (mrros, hollow (a cell) ; acrrqp, star), the same as Aster, 1. See Karyaster. (Flemming, 1882.)

[Cy'toblast] (mrros, hollow (a cell); /JAaoros, germ). 1. The cell-nucleus. (Schleiden, 1838.) 2. One of the hypothetical ultimate vital units (bioblasts or "granula") of which the cell is built up. (Altmann, 1890.) 3. A naked cell or " protoblast." (Kolliker. )

fCytoblaate'ma] (see Cytoblast), the formative material from which cells were supposed to arise by "free cell-formation." (Schleiden, 1838.)

[Cytochyle'ma] (mrros, hollow (a cell) : xvAos juice), the ground-substance of the cytoplasm as opposed to that of the nucleus. (Strasburger, 1882.)

Cy'tode (kvtos, hollow (a cell) ; ci8o<», form), a non-nucleated cell. (Hackel, 1866.)

Cytodie'resia (kvtos, hollow (a cell) ; (Wpccris, division), the same as Mitosis. (Henneguy, 188?.)

Cytohy'aloplaama (jcvtos, hollow (a cell) ; vaAo?, glass ; 7rAao-/xa, anything formed), the substance of the cytoreticulum in which are embedded the microsomes; opposed to nucleohyaloplasma. (Strasburger, 1882.) •

Cy'tolymph (kvtos. hollow (a cell) ; lympha, clear water), the cytoplasmic groundsubstance. (Hackel, 1891.)


440 GLOSSARY

Cytomi'crosomes (see Microsome), microsomes of the cytoplasm ; opposed to nucleomicrosomes. (Strasburger, 1882.)

Cytomi'tome (kvtos. hollow (a cell) ; /u'ra>/ua, from furos, thread), the cytoplasmic as opposed to the nuclear thread-work. (Flemming, 1882.)

Cy'toplasm (kvtck, irXaxTyua). 1. The protoplasmic ground-substance as opposed to the granules. (Kolliker, 1863.) 2. Equivalent to protoplasm. (Kolliker. 1867.) 3. The substance of the cell-body as opposed to that of the nucleus. (Strasburger, 1882.)

Cytoretic'ulum, the same as Cytomitome. (Strasburger, 1882.)

Cy'tosome (kvtogj hollow (a cell) ; <ru>/xa, body). 1. The cell-body or cytoplasmic mass as opposed to the nucleus. (Hackel, 1891.) 2. A term used as parallel to chromosome to denote deeply staining definitely organized cytoplasmic filaments or other cytoplasmic structures composed of ** cytochromatin." ( Prenant, i 898. )

Der'matoplasm (Scp/xa, skin), the living protoplasm asserted to form a part of the cell-membrane in plants. (Wiesner, 1886.)

Der'matosomes (Sepfia, skin ; ow/ao, body), the plasomes which form the cell-membrane. (Wiesner, 1886.)

Determinant, a hypothetical unit formed as an aggregation of biophores, determining the development of a single cell or independently variable group of cells. (Weismann, 1 891.)

[Deuthy alosome] (8evr(cpos), second ; see Hyalosome), the nucleus remaining in the egg after formation of the first polar body. (Van Beneden, 1883.)

Deu'toplasm (8evr(cpos), second ; irkdo-fAa, anything formed), yolk, lifeless foodmatters deposited in the cytoplasm of the egg ; opposed to "protoplasm." (Van Beneden, 1870.)

Diakine'sis (81a, through), the segmented-spireme-stage, following the synapsis, in the primary oocyte or spermatocyte, during which the chromosomes persist for a considerable period in the form of double rods. (Hacker, 1897.)

Directive bodies, the polar bodies. (Fr. MCller, 1848.)

Directive sphere, the attraction-sphere. (Guignard, 1891.)

Dispermy, the entrance of two spermatozoa into the egg.

Dispi'reme (see Spireme), that stage of mitosis in which each daughter-nucleus has given rise to a spireme. (Flemming, 1882.)

Dy'aster (8vas. two; see Aster, 2), the double group of chromosomes during the anaphases of cell-division. (Flemming, 1882.)

Ectosphere (£ktos, outside), the outer or cortical zone of the attraction-sphere. (Ziegler, 1899.)

Egg-nucleus, the nucleus of the egg after formation of the polar bodies and before its union with the sperm-nucleus. Equivalent to the "female pronucleus'" of Van Beneden. (O. Hertwig, 1875.)

Enchyle'ma (cV, in ; xvAos, juice). 1 . The more fluid portion of protoplasm, consisting of " hyaloplasma." (Hanstein, 1880.) 2. The ground-substance (cytolymph) of cytoplasm as opposed to the reticulum. (Carnoy, 1883.)

Endoplast, the cell-nucleus. (Huxley, 1853.)

Ener'gid, the cell-nucleus together with the cytoplasm lying within its sphere of influence. (Sachs, 1892.)

Untosphere, (cVrds, inside), the inner or medullary zone of the attraction-sphere. (Ziegler, 1899.)

Equatorial plate, the group of chromosomes lying at the equator of the spindle during mitosis. (Van Beneden, 1875.)

Ergastic ( epyafo/uai, to work). Applied to relatively passive substances "formed anew through activity of the protoplasm. ,, Equivalent to metaplasmic. Cf* alloplasmatic. (A. Meyer, 1896.)


GLOSSARY 44I

Ergastoplasm (cpya£o/iai, to work). Nearly equivalent to the " kinoplasm n of

Strasburger and the *' ergoplasm " of Davidoff. The more active protoplasmic

substance from which fibrillar formations arise. (Garnier, 1897.) Ergoplasm (cpyov, work). The active protoplasm of the egg (in tunicates), mainly

derived from the achromatic part of the germinal vesicle, and giving rise in part

or wholly to the polar spindle. Analogous to archoplasm and kinoplasm.

(Davidoff, 1889.) Erythro'philous (ipvQpos, red ; <^iyc?v, to love), having an especial affinity for red

dyes. (Auerbach.) Ga'mete (yafienj, wife ; yafierrp, husband), one of two conjugating cells. Usually

applied to the unicellular forms. Gem/mule (see Pangen), one of the ultimate supra-molecular germs of the cell

assumed by Darwin. (Darwin, 1868.) [Ge'noblasts] (yews, sex ; /ftooros, germ), a term applied by Minot to the mature

germ-cells. The female genoblast (egg or " thelyblast ") unites with the male

(spermatozoon or ** arsenoblast ") to form an hermaphrodite or indifferent cell.

(Minot, 1877.) Germinal spot, the nucleolus of the germinal vesicle. (Wagner, 1836.) Germinal vesicle, the nucleus of the egg before formation of the polar bodies.

(Purkinje, 1825.) Germ-plasm, the same as idioplasm. (Weismann.) Heterokine'sis (crcpos, different), qualitative nuclear division ; a hypothetical mode

of mitosis assumed to separate chromatins of different quality; opposed to

homookinesis or equation-division. (Weismann, 1892.) Heterole'cithal (ercpos, different; A.c'ja0O9, yolk), having unequally distributed

deutoplasm (includes telolecithal and centrolecithal) . (Mark, 1892.) Heterotyp'ical mitosis (crcpos, different ; see Mitosis), that mode of mitotic

division in which the daughter-chromosomes remain united by their ends to form

rings. (Flemming, 1887.) [Holoschi'sis] (0A09, whole ; crxt£civ, to split), direct nuclear division. Amitosis.

(Flemming, 1882.) Homole'cithal (ofxoq, the same, uniform ; Acfcttfo?, yolk), equivalent to alecithal.

Having little deutoplasm, equally distributed, or none. (Mark, 1892.) Homottkine'sis or Homasokine'sis (6/109, the same), equation-division, separating

equivalent chromatins; opposed to heterokinesis. (Weismann, 1892.) Homceotyp'ical mitosis (0/1010?, like ; see Mitosis), a form of mitosis occurring

in the secondary spermatocytes of the salamander, differing from the usual type

only in the shortness of the chromosomes and the irregular arrangement of

the daughter-chromosomes. (Flemming, 1887.) Hy'aloplasma (uoAoc, glass ; 7rAao>ui, anything formed). 1. The ground-substance

of the cell as distinguished from the granules or microsomes. [Hanstein, 1880.]

2. The achromatic substance of the nucleus in which the chromatin-particles are

embedded. (Strasburger, 1882.) 3. The ground-substance as distinguished

from the reticulum or "spongioplasm." (Leydig, 1885.) 4. The exoplasm or

peripheral protoplasmic zone in plant-cells. (Pfeffer.) Hy'alosomes (vaAos, glass ; ow/ia, body), nucleolar-like bodies but slightly stained

by either nuclear or plasma stains. (Lukjanow, 1888.) [Hy'groplasma] (vypos, wet; 7rAao>ia, something formed), the more liquid part

of protoplasm as opposed to the firmer stereoplasm. (Nageli, 1884.) Id, the hypothetical structural unit resulting from the successive aggregation of

biophores and determinants. Identified by Weismann as the chromomere, or

chromatin-granule. (W t eismann. 1891.) Idant, the hypothetical unit resulting from the successive aggregation of biophores,


442 GLOSSARY

determinants, and Ids. Identified by Weismann as the chromosome. (Weismakn, 1 89 1.)

Id'ioblasts (18109* one's own ; pXaaros, germ), the hypothetical ultimate units of the cell ; the same as biophores. (O. Hertwig, 1893.)

Idioplasm (18109, one's own ; irXatrfm, a thing formed), equivalent to the germplasm of Weismann. The substance, now generally identified with chromatin, which by its inherent organization involves the characteristics of the species. The physical basis of inheritance. (Nageli, 1884.)

Id'iosome (18109, one's own; oxu/ia, body), the same as idioblast or plasome. (Whitman, 1893.)

Idiozome (18109, specially formed ; £o>/ia, girdle) . The sphere, often called attraction-sphere and usually enclosing the centrosomes, found in the spermatids of animals. (Meves, 1897.)

Interfilar substance, the ground-substance of protoplasm as opposed to the threadwork. (Flemming, 1882.)

Interzonal fibres (" Filaments reunissants " of Van Beneden. " Verbindungsfasern" of Flemming and others). Those spindle-fibres that stretch between the two groups of daughter-chromosomes during the anaphase. Equivalent in some cases to the central spindle. (Mark, 1881.)

Iso'tropy (10-09, equal; tooth}, a turning), the absence of predetermined axes (as applied to the egg). (Pfluger, 1883.)

[Ka'ryaster] (tcapvov, nut, nucleus ; see Aster, 2), the star-shaped group of chromosomes in mitosis. Opposed to cytaster. (Flemming, 1882.)

Karyenchy'ma (tcdpvov, nut, nucleus; iv, in; \ v H^y juice), the "nuclear sap." (Flemming, 1882.)

Karyokine'sis {napvov, nut, nucleus; kivtjctls, change, movement), the same as mitosis. (Schleicher, 1878.)

[Karyoly'ma], the " karyolytic " (mitotic) figure. (Auerbach, 1876.)

Ka'ryolymph. The nuclear sap. (Hackel, 1891.)

[Karyo'lysis] (xapvov, nut, nucleus ; A.ixn9, dissolution), the supposed dissolution of the nucleus during cell-division. (Auerbach, 1874.)

[Karyoly'tic figure] (see Karyolysis), a term applied by Auerbach to the mitotic figure in living cells. Believed by him to result from the dissolution of the nucleus. (Auerbach, 1874.)

Karyomi'crosome (see Microsome), the same as nucleo-microsome.

Karyomi'tome (xapvov, nut, nucleus ; /x/royia, from /X1Y09, a thread), the nuclear as opposed to the cytoplasmic thread-work. (Flemming, 1882.)

Karyomito'sis (xapvov, nut, nucleus; see Mitosis), mitosis. (Flemming, 1882.)

L'ryon (xapvov, nut, nucleus), the cell-nucleus. (Hackel, 1891.) L'ryoplasm (Kapvov, nut, nucleus ; 7r\d<Tfia, a thing formed), nucleoplasm. The nuclear as opposed to the cytoplasmic substance. (Flemming, 1882.) t'ryosome (tcdpvov, nut, nucleus; ctu/uta, body). 1. Nucleoli of the *• net-knot " type, staining with nuclear dyes, as opposed to plasmosomes or true nucleoli. (Ogata, 1883.) 2. The same as chromosome. (Platner, 1886.) 3. Caryosome. The cell-nucleus. (Watase, 1894.)

[Karyo'ta] (napvov, nut, nucleus), nucleated cells. (Flemming, 1882.)

Karyothe'ca (Kapvov, nut, nucleus ; Orjurj, case, box), the nuclear membrane. (Hackel, 1891.) li'noplasm (klvuv, to move* irXavpa, a thing formed), nearly equivalent to archoplasm, but used in a broader sense to denote in general the more active elements of protoplasm from which arise fibrillar the substance of cilia, and (in plants) the peripheral " Hautschicht " from which the membrane is


GLOSSARY


443


formed; opposed to the " trophoplasm " or nutritive plasm. (Strasburger, 1892.)

[Lanthanin] (\avOdv€iv, to conceal), equivalent to oxychromatin. (Heidenhain, 1892.)

Leucoplaa'tids (acvkos, white; irAooros, form), the colourless plastids of plantcells from which arise the starch-formers (amyloplastids), chloroplastids, and chromoplastids. (Schimper, 1883.)

Li'nin (linum, a linen thread), the substance of the " achromatic " nuclear reticulum. (Schwarz, 1887.)

Lininoplast, the true nucleolus or plasmosome. (Eisen, 1899.)

Macrocentroaome, a term applied to the 4 * centrosome " in Boverfs sense, i.e. to the larger body in which lies the central granule. (Ziegler, 1898.) Probably synonymous with entosphere.

Maturation, the final stages in the development of the germ-cells. More specifically, the process by which the reduction of the number of chromosomes is effected.

Metakine'sis (see Metaphaae) (furd, beyond (/>. further) ; kiwtjctk;, movement), the middle stage of mitosis, when the chromosomes are grouped in the equatorial plate. (Flemming, 1882.)

Metanu'cleus, a term applied to the nucleolus after its extrusion from the germinal vesicle. (Hacker, 1892.)

Metaphase, the middle stage of mitosis during which occurs the splitting of the chromosomes in the equatorial plate. (Strasburger, 1884.)

Met'aplasm (/xera, after, beyond; irAac/Aa, a thing formed), a term collectively applied to the lifeless inclusions (deutoplasm, starch, etc.) in protoplasm as opposed to the living substance. (Hanstein, 1868.)

Micella, one of the ultimate supra-molecular units of the cell. (Nageli, 1884.)

Microcentrosome, equivalent to the central granule or centriole of Boveri. (Ziegler, 1898.)

Microcen'trum, the centrosome or group of centrosomes united by a " primary centrodesmus," forming the centre of the astral system. (Heidenhain, 1894.)

Mi'cropyle (fwcpo^, small; wvkq, orifice), the aperture in the egg-membrane through which the spermatozoon enters. [First applied by Turpin, in 1806, to the opening through which the pollen-tube enters the ovule. /. Robert Brown.]

Microsome (juicpo?, small ; ow/ua, body), the granules as opposed to the groundsubstance of protoplasm. (Hanstein, 1880.)

Microsphere, the central region of the aster (centrosphere) at the centre of which lie the centrosomes. (Kostanecki and Siedlecki, 1896.)

Middle-piece, that portion of the spermatozoon lying behind the nucleus at the base of the flagellum. (Schweigger-Seidel, 1865.)

Mid-body ("Zwischenkbrper"), a body or group of granules, probably comparable with the cell-plate in plants, formed in the equatorial region of the spindle during the anaphases of mitosis. (Flemming, 1890.)

Mi'tome (/u'rayia, from /u'ro?, a thread), the reticulum or thread-work as opposed to the ground-substance of protoplasm. (Flemming, 1882.)

[Mitoschi'ais (/u'tos, thread ; <rxi'£civ, to split), indirect nuclear division; mitosis. (Flemming, 1882.)

Mitosis (furoq, a thread), indirect nuclear division typically involving: a, the formation of an amphiaster; 6, conversion of the chromatin into a thread (spireme) ; c, segmentation of the thread into chromosomes ; d, splitting of the chromosomes. (Flemming, 1882.)

Mi'tosome (furos, a thread ; cra>/xa, body), a body derived from the spindle-fibres


444 GLOSSARY

of the secondary spermatocytes, giving rise, according to Platner, to the middle-piece and the tail-envelope of the spermatozoon. Equivalent to the Ne benkern of La Valette St. George. (Platner, 1889.)

Nebenkern (Paranucleus), a name originally applied by Biitschli (1871) to an extranuclear body in the spermatid ; afterwards shown by La Valette St. George and Platner to arise from the spindle-fibres of the secondary spermatocyte. Since applied to many forms of cytoplasmic bodies (yolk-nucleus, etc.) of the most diverse nature.

Nuclear plate. 1. The equatorial plate. (Strasburger, 1875.) 2. The partition-wall which sometimes divides the nucleus in amitosis.

Nuclein, the chemical basis of chromatin ; a compound of nucleinic acid and albumin or albumin radicles. (Miescher, 1871-)

Nucleinic or nucleic acid, a complex organic acid, rich in phosphorus, and an essential constituent of chromatin.

Nucleo-albumin, a nuclein having a relatively high percentage of albumin. Distinguished from nucleo-proteids by containing paranucleinic acid which yields no xanthin-bodies.

[Nucleoohyle'ma] (xvAos, juice), the ground-substance of the nucleus as opposed to that of the cytoplasm. (Strasburger, 1882.)

Nucleohy'aloplasma (see Hyaloplasm), the achromatic substance (linin) in which the chromatin-granules are suspended. (Strasburger, 1882.)

Nucleomi'crosomes (see Microsome), the nuclear (chromatin) granules as opposed to those of the cytoplasm. (Strasburger, 1882.)

Nucleoplasm. 1. The reticular substance of the (egg-) nucleus. (Van Beneden, 1875.) 2. The substance of the nucleus as opposed to that of the cellbody or cytoplasm. (Strasburger, 1882.)

Nucleo-pro'teid, a nuclein having a relatively high percentage of albumin. May be split into albumin and true nucleinic acid, the latter yielding xanthin-bodies.

GSde'matin (otS^/ua, a swelling), the granules or microsomes of the nuclear groundsubstance. (Reinke, 1893.)

O'Scyte (Ovocyte) (o>ov. egg; kvtos, hollow (a cell)), the ultimate ovarian egg before formation of the polar bodies. The primary oocyte divides to form the first polar body and the secondary oocyte. The latter divides to form the second polar body and the mature egg. (Boveri, 1891.)

Oogen esis. Ovogenesis (aidy, egg ; yc'vco-is, origin), the genesis of the egg after its origin by division from the mother-cell. Often used more specifically to denote the process of reduction in the female.

Oogonium, Ovogonium (o>ov, egg ; yovrj, generation). 1 . The primordial mothercell from which arises the egg and its follicle. (Pfluger.) 2. The descendants of the primordial germ-cell which ultimately give rise to the oocytes or ovarian eggs . ( Bo veri, 1 89 1 . )

Ookinesis (a)dv, egg; ku^o-is, movement), the mitotic phenomena of the egg during maturation and fertilization. (Whitman, 1887.)

O'vocentre, the egg-centrosome during fertilization. (Fol, 1891.)

Oxychi o'matin (o£ vs, acid ; see Chromatin), that portion of the nuclear substance stained by acid tar-colours. Equivalent to " linin " in the usual sense. (Heidenhain, 1894.)

Pangen'esis (7ras (7rav-), all; •ycyecris, production), the theory of gem mules, according to which hereditary traits are carried by invisible germs thrown off by the individual cells of the body. (Darwin, 1868.)

Pangens (7ra« (7rav-), all ; -yev^s, producing), the hypothetical ultimate supra-molecular units of the idioplasm, and of the cell generally. Equivalent to gemmules, micellae, idioblasts, biophores, etc. (De Vries, 1889.)


GLOSSARY 445

Farachro'matin (see Chromatin), the achromatic nuclear substance (linin of Schwarz) from which the spindle-fibres arise. (Pfitzner, 1883.)

Parali'nin (see Linin), the nuclear ground-substance or nuclear sap. (Schwarz, 1887.)

Parami'tome (see Mitome), the ground-substance or interfilar substance of protoplasm, opposed to mitome. (Flemming, 1892.)

Paranu'clein (see Nuclein), the substance of true nucleoli or plasmosomes. Pyrenin of Schwarz. (O. Hertwig, 1878.) Applied by Kossel to "nucleins" derived from the cytoplasm. These are compounds of albumin and paranucleic acid which yields no xanthin-bodies.

Paranucleus (see Nebenkern).

Paraplasm (wapd, beside ; irAac/io, something formed), the less active portion of the cell-substance. Originally applied by Kupffer to the cortical region of the cell (exoplasm), but now often applied to the ground-substance. (Kupffer,

1875.) Periplast (irtpu around; ir\aor6s y form). 1. The peripheral part of the cell,

including those parts outside the nucleus or "endoplast." (Huxley, 1853.)

2. A term somewhat vaguely applied to the attraction-sphere. The term

daughter-periplast is applied to the centrosome. (Vejdovsky, 1888.)

Perisphere (ircpt, around), a term applied to the outer region of the attractionsphere in nerve-cells, and opposed to an inner " centrosphere." (Lenhoss£k, 1895.)

Plasmocytes (irXdo-fjuL, kvtos), colourless blood-corpuscles supposed to be free attraction-spheres. (Eisen, 1897.)

Plasmosphere, the same as Perisphere.

Plaa'mosome (7rAaoyza, something formed (i.e. protoplasmic) ; o-w/tia, body), the true nucleus, distinguished by its affinity for acid tar-colours and other " plasmastains." (Ogata, 1883.)

Pla'some (irXda-fm, a thing formed; crwfia, body), the ultimate supra-molecular vital unit. See Biophore, Pangen. (Wiesner, 1890.)

Plas'tid (irAasros, form). 1. A cell, whether nucleated or non-nucleated. (Hackel. 1866.) 2. A general term applied to permanent cell-organs (chloroplasts, etc.) other than the nucleus and centrosome. (Schimper, 1883.)

Plas'tidule, the ultimate supra-molecular vital unit. (Elssberg, 1874; Hackel, 1876.)

Plas'tin, a term of vague meaning applied to a substance related to the nucleoproteids and nucleo-albumins constituting the linin-network (Zacharias) and the cytoreticulum (Carnoy). (Reinke and Rodewald, 1881.)

Pluri'valent (p/us, more; vaUre. to be worth), applied to chromatin-rods that have the value of more than one chromosome sensu strictu. (Hacker, 1892.)

Polar bodies (Polar globules), two minute cells segmented off from the ovum before union of the germ-nuclei. (Disc, by Carus, 1824; so named by Robin, 1862.)

Polar corpuscle, the centrosome. (Van Beneden, 1876.)

Polar rays (Polradien), a term sometimes applied to all of the astral rays as opposed to the spindle-fibres, sometimes to the group of astral rays opposite to the spindle-fibres.

Pole-plates (End-plates), the achromatic spheres or masses at the poles of the spindle in the mitosis of Protozoa, probably representing the attraction-spheres. (R. Hertwig, 1877.)

Polyspermy, the entrance into the ovum of more than one spermatozoon.

[Prochro'matin] (see Chromatin), the substance of true nucleoli, or plasmosomes. Equivalent to paranuclei of O. Hertwig. (Pfitzner, 1.883.)


446 GLOSSAKY

Pronuclei, the germ-nuclei during fertilization ; i.e. the egg-nucleus (female pronucleus) after formation of the polar bodies, and the sperm-nucleus (male pronucleus) after entrance of the spermatozoon into the egg. (Van Bexedex,

1875.)

[Prothy'alosome] (see Hyalosome), an area in the germinal vesicle (of A scans) by which the germinal spot is surrounded, and which is concerned in formation of the first polar body. (Van Beneden, 1883.)

Pro'toblast (7rpa>ros, first; /JAooros, a germ). 1. A naked cell, devoid of a membrane. (Kolliker.) 2. A blastomere of the segmenting egg which is the parent-cell of a definite part or organ. (Wilson, 1892.)

Protoplasm (7rpo>Tos, first ; irXdtrfw^ a thing formed or moulded) . The active or " living " cell-substance. By all earlier and some present writers applied only to the substance of the cell-body (equivalent to Strasburger's cytoplasm). By many later writers applied to the entire active substance of the cell (karyoplasm plus cytoplasm). (Purkinje, 1840; H. von Mohl, 1846.)

Protoplast (irpwros, first; wAixotos, formed). 1. The protoplasmic body of the cell, including nucleus and cytoplasm, regarded as a unit. Nearly equivalent to the energid of Sachs. (Hanstein, 1880.) 2. Used by some authors synonymously with plastid.

[Pseudochro'matln] (see Chromatin), the same as prochromatin. (Pfitzner, 1886.)

Pseudonu'clein (see Nuclein), the same as the paranuclein of Kossel. (Ham MARSTEN, 1894.)

Pseudo-reduction, the preliminary halving of the number of chromatin-rods as a . prelude to the formation of the tetrads and to the actual reduction in the number

of chromosomes in maturation. (Ruckert. 1894.) Pyre'nin (jrvprfv, the stone of a fruit ; i.e. relating to the nucleus), the substance of

true nucleoli. Equivalent to the paranuclein of Hertwig. (Schwarz, 1887.) Pyre'noid (wvprjv, the stone of a fruit ; like a nucleus), colourless plastids (leuco plastids). occurring in the chromatophores of lower plants, forming centres for

the formation of starch. (Schmitz, 1883.) Reduction, the halving of the number of chromosomes in the germ-nuclei during

maturation. Sarcode (<rap£, flesh). The protoplasm of unicellular animals. (Du Jardin.

1835) Sertoli-cells, the large, digitate, supporting, and nutritive cells of the mammalian

testis to which the developing spermatozoa are attached. (Equivalent to u spermatoblast " as originally used by Von Ebner, 1871.)

Spermatid (oTrc'p/uLa, seed), the final cells which are converted without further division into spermatozoa ; they arise by division of the secondary spermatocytes or " SamenmUtterzellen. ,, (La Valete St. George, 1886.)

Spermatoblasts (<nrcp/Lia, seed; yftAaoros, germ), a word of vague meaning, originally applied to the supporting cell or Sertoli-cell, from which a group of spermatozoa was supposed to arise. By various later writers used synonymously with spennatid. (Von Ebner, 1871.)

Sper'matocyst (cnrc'p/ua, seed : kvottis, bladder), originally applied to a group of sperm-producing cells ("spermatocytes ■ 1 ), arising by division from an "Ursamenzelle" or "spermatogonium.'" (La V alette St. George, 1876.)

Sper'matocyte (oircp/ia, seed; kvtos, hollow (a cell)), the cells arising from the spermatogonia. The primary spermatocyte arises by growth of one of the last generation of spermatogonia. By its division are formed two secondary spermatocytes, each of which gives rise to two spermatids (ultimately spermatozoa). (La Valette St. George, 1876.)


GLOSSARY 447

[Sperm atogem'ma] (cnrcp/ia, seed ; gemma, bud), nearly equivalent to spermatocyst. Differs in the absence of a surrounding membrane. [In mammals, La Valette St. George, 1878.]

Spermatogenesis (cnrcppo, seed; ycrccri?, origin), the phenomena involved in the formation of the spermatozoon. Often used more specifically to denote the process of reduction in the male.

Spermatogo'nium (" Ursamenzelle ") (cnrcp/ia, seed ; yovrj, generation), the descendants of the primordial germ-cells in the male. Each ultimate spermatogonium typically gives rise to four spermatozoa. (La Valette St. George, 1876.)

Bpermatome'rites (cnrcppa, seed ; /icpos, a part), the chromatin-granules into which the sperm-nucleus resolves itself after entrance of the spermatozoon. (In Petromyzon, Bohm, 1887.)

Bper'matosome (cnrcp/ia, seed; crw/ia, body), the same as spermatozoon. (La Valette St. George, 1878.)

Spermatozo Id (see Spermatozoon), the ciliated paternal germ-cells in plants. The word was first used by von Siebold as synonymous with spermatozoon.

Spermatozoon (<r7rcppo, seed ; (<£ov, animal), the paternal germ-cell of animals. (Leeuwenhoek, 1677.)

Sperm-nucleus, the nucleus of the spermatozoon ; more especially applied to it after entrance into the egg before its union with the egg-nucleus. In this sense equivalent to the " male pronucleus " of Van Beneden. (O. Hertwig,

1875.) Bper'mocentre, the sperm-centrosome during fertilization. (Fol, 1891.) Spi'reme (cnrctpi/fia, a thing wound or coiled ; a skein), the skein or " Knauel "

stage of the nucleus in mitosis, during which the chromatin appears in the form

of a thread, continuous or segmented. (F lemming, 1882.) Spon'gioplasm (cnroyytov, a sponge ; irAacrpa, a thing formed), the cytoreticulum.

(Leydig, 1885.) Ste'reoplasm (orcpco?, solid), the more solid part of protoplasm as opposed to the

more fluid " hygroplasm." (Nageli, 1884.) Substantia hyalina, the protoplasmic ground-substance or "hyaloplasm."

(Leydig, 1885.) Substantia opaca, the protoplasmic reticulum or " spongioplasm." (Leydig,

1885.) Synap'sis (<rwa7rrci>, to fuse together). A stage in the nucleus preceding the first

maturation-division, characterized by the massing of the chromatin at one side

of the nucleus. From it the chromatin-masses emerge in the reduced number.

(Moore, 1895.) Te'loblast (tcAos, end : /JAaoros, a germ), large cells situated at the growing end

of the embryo (in annelids, etc.), which bud forth rows of smaller cells. (Whitman, Wilson, 1887.) Telole'cithal (tcXos, end ; AcVctlo?, yolk), that type of ovum in which the yolk is

mainly accumulated in one hemisphere. (Balfour, 1880.) Telophases, Telokine'sis (rcAo?, end), the closing phases of mitosis, during

which the daughter-nuclei are re-formed. (Heidenhain, 1894.) To'noplasts (rovos, tension ; wXaoros, form), plastids from which arise the vacuoles

in plant-cells. (De Vries, 1885.) Trophoplasm (rpo<t>^ nourishment; 7rAan-pa). 1. The nutritive or vegetative

substance of the cell, as distinguished from the idioplasm. (Nageli, 1884.)

2. The active substance of the cytoplasm other than the "kinoplasm " or archo plasm. (Strasburger, 1892.) Tro'phoplasts (Tpotjrrj, nourishment ; trAooros, form), a general term, nearly equiv


448 GLOSSARY

alent to the "plastids" of Schimper, including "anaplasts" (amyloplasts),

"autoplasts" (chloroplasts), and chromoplasts. (A. Meyer, 1882-83.) Yolk-nucleus, a word of vague meaning applied to a cytoplasmic body, single or

multiple, that appears in the ovarian egg. [Named " Dotterkern " by Carus.

1850.) Zy'gote or Zy'gospore (£vyw, a yoke), the cell produced by the fusion of two

conjugating cells or gametes in some of the lower plants.


GENERAL LITERATURE-LIST

i

The following list includes only the titles of works actually referred to in the text and those immediately related to them. For more complete bibliography the reader Is referred to the literature-lists in the special works cited, especially the following. For reviews of the early history of the cell-theory see Remak's Untersuchungen ( ? 50-'55), Huxley on the Cell-theory ("53), Sach's History of Botany and Tyson's Cell-doctrine O78) . An exhaustive review of the earlier literature on protoplasm, nucleus, and cell-division will be found in Flemming's Zellsubstanz ('82), and a later review of theories of protoplasmic structure in BUtschli's Protoplasma ('92) and in Fischers Fixierung* etc., des Protoplasmas C99). The earlier work on mitosis and fertilization is very thoroughly reviewed in Whitman's Clepsine ('78), Fol's Hhiogenie ('79)« an d Mark's Umax ("81). For more recent general literature-lists see especially Hertwig's Zelle und Gewcbe ('93, '98), Yves Delage (95), Henneguy's Cellule ("96), Hacker's Praxis und Theorie der Zelle n und Befruchtungslehre ('99), and the admirable reviews by Flemming, Boveri, RUckert, Meves, Roux, and others in Merkel and Bonnet's Ergebnisse ('9 1-98).

The titles are arranged in alphabetical order, according to the system adopted in Minot's Human Embryology. Each author's name is followed by the date of publication (the first two digits being omitted, except in case of works published before the present century), and this by a single number to designate the paper, in case two or more works were published in the same year. For example, Boverl, Th., '87. 2, denotes the second paper published by Boveri in 1887.

In order to economize space, the following abbreviations are used for the journals most frequently referred to : —

ABBREVIATIONS

A. A. Anatomischcr Anzeiger.

A. B. Archives dc Biologic

A. A. P. Archiv fur Anatomie und Physiologic

A . m. A. Archiv fur mikroscopische Anatomie.

A. Entwm. Archiv fur Entwicklungsmechanik.

B. C. Biologisches Ccntralblatt.

C. R. Comptes Rendus.

/. M. Journal of Morphology.

J. w. Bo/. Jahrhuch fur wissenschaftliche Botanik.

J. Z. Jenaische Zeitschrift.

M. A. Muller's Archiv.

M.J. Morphologisches Jahrbuch.

Q.J. Quarterly Journal of Microscopical Science.

Z. A. Zoologischer Anzeiger.

Z. w. Z. Zeitschrift fur wissenschaftliche Zoologie.

ALBRECHT, B., '98. Untersuchungen zur Structur des Seeigeleies : Sitzb. Ges.

Aforph. P/iys. MYmchen.. 3. — Altman, R., *86. Studien iiber die Zelle, I. : Leipzig.

— Id., '87. Die Genese der Zellen: Leipzig. — Id., '89. Cber Nucleinsaure : A.

A. P., p. 524. — Id., '90, '94. Die Elementarorganismen und ihre Beziehung zu

2G 449


450 GENERAL LITERATURE-LIST

den Zellen : Leipzig. — Amelung, B., -93. Uber mittlere Zellgrosse : Flora* p. 176.

— Andrews, B. A., 98, 1. Filose Activities in Metazoan Eggs : Zool. Bull., II., 1.

— Id., '98, 2. Activities of Polar Bodies of Cerebratulus : Arch. Entivm., VI., 2. — Andrews, Q-. P., '97. The Living Substance as Such and as Organism : J. M.* XII., 2, Suppl. — Arnold, J., '79. Uber feinere Struktur der Zellen, etc. : Virchows Arch.* 1879. (See earlier papers.) — Atkinson, O. P., '99. Studies on Reduction in Plants : Bo/. Gaz.* XX VIII., 1, 2. — Auerbach, L., '74. Organologische Studien :

.Breslau. — Id., '91. Uber einen sexuellen Gegensatz in der Chromatophilie der Keimsubstanzen : Sitzungsber . der Konigl. preuss. A had. d. Wiss. Berlin* XXXV.

— Id. "96. Untersuchungen liber die Spermatogenese von Paludina : J. Z. % XXX.

VON BAER, C. B., '28, '37. Uber Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion: I. Konigsberg* 1828; II. 1837. — Id., -34. Die Metamorphose des Eies der Batrachier : Mutter's Arch. — Balbiani, B. O.. '61. Recherches sur les ph^nomenes sexuels des Infusoires : Journ. de la Phys.* IV. — Id., *64. Sur la constitution du germe dans Toeuf animal avant la fecondation: C. R.* LVIII. — Id., '76. Sur les phdnomenes de la division du noyau cellulaire: C. R.* XXX., October, 1876. — Id., '81. Sur la structure du noyau des cellules salivares chez les larves de Chironomus : Z. A.* 1881, Nos. 99, 100. — Id., '89. Recherches experimen tales sur la merotomie des Infusoires cilies : Recueil Zool. Suisse* January, 1889.

— Id., '91, 1. Sur les regenerations successives du peristome chez les Stentors et sur le r6ie du noyau dans ce phe'nomene : Z. A.* yj2, 373. — Id., *91, 2. Sur la structure et division du noyau chez les Spirochona gemmipara: Ann. d. Micrographie. — Id., *93. Centrosome et Dotterkern : Journ. de Fanat. el de la physiol.* XXIX.— Balfour, P. M., '80. Comparative Embryology: I. 1880.— Ballowitz, '88-'91. Untersuchungen liber die Struktur der Spermatozoen : 1 . (birds) A. m. A.. XXXII.. 1888; 2. (insects) Z.w.Z.* LX., 1890; 3. (fishes, amphibia, reptiles) A. m. A., XXXVI., 1890; 4. (mammals) Z. w. Z.* 1891. — Id., '89. Fibrillare Struktur und Contractilitat : Arch. ges. Phys.* XLVI. — Id., '91, 2. Die inncre Zusammensetzung des Spermatozoenkopfes der Saugetiere : Centralb.f. Phys.. V. — Id., '95. Die Doppelspermatozoa der Dytisciden : Z. w. Z.* XLV.. 3. — Id., '97. Uber Sichtbarkeit und Aussehen der ungetarbten Centrosomen in ruhenden Gewebszellen : Z. w. Mic. XIV. — Id., '98. Zur Kenntniss der Zellsphare: Arch. Anal. Phys.* 98, II., III. — Van Bambeke, C, '93. Elimination dWments nucleaires dans l'oeuf ovarien de Scorpa^na scrofa: A. B.* XIII.. 1. — Id., '96. De Pemploi du terme Protoplasma : Bull. Soc. Beige. Mic, XXII. — Id., '97. A propos de la delimitation cellulaire: Ibid., XXIII. — Id., '98. Recherches sur roocyte de Pholcus phalangioides : --/. />'., XV. — De Bary, '58. Die Conjugaten. — Id., '62. Uber den Ban und das VVesen der Zelle: Flora, 1862. — Id., '64. Die Mycetozoa: 2d Ed., Leipzig. — Barry, M. Spermatozoa observed within the Mammiferous Ovum : Phil. Trans., 1843. — Beale, Lionel S., '61. On the Structure of Simple Tissues of the Human Body : London. — B^ champ and Bstor, '82. De la constitution ele'mentaire des tissues: Mont pettier. — Belajeff, W., '89. Mittheilung Uber Bau und Entwicklung der Spermatozoiden : Bcr. D. Bot. Ges. — Id., '92. 1. Uber den Bau und die Entwicklung der Antherozoiden, I., Characeen. — Id., '92. 2. Uber die Karyokinesis in den Pollenmutterzellen bei luirix und Fritillaria: Sitzb. li'arsch. Naturf. Ges. — Id., '94, 1. Zur Kenntniss der Karyokinese bei den Pfianzen: Flora. 1894, Erganzungsheft. — Id., '94,2. Uber Bau und Entwicklung der Spermatozoiden der Pfianzen : Flora, LIV. — Id., '97, 1. Uber den Nebenkern in Spermatogenen Zellen und die Spermatogenese bei den Farnkrauten : Ber. I). Bot. Ges.* XV. — Id., '97, 2. Uber die Spermatogenese bei den Schachtelhalmen : Ibid. — Id., '97, 3. Uber die Aehnlichkeit einiger Erscheinungen in der Spermatogenese bei Thieren und Pfianzen: Ibid. — Id., '97, 4. Einige Streit


GENERAL LITERATURE-LIST 45 1

fragen in den Untersuchungen liber die Karyokinese: Ibid. — Id., '98, 1. Uber die Reductionstheilung des Pflanzenkerns : Ibid., XVI. — Id., '98, 2. t)ber die Cilienbildrfer in den spermatogenen Zellen : Ibid. — Id., '99. Uber die Centrosomen in den spermatogenen Zellen: Ibid., XVII., 6. — Benda, C, *87. Untersuchungen liber den Bau des funktionirenden Samenkenkanalchens einiger Saugethiere: A. m. A. — Id., '93. Zellstrukturen und Zelltheilungen des Salamanderhodens : Verh. d. Anat. Ges., 1893. — Van Beneden, B., '70. Recherches sur la composition et la signification de Poeuf : Mhn. cour. de VAc. roy. d. S. de Belgique, 1870. — Id., '75. La maturation de Toeuf, la fe'condation et les premieres phases du de'veloppement embryonnaire des mammiferes d'aprcs des recherches faites chez le lapin: Bull. Ac. roy. de Belgique, XI. — Id., '76, 1. Recherches sur les Dicye'mides: Bull. Ac. roy. Belgique* XLI., XLII. — Id., '76, 2. Contribution a Thistoire de la vdsicule germinative et du premier noyau embryonnaire : Ibid., XLI. ; also Q.J., XVI. — Id., '83. Recherches sur la maturation de l'oeuf, la fe'condation et la division cellulaire: A. B., IV. — Van Beneden and Julin. '84, 1. La segmentation chez les Ascidiens et ses rapports avec l'organisation de la larve: Ibid.,V. — Id., '84, 2. La spermatogenese chez TAscaride mdgaloc^phale : Bull. Ac. roy. Belgique, 3me ser.. VII. — Van Beneden, E., et Neyt, A.. '87. Nouvelles recherches sur la fdcondation et la division mitosique chez TAscaride me'galoce'phale : Ibid., 1887. — Bergh, R. S., '89. Recherches sur les noyaux de TUrostyla : A. B. IX. — Id., '94. Vorlesungen liber die Zelle und die einfachen Gewebe : Wiesbaden. — Id., '95. Uber die relativen Theilungspotenzen einiger Embryonalzellen : A. Entm., II., 2. — Bernard, Claude. Lecons sur les Phe'nomenes de la Vie: 1st Ed. 1878, 2d Ed. 1885, Paris. — Berthold, 0^86. Studien Uber Protoplasma-mechanik : Leipzig. — Bickford, E. E., '94. Notes on Regeneration and Heteromorphosis of Tubularian Hydroids: /. J/., IX., 3. — Biondi. D., '85. Die Entwicklung der Spermatozoiden : A. m. A., XX V. — Blanc, H., -93. Etude sur la fe'condation de Toeuf de la truite : Ber. Naturforsch. Ges. zu Freiburg, VIII. — Blochmann,F.,'87, 2. Uber die Richtungskorper bei lnsekteneiern : M. /., XII. — Id., '88. Uber die Richtungskorper bei unbefruchtet sich entwickelnden Insekteneiern : Verh.naturh. wed. Ver. Heidelberg N. F., IV., 2. — Id., *89. Uber die Zahl der Richtungskorper bei befruchteten und unbefruchteten Bieneneiern : M.J. — Id., '94. Uber die Kerntheilung bei Euglena : B. C.„ X I V. — Btthm, A.. '88. Uber Reifung und Befruchtung des Eies von Petromyzon Planeri : A. ///. A., XXXII. — Id.. '91. Die Befruchtung des Forelleneies : Sitz.-Ber. d. Ges. f. Morph. u. Phys. Munchen, VII. — Boll, Pr., '76. Das Princip des Wachsthums : Berlin. — Bonnet, C 1762. Considerations sur les Corps organ isds : Amsterdam. — Born, O., ^85. Uber den Einfluss der Schvvere auf das Froschei : A. m. A., XXIV. — Id., "94. Die Structur des Keimblaschens im Ovarialei von Triton t«niatus : A. m. A.. XLIII. — Bourne, O. C, '95. A Criticism of the Cell-theory ; being an Answer to Mr. Sedgwick's Article on the Inadequacy of the Cellular Theory of Development; (J. J. XXXVIII., 1— Boveri, Th., '86. Uber die Bedeutung der Richtungskorper : Sitz.-Ber. Ges. Morph. u. Phys. Miinchen, II. — Id., '87, 1. Zellenstudien. Heft I. ; /. Z., XXI. —Id., '87, 2. Uber die Befruchtung der Eier von Ascaris me^alocephala : Sitz.-Ber. Ges. Morph. Phys. Miinchen, III. — Id., '87, 2. Uber den Anteil des Spermatozoon an der Teilung des Eies: Sitz.-Ber. Ges. Morph. Phys. Miinchen* III., 3. — Id., '87,3. Uber Differenzierung der Zellkerne wahrend der Furchung des Eies von Ascaris meg.: A. A., 1887. — Id., '88, 1. Uber partielle Befruchtung : Sitz.-Ber. Ges. Morph. Phys. Miinchen, IV., 2. — Id., '88. 2. Zellenstudien, II. : /. Z., XXII. — Id., '89. Ein geschlechtlich erzeugter Organismus ohne mlitterliche Eigenschaften : Sitz.-Ber. Ges. Morph. Phys. Miinchen. V. Trans, in Am. A r at., March, '93. — Id., '90. Zellenstudien, Heft III . : J. Z., XXIV. — Id., '91. Befruchtung: Merkel und Bonnets Ergebnisse, I. — Id.,


452 GENERAL LITERATURE-LIST

  • 95, 1. Uber die Befruchtungs- und Entwickelungsfahigkeit kernloser Seeigel-Eier,

etc. : A. Entwm. II., 3. — Id., '95, 2. tlber das Verhalten der Centrosomen bei der Befruchtung des Seeigeleies, nebst allgemeinen Bemerkungen iiber Centrosomen und Verwandtes : Verh. d. Physikal.-med. Gesellschaft zu Wurzburg, N. F., XXIX., 1. — Id., '96. Zur Physiologie der Kern- und Zellteilung: Sitsb. Phys.Med. Ges. Wurzburg. — Braem, P., '93. Das Prinzip der organ bilden den Keimbezirke und die entwicklungsmechanischen Studien von H. Driesch : B. C, XI 11., 4, 5. — Brandt, H., '77. Ober Actinosphaerium Eichhornii: Dissertation, Halle, 1877. — Brass, A., 7 83-4. Die Organisation der thierischen Zelle : Halle.— Brauer, A., '92. Das Ei von Branchipus Grubii von der Bildung bis zur Ablage: Abh. preuss. Akad. Wiss., '92. — Id., '93,1. Zur Kenntniss der Reifung des parthenogenetisch sich entwickelnden Eies von Artemia Salina: A. m. A., XLIII.— Id., '93, 2. Zur Kenntniss der Spermatogenese von Ascaris megalocephala : A. m. A., XL1I. — Id., '94. tJber die Encystierung von Actinosphaerium Eichhornii: Z. w. Z., LVIII., 2. — Braus, '95. Uber Zellteilung und Wachstum des Tritoneies: /. Z., XXIX. — Brooks, W. K., '83. The Law of Heredity: Baltimore. Brown, H. H. f '85. On Spermatogenesis in the Rat : Q. /., XXV. — Brown, Robert, '33. Observations on the Organs and Mode of Fecundation in Orchideae and Asclepiadese : Trans. Linn. Soc, 1833. Brticke, C, '61. Die Elementarorganismen : Wiener Sitzbcr., XLIV., 1861. Brunn,M. von, '89. Beitrage zur Kenntniss der Samenkorper und ihrer Entwickelung bei Vogeln und Saugethieren : A. m. A., XXXIII. — De Bruyne, C, '95. La sphere attractive dans les cellules fixes du tissu conjonctif: Bull. Acad. Sc. de Belgian e, XXX. — Biirger, O., '91. Uber Attractionsspharen in den Zellkorpern einer Leibesflussigkeit : A. A., VI. — Id., '92. Was sind die Attractionsspharen und ihre Centralkorper? A. A., 1892. — BUtschli, O., '73. Beitrage zur Kenntniss der freilebenden Nematoden: Nova acta acad. Car. I^eopold, XXXVI. — Id., '75. Vorlaufige Mitteilungen iiber Untersuchungen betreflfend die ersten Entwickelungsvorgange im befruchteten Ei von Nematoden und Schnecken : Z. w. Z., XXV. — Id., '76. Studien iiber die ersten Entwickelungsvorgange der Eizelle, die Zellteilung und die Konjugation der Infusorien : Abh. des Senckcnb. Naturforscher-Ges.. X. — Id., '85. Organisationsverhaltnisse der Sog. Cilioflagellaten und der Noctiluca: M. /., X. — Id., '90. t*ber den Bau der Bakterien, etc.: Leipzig. — Id., '91. tjber die sogenannten Centralkorper der Zellen und ihre Bedeutung: Verh. Naturhist. Med. Ver. Heidelberg 1891. — Id., '92, 1. tlber die kiinstliche Nachahmung der Karyokinetischen Figuren : /bid., N. F\, V. — Id., '92, 2. Untersuchungen iiber mikroskopische Schaume und das Protoplasma (full review of literature on protoplasmic structure) : Leipzig {En^eltnann). — Id., '94. Vorlaufige Berichte iiber fortgesetzte Untersuchungen an Gerinnungsschaumen, etc. : Verh. Naturhist. Ver. Heidelberg V. — Id., '96. Weitere Ausfuhrungen iiber den Bau der Cyanophyceen und Bakterien : Leipzig. — Id., '98. Untersuchungen iiber Strukturen : Leipzig (Engebnann).

CALKINS, O. N., '95, 1. Observations on the Yolk-nucleus in the Eggs of Lumbricus : Trans. N.Y. Acad. So'., June, 1895. — Id., '95, 2. The Spermatogenesis of Lumbricus : /. A/., XL, 2. — Id., '97. Chromatin-reduction and Tetradformation in Pteridophytes : Bull. Torrey Bot. Club, XXIV. — Id., *98, 1. The Phylogenetic Significance of Certain Protozoan Nuclei : Ann. N V. Acad. Sci.. Xl«  16. —Id., '98, 2. Mitosis in Noctiluca: Ginn & Co., Boston, also/. /!/., XV., 3.— Calberla, E., '78. Der Befruchtungsvorgang beim Ei von Petromyzon Planeri: Z.w.Z., XXX. — Campbell, D. H., '88-9. On the Development of Pilularia globulifera : Ann. Bot.. II. — Carnoy, J. B., '84. La biologie cellulaire : Lierre. — Id., '85. La cytodidrese des Arthropodes : La Cellule. I. — Id., '06. La cytodi£rese de Tceuf: La Cellule, III. — Id., '86. La ve'sicule germinative et les globules


GENERAL LITERATURE-LIST 453

polaires chez quelques Nematodes: La Cellule, III. — Id., '86. La segmentation de Toeuf chez les Nematodes: La Cellule* III., 1. — Carnoy and Le Bran, '97, 1, '98, '99. La vdsicule germinative et les globules polaires chez les Batraciens : La Cellule. XII, XIV, XVI. — Id., '97, 2. La fe'eondation chez l'Ascaris megalocephala : La Cellule* XIII. — Castle, W. E., '96. The Early Embryology of Ciona intestinalis : Bull. A/us. Comp. Zoo/., XXVII., 7. — Chabry, L., '87. Contributions a Tembryologie normale et pathologique des ascidies simples: Pan's. 1887.

— Child, C. M., '97. The Maturation and Fertilization of the Egg of Arenicola : Trans. N. Y. Acad. Sci.. XVI. — Chittenden, R. H., '94. Some Recent Chemicophysiological Discussions regarding the Cell: Am. Na/., XXVIII., Feb., 1894. — Chun, C, '90. Uber die Bedeutung der direkten Zelltheilung : Si/zb. Schr. Physik.Ofcon. Ges. K'dnigsberg. 1890. — Id., *92, 1. Die Dissogonie der Rippenquallen : Fes/schr. f. Leuckar/, Leipzig, 1892. — Id., '92, 2. (In Roux, '92, p. 55) : Verh. d. Ana/. Ges., VI., 1892. — Clapp, C. M., '91. Some Points in the Development of the Toad-Fish : /. A/.. V. — Clarke, J. Jackson, '95. Observations on various Sporozoa: Q.J., XXXVII., 3. — Coe, W. R., '99. The Maturation and Fertilization of the Egg of Cerebratulus : Zoo/.Jahrb., XII. — Cohn, Ferd., '51. Nachtrage zur Naturgeschichte des Protococcus pluvialis : Nova Ac/a. XXII. — Conklin, E. O., '94. The Fertilization of the Ovum : Biol. Lee/.. Marine Biol. Lab., Wood's Noll, Bos/on, 1894. — Id., '96. Cell-size and Body-size: Rep/, of Am. A/orph. Soc. Science. III., Jan. 10, 1896. — Id., *97, 1. Nuclei and Cytoplasm in the Intestinal Cells of Land Isopods : Am. Nat.. Jan. — Id., '97, 2. The Embryology of Crepidula : /. A/., XIII., 1.-— Id., '98. Cleavage and Differentiation: Wood's Noll Biol. Uc/ures. — Id., '99. Protoplasmic Movement as a Factor in Differentiation : Wood's Ho// Biol. Lec/ures. — Cramp ton, H. B., '94. Reversal of Cleavage in a Sinistral Gasteropod: Ann. N. Y. Acad. Sci., March, 1894. — Id., '97. The Ascidian HalfEmbryo: Ibid., June 19. — Id., '99. The Ovarian History of the Egg of Molgula: /. J/., XV., Suppl. — Crampton and Wilson, *96. Experimental Studies on Gasteropod Development (H. E. Crampton). Appendix on Cleavage and MosaicWork (E. B. Wilson) : A. En/wm.. III., 1. — Czermak, N., '99. Uber die Desintegration und die Reintegration des Kernkorperchens, etc. : A. A., XV., 22.

DARWIN, P., '77. On the Protrusion of Protoplasmic Filaments, etc. : Q. J. XVII. — Davis, B. M., '99. The Spore-mother-cell of Anthoceros: Bo/. Gaz., XXVIII., 2. — Debski, B., '97. Beobachtungen Uber Kerntheilung bei Chara: J. w. B., XXX.— Id., '98. Weitere Beobachtungen an Chara: Ibid., XXXII., 4.

— Delage, Yves, '95. La Structure du Protoplasma et les Theories sur l'hdre'dite' et les grands Problemes de la Biologie Gdndrale : Paris, 1895. — Id., '98. Embryons sans noyau maternel: C. R.. CXXVII., 15. — Id., '99. La fdcondation me'rogonique et ses rdsultats: C. R.. Oct. 23. — Demoor, J., '95. Contribution a Te'tude de la physiologie de la cellule (inddpendance fonctionelle du protoplasme et du noyau) : A. B., XIII. — Dendy, A., '88. Studies on the Comparative Anatomy of Sponges: Q.J.% Dec, 1888. — Dixon, H. H., '94. Fertilization of Pinus: Ann. Bo/., VIII. — Id., '96. On the Chromosomes of Lilium longliflorum : Proc. R. Ir. Ac, III. — Doflein, P. J., *97, 1. Die Eibildung bei Tubularia: Z. w. Z., LXII., 1. — Id., '97, 2. Karyokinesis des Spermakerus: A. m. A., L, 2. — Dogiel, A. S., '90. Zur Frage uber das Epithel der Harnblase : A. m. A., XXXV. — Driesch, H., '92, 1. Entwickelungsmechanisches : A. A., VII., 18. — Id. Entwicklungsmechanische Studien, I., II., 1892, Z. w. Z., LIU. ; 1 1 1. -VI., 1893, Ibid.. LV. ; VII.-X., 1893: A/itt. Zo'dl. St. Neapel. XL, 2.— Id., '94. Analytische Theorie der organischen Entwicklung: Leipzig. — Id., '95. 1. Von der Entwickelung einzelner Ascidienblastomeren : A. Entwm., L, 3. — Id., '95, 2. Zur Analysis der Potenzen embryonaler Organzellen: Ibid., II. — Id., '98,1. Uber den Organisation des


454 GENERAL LITERATURE-LIST

Eies: Entwm., IV. — Id.. '98, 2. Von der Beendigung morphogener Elementarprocesse: Arch. Entwm. , VI. — Id., '98, 3. Ueber rein-miitterliche Charaktere an Bastardlarven von Echiniden: Ibid., VII., i. — Id., '99. Die Localisation morphogenetischer Vorgange: Ibid., VIII., I. — Drieach and Morgan, '95, 2. Zur Analysis der ersten Entwickelungsstadien des Ctenophoreneies : Ibid.* II., 2.— Drtiner, L., '94. Zur Morphologie der Centralspindel : /. Z., XXVIII. (XXI.). — Id., '95. Studien iiber den Mechanismus der Zelltheilung: Ibid., XXIX., 2. — Dflsing, C, 1 84. Die Regulierung des Geschlechtsverhaltnisses : Jena, 1884.

VON EBNER, V., '71. Untersuchungen iiber den Bau der Samencanalchen und die Entwicklung der Spermatozoiden bei den Saugethieren und beim Menschen: Inst. Phys. u. Hist. Graz., 1871 (Leipzig). — Id., '88. Zur Spermatogenese bei den Saugethieren: A. m. A., XXXI. — Ehrlich, P., '79. Ober die specifischen Granulationen des Blutes: A. A. P. (Phys.), 1879, P- 573- — Bi»en, Gk, *97. Plasmocytes: Proc. Cat. Acad. Sci., I., 1. — Id., '99. The Chromoplasts and the Chromioles: B. C, XIX., 4. — Eismond, J., '95. Einige Beitrage zur Kenntniss der Attraktionsspharen und der Centrosomen: A. A., X. — Endres and "Walter, '95. Anstichversuche an Eiern von Rana fusca : A. Entwm.^ II., 1 . — Engelmann. T. W., '80. Zur Anatomie und Physiologie der Flimmerzellen : Arch. ges. Phys., XXIII. — von Erlanger, R., '96, 1. — Die neuesten Ansichten liber die Zelltheilung und ihre Mechanik: Zoo/. Centralb., III., 2. — Id., '96, 2. Zur Befruchtung des Ascariseies nebst Bemerkungen iiber die Struktur des Protoplasmas und des Centrosomas: Z. A., XIX. — Id., '96, 3. Neuere Ansichten iiber die Struktur des Protoplasmas: Zoo/. Centralb., III., 8,9. — Id., '96, 4. Zur Kenntniss des feineren Baues des Regenwurmhodens, etc. : A. m. A., XLVII. — Id., '96, 5. Die Versonische Zelle : Zo'dl. Centralb., III., 3. — Id., '96, 6. Die Entwicklung der mannlichen Geschechtszellen : Ibid., III., 12. — Id., '97, 1. tlber Spindelreste und den echten Nebenkern, etc.: Zo'dl. Centralb., IV., 1. — Id., '97, 2. Uber die sogenannte Sphare in den mannlichen Geschlechtszellen : Ibid., IV., 5. — Id., '97, 3. t*ber die Chromatinreduktion in der Entwicklung der mannlichen Geschlechtszellen : Ibid., IV., 8. — Id., '97, 4. Beitrage zur Kenntniss des Protoplasmas, etc. A. m. A., XLIX. — Id., '97,5. tU)er die Spindelbildung in den Zellen der Cephalopoden Keimscheibe : /?. C, XVII., 20. — Id., '98. Tber die Befruchtung, etc., des Seeigeleies : B. C, XVIII., 1. — Errera, *86. Eine fundamentale Gleichgewichtsbedingung organischen Zellen: Ber. Deutsch. Bot. Ges., 1886. — Id., '87. Zellformen und Seifenblasen : TagcbL der 60 Versammlung deutschcr Naturforscher und AerzU zu Wiesbaden, 1887.

FAIRCHILD, D. O., '97. t'ber Kerntheilung und Befruchtung bei Basidiobolus : Jahrb. wiss. Bot.. XXX. — Farmer, J. B., '93. On nuclear division of the pollen-mother-cell of Lilium Martagon : Ann. Bot. VII., 27. — Id.. '94. Studies in Hepaticae: Ibid., VIII., 29. —Id., '95, 1. Uber Kernteilung in Lilium-Antheren, besonders in Bezug auf die Centrosomenfrage : Flora, 1895, p. 57. — Id., 95, 2. On Spore-formation and Nuclear Division in the Hepaticae: Ann. Bot., IX. — Farmer and Moore, '95. On the essential similarities existing between the heterotype nuclear divisions in animals and plants: A. A., XI., 3. — Farmer and Williams, '96. On Fertilization, etc., in Fucus : Ann. Bot., X. — Pick, R., '93. t*ber die Reining und Befruchtung des Axolotleies : Z.w.Z., LVI., 4. — Id., '97. Bemerkungen zu M. Heidenhain's Spannungsgesetz : Arch. Anat. //. Phys. (Anat.). — Fiedler, C, '91. Entwickelungsmechanische Studien an Echinodermeneiern : Festschr. Nagcli u. Kolliker. Zurich, 1891. — Field, O. W., '95. On the Morphology and Physiology of the Echinoderm Spermatozoon: /. J/., XI. — Fischer, A., '94, 1. Zur Kritik der Fixierungsmethoden der Granula: A. A., IX., 22.—


GENERAL LITERATURE-LIST 455

Id., 94, 2. — t'ber die Geisseln einiger Flagellaten: /. w. B. XXVII. — Id., '95. Neue Beitrage zur Kritik der Fixierungsmethoden : A. A., X. — Id., '97. Untersuchungen iiber den Bau der Cyanophyceen und Bakterien : Jena* Fischer. — Id., '99. Fixierung, Farbung und Bau des Protoplasmas : Ibid. — Flemming, W., '75. Studien in der Entwicklungsgeschichte der Najaden: Sitzb. d. k. k. Akad. IViss. IVien, LXXI., 3. — Id., '79,1. Beitrage zur Kenntniss der Zelle und ih re Lebenserscheinungen, I. : A. m. A., XVI. — Id., '79, 2. Uber das Verhalten des Kerns bei der Zelltheilung, etc.: Virchow's Arch., LXXVII. — Id.. '80. Beitrage zur Kenntniss der Zelle und ihrer Lebenserscheinungen, II. : A. m. A., XIX. — Id., '81. Beitrage zur Kenntniss der Zelle und ihrer Lebenserscheinungen, III. : Ibid., XX.— Id., '82. Zellsubstanz, Kern und Zellteilung: Leipzig, 1882. — Id., '87. Neue Beitrage zur Kenntniss -der Zelle: A. m. A., XXIX. — Id., '88. Weitere Beobachtungen iiber die Entwickelung der Sperm atosomen bei Salamandra maculosa : Ibid., XXXI. — Id., '91-'97. Zelle, I.-VI. : Ergebn. Anat. u. Entwicklungsgesch. {Merkel and Bonnet), 1891-97. — Id., '91,1. Attraktionsspharen u. Centralkorper in Gewebs- u. Wanderzellen : A. A. — Id., '91, 22. Neue Beitrage zur Kenntniss der Zelle, II. Teil: A. m. A., XXXVII.— Id., '95, 1. Uber die Struktur der Spinaiganglienzellen : Verhandl. der anat. Gesellschaft in Basel, 17 April, 1895, p. 19. — Id., '95, 2. Zur Mechanik der Zelltheilung -.A.m. A., XLVI. — Id., '97, 2. Ueber den Bau der Bindegewebszellen, etc.: Zeit. Biol., XXXIV '. — Floderus, M., '96. tJber die Bildung der Follikelhullen bei den Ascidien : Z. w. Z., LXI., 2. — Pol, H., '73. Die erste Entwickelung des Geryonideies : /. Z., VII. — Id., '75. Etudes sur le deVeloppement des Mollusques. — Id., '77. Sur le commencement de Thdnogenie chez divers animaux : Arch. Sci. Nat. et Phys. Geneve, LVIII. See also Arch. Zool. Exp., VI. — Id., '79. Recherches sur la fecondation et la commencement de The'nogenie: Mbn. de la Soc. de physique et d^hist. nat., Geneve. XXVI. — Id., '91. Le Quadrille des Centres. Un episode nouveau dans Thistoire de la fe'condation : Arch, des sci. phys. et nat., 15 Avril, 1891 ; also, A. A., 9-10. 1891. — Foot, K., '94. Preliminary Note on the Maturation and Fertilization of Allolobophora : J. M., IX., 3, '94. — Id., '96. Yolk-nucleus and Polar Rings: Ibid., XII., 1. — Id., '97. The Origin of the Cleavage Centrosomes: /. M., XII., 3. — Francotte, P., '97. Recherches sur la maturation, etc., chez les Polyclades : Mem. cour. Acad. Sci. Belg.

— Frenzel, J., '93. Die Mitteldarmdruse des Flusskrebses und die amitotische Zelltheilung: A. m. A., XLI. — Fromman, C, '65. t)ber die Struktur der Bindesubstanzzellen des RUckenmarks: Centrl. f. med. Wiss., III., 6. — Id., '75. Zur Lehre von der Structur der Zellen: /. Z., IX. (earlier papers cited). — Id., '84. Untersuchungen iiber Struktur, Lebenserscheinungen und Reactionen thierischer und pflanzlicher Zellen: J.Z., XVII. — Fiirst, E., '98. Uber Centrosomen bei Ascaris: A. m. A., LII. — Fulmer, E. L., '98. Cell-division in Pine Seedlings: Bet. Gaz., XXVI., 4.

GAIiEOTTI, GINO, '93. t!ber experimentelle Erzeugung von Unregelmassigkeiten des karyokinetischen Processes: Bei. zur patholog. Anat. u. z. Altg. Pathol., XIV '.,2, Jena, Fischer, 1893. — Gallardo, Angel, '96. La Carioquinesis : Ann. Soc. Cientif. Argentina, XLII. — Id., '97. Significado Dinaraico de las Figuras Cariocineticas : Ibid., XLIV. — Gardiner, E. G., '98. The Growth of the Ovum, etc., in Polvchoerus: /. M., XV., 1. — Gardiner, W., '83. Continuity of Protoplasm in Vegetable Cells: Phil. Trans., CLXXIV. — Garnault, '88, '89. Sur les phe'nomenes de la fe'condation chez Helix aspera et Arion empiricorum : Zool. Anz., XI., XII. — Geddes and Thompson. The Evolution of Sex: London, 1899. —

— Gegenbaur, C, '54. Beitrage zur naheren Kenntniss der Schwimmpolypen : Z. w. Z., V. — Van Gehuchten, A., '90. Recherches histologiques sur Tappareil digestif de la larve de la Ptychoptera contaminata : La Cellule, VI. — Giard, A., '77.


456 GENERAL LITERATURE-LIST

Sur la signification morphologique des globules polaires : Revue scientifiqut, XX. — Id., '90. Sur les globules polaires et les homologues de ces e'le'ments chez les infusoires cilids : Bulletin scientifique de la France et de la Belgique, XXII. — Godlewsky, E., '97, 1. t v ber mehrfache bipolar Mitose bei der Spermatogenese von Helix: Anz. Akad. Wiss. Krakau. — Id., '97, 2. Weitere Untersuchungen iiber die Umwandlung der Spermatiden, etc. : Am. Akad. Wiss. Krakau., Nov., "97.

— Goroschanktin, J., '83. Zur Kenntniss der Corpuscula bei den Gymnospermen : Bot. Zeit., LXI. — Graf, A., '97. The Individuality of the Cell : N. Y. State Hosp. Bull., April. — Gr6goire, V., "99. Les cineses polliniques dans les LiJiacees: Bot. Centb., XX., 1 ; La Cellule. XVI., 2. — Griffin, B. B., , 96. The History of the Achromatic Structures in the Maturation and Fertilization of Thalassema : Trans. N. Y. Acad. Set. — Id., '99. Studies on the Maturation, Fertilization, and Cleavage of Thalassema and Zirphaea : /. AI., XV. — Gierke, H., -85. Farberei zu mikroskopischen Zwecken : Zeit. Wiss. Alik., II. — Grobben, C, '78. Beitrage zur Kenntniss der mannlichen Geschlechtsorgane der Dekapoden : Arb. Zool. Inst. Witn* I.

— Gruber, A., '84. tJber Kern und Kerntheilung bei den Protozoen : Z. w. Z~> XL. — Id., '85. liber kiinstliche Teilung bei Infusorien: B. C, IV., 23; V., 5.— Id., '86. Beitrage zur Kenntniss der Physiologie und Biologie der Protozoen : Ber. Naturf. Ges. Freiburg,!. — Id., '93. Mikroscopische Vivisektion : Ber. d. IVafnrf. Ges. zu Freiburg, VII., 1. — Id., *97. Weitere Beobachtungen an vielkernigen Infusorien : Ber. Naturf. Ges. Freiburg, III. — Guignard, L., '89. De'veloppement et constitution des Antherozoides : Rev. gen. Bot., I. — Id., '91, 1. Nouvelles eludes sur la fe'eondation : Ann. d. Sciences Nat. Bot., XIV. — Id., '91, 2. Sur Texistence des '* spheres attractives" dans les cellules ve'ge'tales: C.R., 9 Mars. — Id., ^98, 1. Les centres antiques chez les ve*ge*taux : Ann. Sci. Nat. Bot., (VIII.) V. ; also, Bot. Gaz., XXV. — Id., '98, 2. Le developpement du pollen et la reduction chromatique dans le Nat's major : Arch. Anat. Mik., II., 4. — Id., '99. Sur les antherozoides et la double copulation sexuelle chez les ve'ge'taux angiospermes : C. R., CXXVIII., 14.

HABERLANDT, G.. '87. tJber die Beziehungen zwischen Funktion und Lage des Zellkerns: Fischer, 1887. — Hackel, E., '66. Generelle Morphologie. — Id.. '91. Anthropogenic 4th ed.. Leipzig, 1891. — Hacker, V., 92, 1. Die Furchung des Eies von /Equorea Forskalea: A. tn. A., XL. — Id., "92, 2. Die Eibildung bei Cyclops unci Canthocamptus : Zool. fahrb., V. — Id., '92, 3. Die heterotypische Kerntheilung im Cyclus der generativen Zellen : Ber. naturf. Ges. Freiburg. VI. — Id, '93. Das Keimblaschen, seine Elemente und Lageveranderungen : A. tn. A.. XLI. — Id.. '94. t*ber den heutigen Stand der Centrosomenfrage : Verhandl. d. deutschen Zool. Ges.. 1894, p. 11. — Id.. *95. 1. Die Vorstadien der Eireifung: A. t/t. A., XLV., 2. — Id., '95, 2. Zur frage nach dem Vorkommen der Schein-Reduktion bei den Pflanzen : Ibid., XLVI. Also Ann. Bot., IX. — Id., '95, 3. Uber die Selbstandigkeit der vaterlichen und miitterlichen Kernsbestandtheile wahrend der Embryonalentwicklung von Cyclops: A. m. A., XLVI.. 4.

— Id., '97,1. Die Keimbahn von Cyclops : A. tn. A., XLIX. — Id., "97, 2. Tber weitere t^bereinstimmungen zwischen den Fortpflanzungsvorgangen der Thiere und Pflanzen : B. C, XVII. — Id., "98. t v ber vorbereitende Theilungsvorgange bei Thieren und Pflanzen: Verh. d. Zool. Ges., VIII. — Id.. '99. Praxis und Theorie der Zellen und Befruchtungslehre : fena, Fischer. — Hallez. P.. *86. Sur la loi de l'orientation de l'embryon chez les insectes : C. R., 103. 1886. — Halliburton, W. D.. '91. A Text-book of Chemical Physiology and Pathology: London.

— Id.. '93. The Chemical Physiology of the Cell: (Gouldstonian Lectures) Brit. Aled. fount. — Hammar, J. A., '96. Cber einen primaren Zusammenhang zwischen den Furchungszellen des Seeigeleies : A. tn. A.. XLVII., 1. — Id.. '97. t*ber einc allgemein vorkommende primare Protoplasmaverbindung zwischen den Bias


GENERAL LITERATURE-LIST 457

tomeren: A. m. A., XL1X. — Hammarsten, O., ^4. Zur Kenntniss der Nucleoproteiden: Zeit. Phys. Chem., XIX. — Id., '95. Lehrbuch der physiologischen Chemie, 3e Ausgabe : Wiesbaden, 1895. — Hansemann, D., '91. Karyokinese und Cellularpathologie : Berl. Klin. Wochenschrift, No. 42. — Id., '93. Spezificitat, Altruismus und die Anaplasie der Zellen: Berlin, 1893. — Hanstein, J., ? 80. Das Protoplasma als Trager der pflanzlichen und thierischen Lebensverrichtungen. Heidelberg. — Harper, R. A., , 96. Cber das Verhalten der Kerne bei der Fruchtentwickelung einiger Ascomyceten: Jahrb. wiss. Bot., XXIX. — Id., '97. Kernteilung und freie Zellbildung im Ascus: Ibid., XXX. — Hardy, W. B., '99. On the Structure of Cell-protoplasm: Jour. Phys.. XXIV., 2. — Harvey, Wm., 1651. Exercitationes de Generatione Animalium : London. Trans, in Sydenham Soc, X., 1847. — Hartog, M. M., '91. Some Problems of Reproduction, etc. : Q.J., XXXIII. — Id., '96. The Cytology of Saprolegnia: Ann. Bot., IX. — Id., '98. Nuclear Reduction and the Function of Chromatin: Nat. Sci., XII. — Hatschek, B., '87. Cber die Bedeutung der geschlechtlichen Fortpflanzung: Prager Med. Wochenschrift, XLVI. — Id., , 88. Lehrbuch der Zoologie.— Heath, H., '99. The Development of Ischnochiton : Jena, Fischer. — Heidenhain, M., ^93. liber Kern und Protoplasma : Festchr. z. jo-Jdhr. Doctorjub. von v. Kolliker : Leipzig. — Id., , 94. Neue Untersuchungen liber die Centralkorper und ihre Beziehungen zum Kern und Zellenprotoplasma : A. m. A.,XLlll. — Id., '95. Cytomechanische Studien : A. Entwm., I., 4. — Id., '96, 1. Ein neues Modell zum Spannungsgesetz der centrirten Systeme : Verh. anat. Gcs. — Id., '96, 2. Uber die Mikrocentren mehrkerniger Ricsenzellen, etc. : Morph. Arb., VII., 1. — Id., '99. Uber eine eigenthtimliche Art Knospung an Epithelzellen, etc. : A. m. A., LIV., 1. — Heidenhain and Conn,' 97. Uber die Mikrocentren in den Geweben des Vogelembryos, etc.: Morph. Arb., VII. — Heitzmann, J., '73. Untersuchungen liber das Protoplasma : Sits. d. k. Acad. Wiss. Wien., LXVII. — Id., '83. Mikroscopische Morphologie des Thierkorpers im gesunden und kranken Zustande : Wien, 1883. — Henking, H. Untersuchungen Uber die ersten Entvvicklungsvorgange in den Eiern der Insekten, I., II., III.: Z. w. Z., XLIX., LI., LIV., 1890-92. — Henle, J., '41. Allgemeine Anatomie: Leipzig. — Henneguy, L. P., '91. Nouvelles recherches sur la division cellulaire indirecte: Joum. Anat. et Physiol., XXVII. — Id., 93. Le Corps vitellin de Balbiani dans Toeuf des Ve'rte'bres : Ibid., XXIX. — Id., ^96. Lecons sur la cellule: Paris. — Id., '98. Sur les rapports des cils vibratils avec les cgntrosomes : Arch. Anat. Mik., 1. — Hensen, V., '81. Physiologic der Zeugung : Hermann's Physiologie, VI. — Herbst, C. Experimented Untersuchungen uber den Einfluss der veranderten chemischen Zusammensetzung des umgebenden Mediums auf die Entwicklung der Thiere, I. ; Z.w. Z., LV., 1892 ; lh,Mitt. Zool. St. Neapet,Xh, 1893; Ul.-Vl., Arch. Entwm., II., 4, 1896. — Id., '94, '95. Uber die Bedeutung der Reizphysiologie fiir die Kausale AufFassung von Vorgangen in der tierischen Ontogenese: Biol. Centralb., XIV., XV., 1894, 1895. — Herla, V., '93. Etude des variations de la mitose chez Tascaride me'galoce'phale : A. B., XIII. — Herlitzka, A., '95. Contributo alio studio della capacita evolutiva dei due primi blastomeri neir uove di Tritone: A. Entwm., II., 3. — Hermann. F., '89. Beitrage zur Histologic des Hodens : A. m. A.. XXXIV. — Id., '91. Beitrag zur Lehre von der Entstehung der karyokinetischen Spindel: Ibid., XXXVII. — Id., "92. Urogenitalsystem, Struktur und Histiogenese der Spermatozoen : Merkel und Bonnet's Ergebnisse, II. — Id., '97. Beitrage zur Kenntniss der Spermatogenese: A. m. A., L. — Hertwig, O., '75. Beitrage zur Kenntniss der Bildung, Befruchtung und Teilung des tierischen Eies, I. : M. J., I. — Id., -77. Beitrage, etc., II. ; Ibid., III. — Id., '78. Beitrage, etc.. III. ; Ibid., IV. — Id., '84. Das Problem der Befruchtung und der Isotropic des Eies, eine Theorie der Vererbung : /. Z., XVIII. — Id., '90, 1. Vergleich der Ei- und Samenbildung bei Nematoden. Eine


45 8 GENERAL LITERATURE-LIST

Grundlage fur cellulare Streitfragen : A. m. A., XXXVI. — Id., "90, 2. Experimentelle Studien am tierischen Ei vor, wahrend und nach der Befruchtung : /. Z., 1890. — Id., '92, 1. — Urmund und Spina Bifida: A, m. A., XXXIX. — Id., 92,2. Aeltere und neuere Entwicklungs-theorieen : Berlin. — Id., '93, 1. Uber den Werth der ersten Furchungszellen fiir die Organbildung des Embryo : A. m, A^ XLII.— Id., 93, 2. Die Zelle und die Gewebe: Fischer, Jena, 1893, 1898.— Id., '94. Zeit und Streitfragen der Biologie: Berlin. — Hertwig, O. and R., "86. Experimentelle Untersuchungen Uber die Bedingungen der Bastard befruchtung : /. Z., X IX . — Id., '87. tlber den Befruchtungs- und Teilungsvorgang des tierischen Eies unter dem Einfluss ausserer Agentien : Ibid., XX. — Hertwig, R., "77. Uber den Bau und die Entwicklung der Spirochona gemmipara: Ibid.* XI. — Id.. "84. Die Kerntheilung bei Actinosphaerium Eichhorni : Ibid., XVII. — Id., '88. Uber Kernstruktur und ihre Bedeutung fiir Zellteilung und Befruchtung: Ibid., IV., 1888.

— Id., '89. Uber die Konjugation der Infusorien: Abh. der bayr. Akad. d. Hiss., II., CI., XVII. — Id., '92. Uber Befruchtung und Conjugation : Verh. deutsch. Zool. Ges., Berlin. — Id., "95. Uber Centrosoma und Centralspindel : Silz.-Ber. Ges. Morph. und Phys., Miinchen, 1805, Heft I. —Id., '96. Uber die Entwicklung des unbefruchteten Seeigeleies, etc. : Festchr. f. Gegenbaur. — Id., *97, 1. tJber die Bedeutung derNucleolen : Silzb. Ges. Morph. Phys. Miinchen, 1898, I. — Id., *97, 2.

— Uber Karyokinese bei Actinosphaerium : Silzb. Ges. Morph. Phys. Miinchen, XIII.,

1. — Id., '98. Kerntheilung, Richtungskorperbildung und Befruchtung von Actinosphaerium: Abh. A", bayer. Akad. Wiss., XIX, 2. — Heuser, E., 84. Beobachtung uber Zelltheilung : Bol. Cent. — Hill, M. D., '95. Notes on the Fecundation of the Egg of Sphcerechinus granularis and on the Maturation and Fertilization of the Egg of Phallusia mammillala: Q. /., XXXVIII. — Hirase, S., 37. Untersuchungen liber das erhalten des Pollens von Gingko biloba : Bol. Centb., LXIX.,

2, 3. — Id., '98. fitudes sur la fe*condation et rembryogc"nie der Gingko: Jour. Coll. Sci., Tokio, XII. — His. W.,'74. Unsere Korperform und das physiologische Problem ihrer Entstehung : Leipzig. — Hofer, B., -89. Experimentelle Untersuchungen iiber den Einfluss des Kerns auf das Protoplasma: J. Z., XXIV. — Hoffman, R. W., '98. Uber Zellplatten und Zellplattenrudimente : Z. w. Z.. LXIII.

— Hofmeister. '67. Die Lehre von der Pflanzenzelle : Leipzig, 1867. — Holl, M.. '90. Uber die Reifung der Eizelle des Huhns : Silzb. Acad. Wiss. H'ien. XCIX., 3. — Hooke, Robt., 1665. Mikrographia, or some physiological Descriptions of minute Bodies by magnifying Glasses: London. — Hoyer. H.. *90. t*ber ein fiir das Studium der " direkten v Zelltheilung vorzliglich geeignetes Objekt : A. A., V. — Hubbard, J. W., '94. The Yolk-Nucleus in Cymatogaster : Proc. Am. Phil. Soc, XXXIII. — Huie. L.. '97. Changes in the Cell-organs of Drosera produced by Feeding with Egg-albumen: Q. J., XXXIX. — Humphrey, J. B., '94. Nucleolen und Centrosomen : Per. deutschen bot. Ges., XII., 5. — Id.. "95. On some Constituents of the Cell: Attn. Bot.. IX. — Huxley. T. H., '53. Re\iew of the Cell-theory: Brit, and Foreign Med.-Chir. Review, XII. — Id., '78. Evolution in Biology, Enc. Brit., 9th ed., 1878; Science and Culture, N. Y., 1882.

IKENO, S., '97. Vorlaufige Mitth. uber die Spermatozoiden bei Cycas: Bot. Ccntb., LXIX., 1. — Id., '98, 1. Zur Kenntniss des sogenannten centrosomahnlichen Korpers im Pollenschlauche der Cycaden : Flora, LXXXV., 1. — Id., '98, 2. Untersuchungen iiber die Entwickelung der Geschlechtsorgane, etc., bei Cycas : Jahrb. wiss. Bot.. XXXII., 4. — Ishikawa, M., '91. Vorlaufige Mitteilungen Uber die Konjugationserscheinungen bei den Noctiluceen : Z. A., No. 353, 1891. — Id., '94. Studies on Reproductive Elements: II., Noctiluca miliar is Sur., Its Division and Spore-formation: Journ. College of Sc. Imp. Univ. Japan, VI. — Id., *97. Die


GENERAL LITERATURE-LIST 459

Entwickelung der Pollenkorner von Allium: Journ. Coll. Sci. Tokyo, X., 2. — Id.,

  • 99. Further Observations on the Nuclear Division of Noctiluca : Ibid., XII., 4.

JENNINGS, H. S., '96. The Early Development of Asplanchna: Bull. Mus. Comp. Zool., XXX. — Jensen, O. S., '83. Recherches sur la spermatoge'nese : A. B., IV. — Johnson, H. P., '92. Amitosis in the embryonal envelopes of the Scorpion: Bull. Mus. Comp. Zool., XXII., 3. — Jordan, E. O., '93. The Habits and Development of the Newt: /. M., VIII., 2. — Jordan and Eycleshymer, '94. On the Cleavage of Amphibian Ova: /. A/., IX., 3, 1894. — Juel, H. O., '97. Die Kerntheilungen in den Pollenmutterzellen, etc. : Jahrb. wiss. Bot., XXX. — Julin, J., "93, 1. Structure et developpement des glandes sexuelles, ovogdnese, spermatoge'nese et fe'eondation chez Styleopsis grossularia : Bull. Sc. de France et de Belgique, XXIV. — Id., '93, 2. Le corps vitellin de Balbiani et les eUe'ments des Me'tazoaires qui correspondent au Macronucle\is des Infusoires cilie's: Ibid., XXIV.

KARSTEN, G, 96. Untersuchungen tiber Diatomeen : Flora, LXXXIL — Keuten, J., ^5. Die Kerntheilung von Euqlena viridis Ehr: Z.w.Z., LX. — Kienitz-Gerloff, P., '91. Review and Bibliography of Researches on Protoplasmic Connection between adjacent Cells: in Bot. Zeitung, XLIX. — Kingsbury, B. P.,

  • 99. The Reducing Divisions in the Spermatogenesis of Desmognathus : Zool.

Bull.. II., 5. — Klebahn, '90. Die Keimung von Closterium und Cosmarium : Jahrb. wiss. Bot., XXII. — Id., '92. Die Befruchtung von CEdigonium : Jahrb. f. wiss. Bot., XXIV. — Id., '96. Beitrage zur Kenntniss der Auxosporenbildung, I., Rhopalodia : JaJtrb. wiss. Bot., XXIX. — Klebs, G., '83. Uber die Organisation einiger Flagellaten-Gruppen, etc. : Bot. Inst. Tubingen, I., 1. — Id., '04. Uber die neueren Forschungen betreffs der Protoplasmaverbindungen benachbarter Zellen : Bot. Zeit., 188.4 — Id., '87. Uber den Einfluss des Kerns in der Zelle: B. C, VII. — Klein, E., , 78- , 79. Observations on the Structure of Cells and Nuclei : Q. J., XVIII., XIX.

— Klinckowstr&m, A. v., '97. Beitrage zur Kenntniss der Eireife und Befruchtung bei Prostheceraeus : A. m. A., XLVIII. — von K&lliker, A., '41. Beitrage zur Kenntniss der Geschlechtsverhaltnisse und der Samenflussigkeit wirbelloser Tiere : Berlin. — Id., '44. Entwicklungsgeschichte der Cephalopoden : Zurich. — Id., '85. Die Bedeutung der Zellkerne fur die Vorgange der Vererbung: Z. w. Z., XLII. — Id., '86. Das Karyoplasma und die Vererbung, eine Kritik der Weismann'schen Theorie von der Kontinuitat des Keimplasmas : Ibid., XLIII. — Id., '89. Handbuch der Gewebelehre, 6th ed. : Leipzig. — Id., 97. Die Energiden von Sachs, etc.: Verh. Phys. Med. Ges., Wurzburg, XXXI., 5. — Korff, 39. Zur Histogenese der Spermien von Helix : A. m. A., LIV. Korschelt, E., '89. Beitrage zur Morphologie und Physiologie des Zell-kernes: Zool. Jahrb. Anat. u. Ontog., IV. — Id., ^3. Uber Ophryotrocha puerilis: Z. w. Z., LIV. — Id., '95. Uber Kerntheilung, Eireifung und Befruchtung bei Ophryotrocha puerilis : Ibid., LX. — Id., '96. Kernstructuren und Zellmembranen in den Spinndrusen der Raupen: A. m. A., XLVII.

— Id., 37. Uber den Bau der Kerne in den Spinndrusen der Raupen : Ibid., XLIX.

— Kossel, A., '91. Uber die chemische Zusammensetzung der Zelle : Arch. Anat. u , Phys. — Id., 33. Uber die Nucleinsaure : Ibid., 1893. —Id., '96. Uber die basischen Stoffe des Zellkernes : Zeit. Phys. Chem., XXII. — von Kostanecki, K., 31 . Uber Centralspindelkorperchen bei karyokinetischer Zellteilung : Anat. Hefte, 1892. dat. 91. — Id., "96. Uber die Gestalt der Centrosomen im befruchteten Seeigelei: Ibid,, VII., 2. — Id., '97, 1. Uber die Bedeutung der Polstrahlung, etc. : A. m. A., LXIX. — Id., '98. Die Befruchtung des Eies von Myzostoma : Ibid., LI. — Kostanecki and Siedlecki, 96. Uber das Verhalten der Centrosomen zum Protoplasma: Ibid., XLIX. — Kostanecki and Wierzejski, 1 96. Uber das Verhalten der sogenannten achromatischen Substanzen im befruchteten Ei : Ibid., XLII., 2. — Ktthne, W., '64. Untersuchungen Uber das Protoplasma und die Con


460 GENERAL LITERATURE-LIST

tractilitat. — Kupffer, C, '75. tfber Differenzierung des Protoplasma an den Zellen thierischer Gewebe: Schr. natur. Ver. Schles.-Holst '.* I., 3. — Id., '90. Die Entwicklung von Petromyzon Planeri: A. m. A., XXXV. — Id., *96. tTber Energiden und paraplastische Bildungen : Rek/oratrede* Munchen* 1896.

LAMEERE, A., *90. Recherches sur la reduction karyogamique : BruxelUs. — Lauterborn, R., '93. Uber Bau und Kerntheilung der Diatomeen : Verh. d. Natur h. Med. Ver. in Heidelberg* 1893. — Id., '95. Protozoenstudien, Kern- und Zellteilung von Ceratium hirundinella O. F. M. : Z. w. Z.* XLIX. — Id., *96. — La Valette St. George, '65. tlber die Genese x der Samenkdrper : A. m. A.* 1.— Id., '67. t)ber die Genese der Samenkorper, II. (Terminology): Ibid.* III.— Id., '76. Die Spermatogenese bei den Amphibien: Ibid.* XII. — Id., ^78. Die Spermatogenese bei den Saugethieren und dem Menschen : Ibid., XV. — Id., , 85-*87. Spermatologische Beitrage, I.-V.: Ibid, XXV., XXVII., XXVIIL, and XXX.

— Lankester, E. Ray, '77. Notes on Embryology and Classification : London.— Lavdovsky, M., 1 94. Von der Entstehung der chromatischen und achromatischen Substanzen in den tierischen und pnanzlichen Zellen : Merkel und Bonnets Anal. Hef/e* IV., 13. — Lawson, A. A., '98. Some Observations on the Development of the Karyokinetic Spindle, etc. : Proc. Cat. Acad. Sci., I., 5. — Lazarus, A., *S8. Die Anaemie: Wien. — Lee, A. Bolles, , 96. Sur le Nebenkern, etc., chez Helix: La Cellule* XI. — Id., '97. Les cineses spermatoge'ne'tiques chez Helix: Ibid., XIII. — von LenhoBse'k, M., 95. Centrosom und Sphare in den Spinalganglien des Frosches: A. m. A., XLVI. — Id., '98, 1. Uber Flimmerzellen : Verh. An. Ges.* XII. — Id., '98, 2. Untersuchungen Uber Spermatogenesis: A. m. A., LI. — Id., '99. Das Mikrocentrum der glatten Muskelzellen : A. A., XVI., 13, 14. — Ley dig, Pr., '54. Lehrbuch der Histologic des Menschen und der Thiere : Frankfurt. — Id., '85. Zelle und Gewebe, Bonn. — Id., '89. Beitrage zur Kenntniss des thierischen Eies im unbefruchteten Zustande : SpengeVsJahrb. A not. On/.. III.

— Lilienfeld, L., '92, ^93. Uber die Verwandtschaft der Zellelemente zu gewissen Farbstoflfen: ' Verh. Phys. Ges., Berlin, 1892-93. —Id., '93. Uber die Wahlverwandtschaft der Zellelemente zu Farbstoflfen : A. A. P.* 1893. — Lillie, P. R.. '95. The Embryology of the Unionidae : /. J/., X. — Id.,. '96. On the Smallest Parts of Stentor capable of Regeneration : /. M.* XII., 1. — Id., '97. On the Origin of the Centres of the First Cleavage-spindle in Unio: Science, V. — Id., *98. Centrosome and Sphere in the Egg of Unio: Zool. Bull.. I.. 6. — Id., '99. Adaptation in Cleavage : Wood's Moll Biol. Led. — List, Th., '96. Beitrage zur Chemie der Zelle und Gewebe, I. : Mitth. Zool. St. Neap.* XII., 3. — Loeb, J., '91-'92. Untersuchungen zur physiologischen Morphologic I. Heteromorphosis : HUrzburg. 180J. II. Organbildung und Wachsthum : Ibid.* 1892. — Id., '92. Experiments on Cleavage : J. M.* VII. — Id., '93. Some Facts and Principles of Physiological Morphology: Wood's H oil Biol. Lectures* 1893. — Id., "94. Uber die Grenzen der Theilbarkeit der Eisubstanz : A. %cs. P.* LIX., 6, 7. — Id., ^5. Cber Kerntheilung ohne Zelltheilung : Arch. Entwm.. II. — Id., "99, 1. Warum istdie Regeneration kernloser Protoplasmastiicken unmoglich, etc.: Ibid.* VIII., 4. — Id., "99. 2. On the Nature of the Process of Fertilization and the Artificial Production of Normal Larv;t\ etc. : Am. Journ. Phys.. III., 3. — Ldwit, M., '91. Uber amitotische Kerntheilung: B. CXI. — Lukjanow, '91. Grundzuge einer allgemeinen Pathologic der Zelle: Leipzig. — Lustig and Galeotti, *93. Cytologische Studien uber pathologische menschliche Gewebe : Bcitr. Path. Ana/.* XIV.

MACALLUM. A. B., '91. Contribution to the Morphology and Physiology of the Cell: Trans. Canad. Inst.. I., 2.— McClung, C. E., '99. A Peculiar Nuclear Element in the Male Reproductive Cells of Insects: Zool. Bull.* II., 4. — MacFar


GENERAL LITERATURE-LIST 46 1

land. F. M., '97. Cellulare Studien an Molluskeneiern : Zool.Jahrb. Anat., X. — McGregor, J. H., '99. The Spermatogenesis of Amphiuma: /. Af., XV., Suppl. — McMurrich, J. P., '86. A Contribution to the Embryology of the Prosobranch Gasteropods: Studies Biol. I jab. Johns Hopkins Univ., III. — Id., '95. Embryology of the Isopod Crustacea: /. Af., XL, 1. — Id., '96. The Yolk-Lobe and the Centrosome of Fulgur : A. A., XII., 23. —Id., '97. The Epithelium of the Midgut of the Terrestrial Isopods: /. Af., XIV., 1. — Maggi, L., '78. I plastiduli nei ciliati ed i plastiduli liberamente viventi: Atti. Soc. Ital. Sc. Nat. Afilano, XXI. (also later papers). — Malfatti, H., *91. Beitrage zur Kenntniss der Nucleine: Zeit. Phys. Chem., XVI. — Mark, E. L., *81. Maturation, Fecundation, and Segmentation of Limax campestris: Bull. Afus. Cotnp. Zool. Harvard College* VI. — Mathews, A. P., '97, 1. Internal Secretions considered in Relation to Variation and Development: Science, V., 122. — Id., '97, 2. Zur Chemie der Spermatozoen : Zeit. Phys. Chem., XX ML, 4, 5. — Id., '98. A Contribution to the Chemistry of Cytological Staining: Am.Journ. Phys., I., 4. — Id., '99, 1. The Origin of Fibrinogen: Ibid., III. — Id., '99,2. The Metabolism of the Pancreas Cell: /. J/., XV., Suppl. — Maupas, M., '88. Recherches expe'rimentales sur la multiplication des Infusoires cilie's: Arch. Zool. Exp., 2me sdrie, VI. — Id., '89. Le rejeunissement karyogamique chez les Cilie's: /bid., 2me se*rie, VII. — Id., '91. Sur le de'terminisme de la sexuality chez THydatina senta: C. R., Paris. — Mayer, P., ^l. Uber das Farben mit Carmin, Cochenille und Hamatein-Thonerde: Afitth. Zool. St. Neapol., X., 3. — Id., '97. Beruht die Farbung der Zellkerne auf einem chemischen Vorgang oder nicht?: A. A., XIII., 12. — Mead, A. D., ? 95. Some Observations on Maturation and Fecundation in Chaetopterus pergamentaceus Cuv. : /. A/., X., 1 . — Id., '97, 1. The Origin of the Egg-centrosomes : /bid., XII. — Id., '97, 2. The early Development of marine Annelids: /bid., V. — Id., ? 98, 1. The Origin and Behaviour of the Centrosomes in the Annelid Egg : /bid., XIV., 2. — Id., '98, 2. The Rate of Cell-division and the Function of the Centrosome: IVood's H oil Biol. Lectures. — Merkel, P., '71. Die Stiitzzellen des menschlichen Hodens: A/tiller's Arch. — Mertens, H., '93. Recherches sur la signification du corps vitellin de Balbiani dans Tovule des Mammiferes et des Oiseaux: A. B., XIII. — Metschnikoff, E., 66. Embryologische Studien an Insecten: Z.w.Z., XVI. — Meves, P., "91. t)ber amitotische Kernteilung in den Spermatogonien des Salamanders, und das Verhalten der Attraktionsspharen bei derselben: A. A., 1891, No. 22. — Id., "94. tfber eine Metamorphose der Attraktionssphare in den Spermatogonien von Salamandra maculosa: A. m. A., XLIV. — Id., '95. Uber die Zellen des Sesambeines der Achillessehne des Frosches (Rana temporaria) und iiber ihre Centralkbrper: /bid.,XLV. — Id., '96. Cber die Entwicklung der mannlichen Geschlechtszellen von Salamandra: /bid., XLVIII. — Id., '97, 1. Zur Struktur der Kerne in den Spinndrlisen der Raupen: /bid., XLVIII. — Id., '97, 2. Cber Struktur und Histiogenese der Samenfaden von Salamandra: /bid., L. — Id., '97, 3. Uber den Vorgang der Zelleinschnurung : Arch. Entwm., V '., 2. — Id., '97, 4. Zelltheilung : Merkel u. Bonnet, Erg., VI. — Id., ? 97, 5. tJber Centralkorper in mannlichen Geschlechtszellen von Schmetterlingen : A. A., XIV., 1. — Id., '98. ITber das Verhalten der Centralkorper bei der Histogenese der Samenfaden vom Mensch und Ratte: Verh. An. Ges., XIV. — Id., '99. tlber Struktur und Histogenesis der Samenfaden des Meerschweinschens : A. m. A., LIV. — Meyer, A., '96. Die Plasmaverbindungen, etc. : Bot. Zeit., 11, 12. — Meyer, O., ? 95. CellularUntersuchungen an Nematodeneiern : /. Z., XXIX. (XXIL). — Michaelis, L., '97. Die Befruchtung des Tritoneies: A. m. A., XLVIII. — Miescher, P., '96. Physiologisch-chemische Untersuchungen iiber die Lachsmilch : Arch. Exp. Path. u.Pharm.y XXXVII. — Mikosch, '94. Uber Struktur im pflanzlichen Protoplasma : Verhandl. d. Ges. deutscher Naturf. und Arste, 1 894 ; Abteil f. P/lanzen


462 GENERAL LITERATURE-LIST

physiologie u. Pflansenanatomie. — Minot, C. S., '77. Recent Investigations of Embryologists : Proc. Post. Soc. Nat. Hist., XIX. — Id., '79. Growth as a Function of Cells : Ibid., XX. — Id., '82. Theorie der Genotlasten : P. C, II., 12. See ako Am. Nat., February, 1880, and Proc. Post. Soc. Nat. Hist., XIX., 1877. — Id., ^1. Senescence and Rejuvenation : Journ. Phys.,Xl\., 2. — Id., , 92. Human Embryology. New York. — von Mohl Hugo, '46. Cber die Saftbewegung im Innern der Zellen : Pot. Zeitung. — Moll, J. W., '93. Observations on Karyokinesis in Spirogyra: Verh. Kon. Akad., Amsterdam, No. 9. — Montgomery, Th. H., "98, 1. The Spermatogenesis of Pentatoma, etc. : Zoo/. Jahrb. — Id., '98, 2. Comparative Cytological Studies, with Especial Reference to the Morphology of the Nucleolus : /. M., XV., 2. — Moore, J. E. S., '93. Mammalian Spermatogenesis: A. A., VIII. — Id., '95. On the Structural Changes in the Reproductive Cells during the Spermatogenesis of Elasmobranchs : Q. J , XXXVIII. — Morgan, T. H., '93. Experimental Studies on Echinoderm Eggs: A. A., IX., 5, 6. — Id., '95. 1. Studies of the " Partial " Larva of Sphaerechinus : A. Entwm., II., 1. — Id., '95, 2. Experimental Studies on Teleost-eggs : A. A., X., 19. — Id., '95, 3. Half-embryos and Whole-embryos from one of the first two Blastomeres of the Frog's Egg: Ibid., X., 19. — Id., '95, 4. The Fertilization of non-nucleated Fragments of Echinoderm-eggs : Arch. Entwm. , II., 2. — Id., ^95, 5. The Formation of the Fishembryo: J. M., X., 2. — Id., '96, 1. On the Production of artificial archoplasmic Centres: Rept. of the Am. Morph. Soc, Science, III., January 10, 1896. — Id., "96, 2. The Number of Cells in Larvae from Isolated Blastomeres of Amphioxus: Arch. Entwm., III., 2. — Id., '96,3. The Production of Artificial Astrosphaeres : Arch. Entwm. , III. — Id., '98, 1. Experimental Studies of the Regeneration of Planaria maculata: Ibid., VII., 2, 3. — Id., "98, 2. Regeneration and Liability to Injury: Zool. Pull., I., 6. — Id., '99, 1. The Action of Salt-solutions on the Unfertilized and Fertilized Eggs of Arbacia and other Animals : Arch. Entwm., VIII., 3. — Id., '99,2. A Confirmation of SpallanzanPs Discovery, etc.: A. A., XV. 21 . — Mottier, D. M., '97, 1. t'ber das Verhalten der Kerne bei der Entwicklung des Embryosacs, etc.: Jahrb. wiss. Pot., XXXI. — Id., '97, 2. Beitrage zur Kenntniss der Kerntheilung in den Pollenmutterzellen. *•/<:. : Ibid., XXX. — Id., '98. Das Centrosoma bei Dictyota : Per. D. Pot. Ges., XVI., 5. — Muller, E., '96. Uber die Regeneration der Augenlinse nach Exstirpation derselben bei Triton : A. m. A., XLVIL, 1.— Munson, J. P., '98. The Ovarian Egg of Limulus, etc.: J. J/., XV., 2. — Murray, J. A., '98. Contributions to a Knowledge of the Nebenkern in the Spermatogenesis of Pulmonata: Zool. Jahrb., XI., 14.

NADSON, G., '95. ttber den Bau des Cyanophyceen-Protoplastes : Script. Potan. Horti. PetropoL, IV. — Nageli, C, '84. Mechanisch-physiologische Theorie der Abstammungslehre : Munchen 11. Leipzig* 1884. — Nageli und Schwendeuer. ? 67. Das Mikroskop. (See later editions.) Leipzig. — Nawaachin, '99. Neue Beobachtungen uber Befruchtung bei Fritillaria und Lilium : Pot. Centb., LXXVIL, 2. — Nemec, B., '97. Tber die Stmktur der Diplopodeneier. A. A.. XIII., 10, n. — Id., '99. Uber die karvokinetischc Kerntheilung in den \Vurzelspitzen von Allium : J. w. P.. XX VIII, 2. — Newport, G. On the Impregnation of the Ovum in the Amphibia: Phil. Trans., 1851, 1853, 1854. — Norman. W. W.. "96. Segmentation of the Nucleus without Segmentation of the Protoplasm: Arch. Entwm., III. — Nussbaum, M., '80. Zur Differenzierung des Geschlcchts im Tierreich : A. m. A., XVIII. — Id., '84, 1. Uber Spontane und Kiinstliche Theilung von Infusorien : Verh. d. naturh. Ver. preus.. Rheinland, 1884. — Id., '84,2. Cber die Verandeningen der Geschlechtsproducte bis zur Eifurchung: A. m. A., XXIII. — Id., '85. Uber die Teilbarkeit der lebendigen Materie, I. : A. m. A., XXVI. —


GENERAL UTERATURE-UST 463

Id., '94. Die mit der Entwickelung fortschreitende Differenzierung der Zellen : Sitz.-Ber. d. niederrhein. Gesellschaft f. Natur- u. Heilkunde, Bonn, 5 Nov., 1894; also/?. C, XVI., 2, 1896. — Id., ? 97. Die Entstehung des Geschlechts bei Hydatina: A. m. A., XLIX.

OBST, P., ^9. Untersuchungen liber das Verhalten der Nucleolen, etc. : Z. w. Z. LXVI., 2. — Ogata, 83. Die Veranderungen der Pancreaszellen bei der Secretion: A. A. P. — Oppel, A., '92. Die Befruchtung des Reptilieneies : A. m. A. XXXIX.— Osterhout, W. J. V., W. Uber Entstehung der karyokinetischen Spindel bei Equisetum: Jahrb. wiss. Bot., XXX. — Oltmanna, P., '95. Uber die Entwickelung der Sexualorgane bei Vaucheria: Flora. — Overton, C. E., , 88. Uber den Conjugationsvorgang bei Spirogyra: Ber. deutsch. Bot. Ges., VI. — Id., W. Beitrag zur Kenntniss der Gattung Volvox : Bot. Centralb., XXXIX.— Id., '93. Uber die Reduktion der Chromosomen in den Kernen der Pflanzen : Vierteljahrschr, naturf. Ges. Zurich, XXXVIII. Also Ann. Bot., VII., 25.

PALADINO, G., '90. I ponti intercellulari tra V uovo ovarico e le cellule follicolari, etc. : A. A., V. — Paulmier, P. C, '98. Chromatin Reduction in the Hemiptera: A. A., XIV. — Id., '99. The Spermatogenesis of Anasa tristis: /. M., XV., Suppl. — Peter, K., '99. Das Centrum flir die Flimmer- und Geisselbewegung: A. A., XV., 14, 15. — Pfeffer, W., '99. Uber die Erzeugung und die physiologische Bedeutung der Amitose : Ber. konigl., sacks., Ges. Hiss. Leipzig., July 3. — Pfitzner, W., '82. Uber den feineren Bau der bei der Zelltheilung auffretenden fadenformigen DifFerenzierungen des Zellkerns : M. J., VII. — Id., '83. Beitrage zur Lehre vom Baue des Zellkerns und seinen Theilungserscheinungen : A. m. A., XXII. — Pfliiger, E., '83. Uber den Einfluss der Schwerkraft auf die Theilung der Zellen : I., Arch. ges. Phys., XXXI.; II., Ibid., XXXII.; abstract in Biol. Centb., III., 1884. — Id., '84. Uber die Einwirkung der Schwerkraft und anderer Bedingungen auf die Richtung der Zelltheilung: Arch. ges. Phys., XXXIV. — Id.* '89. Die allgemeinen Lebenserscheinungen : Bonn. — Platner, G., '86, 1. Zur Bildungder Geschlechtsprodukte bei den Pulmonaten : A. m. A., XXVI. — Id., '86,2. — Uber die Befruchtung von Arion empiricorum : A. m. A., XXVII. — Id., '89, 1. Uber die Bedeutung der Richtungskorperchen : B. C, VIII. — Id., '89, 2. Beitrage zur Kenntniss der Zelle und ihrer Teilungserscheinungen, I. -VI. : A. m. A., XXXIII. — Poirault and Raciborski, '96. Uber konjugate Kerne und die konjugate Kerntheilung : B. C, XVI., 1. — Prenant, A., '94. Sur le corpuscule central: Bull. Soc. Sci. % Nancy, 1894. — Id., "98, '99. Sur le protoplasma superieure (archoplasme, kinoplasme, ergastoplasme) : Jour. Anal. Phys.,XXX\V., XXXV. — Preusse, P., '95. Uber die amitotische Kerntheilung in den Ovarien der Hemipteren : Z. w. Z., LIX., 2. — Provost and Dumas, *24. Nouvelle the'orie de la generation: Ann. Sci. Nat., I., II. — Pringsheim, N., '55. Uber die Befruchtung der Algen : Monatsb. Bert. Akad., 1855-56.

RABL, C, '85. Uber Zellteilung: M. /., X. — Id., '89, 1. Uber Zelltheilung: A. A., IV. — Id., '89,2. Uber die Prinzipien der Histologic: Verh. Anal. Ges., III. — vom Rath, O., '91. Uber die Bedeutung der amitotischen Kernteilung imHoden: Zo'ol. Anz., XIV. — Id., '92. Zur Kenntniss der Spermatogenese von Gryllotalpa vulgaris: A. m. A., XL. — Id., '93. Beitrage zur Spermatogenese von Saiamandra: Z. w. Z., LVII. — Id., '94. Uber die Konstanz der Chromosomenzahl bei Tieren : B. C, XIV., 13. — Id., "95, 1. Neue Beitrage zur Frage der Chromatinreduction in der Samen- und Eireife : A. m. A., XLVI. — Id., '95, 2. Uber den feineren Bau der Drusenzellen des Kopfes von Anilocra, etc. : Z. w. Z., LX., 1. — Rauber, A., ? 83. Neue Grundlegungen zur Kenntniss der Zelle : M.J.,


464 GENERAL LITERATURE-LIST

VIII. — Rawitz, B., '95. Centrosoma und Attraktionsphare in der ruhenden Zelle des Salamanderhodens : A. m. A., XLIV., 4. — Id., '97. Bemerkungen uber Mikrotomschneiden, etc. : A. A., XIII. — Reinke, Pr., '94. Zellstudien, I., A. m. A., XLIII. : III., Ibid., XLIV., 1894. — Id., '95. Untersuchungen uber Befruchtung und Furchung des Eies der Echinodermen : Sitz.-Ber. Akad. d. Wiss. Berlin, 1895, June 20. — Reinke and Rodewald, '81. Studien tiber das Protoplasma: (J titer such. aus. d. bot. Inst. Gdttingen, II. — Remak, R., '41. Uber Theilung rother Blutzellen beim Embryo: Med. Ver. Zeit., 1841. — Id., ^50-55., Untersuchungen uber die Entwicklung der Wirbelthiere : Berlin, 1850-55. — Id., '58. tiber die Theilung der Blutzellen beim Embryo: Mailer's Arch.* 1858.— Retzius, G., '89. Die Intercellularbrucken des Eierstockeies und der Follikelzellen : Vcrh. Anal. Ges. y 1889. — Rhumbler, L., '93. Uber Entstehung und Bedeutung der in den Kernen vieler Protozoen und im Keimblaschen von Metazoen vorkommenden Binnenkorper (Nucleolen) : Z, w. Z., LV1. — Id., '96. Versuch einer mechanischen Erklarung der indirekten Zell- und Kerntheilung : Arch. Eniwm., III. — Id., '97. Stemmen die Strahlen der Astrosphare oder ziehen sie? Arck. Entwm., IV. — Rompel, '94. Kentrochona Nebaliae n. sp., ein neues Infusor aus der Familie der Spirochoninen. Zugleich ein Beitrag zurLehre von der Kernteilung und dem Centrosoma : Z. w. Z., LVIII., 4. — Rosen, ^92. Uber tinctionelle Unterscheidung verschiedener Kernbestandtheile und der Sexual-kerne : Cokns Beitr. z. Biol. d. Pflanzen,\ . — Id., '94. Neueres liber die Chromatophilie der Zellkerne : Schles. Ges. vaterl. Kidt., 1894. — Roux, W., '83, 1. Uber die Bedeutung der Kernteilungsfiguren : Leipzig. — Id., '83, 2. Uber die Zeit der Bestimmung der Hauptrichtungen des Froschembryo : Leipzig. — Id., '85. t?ber die Bestimmung der Hauptrichtungen des Froschembryos im Ei, und tiber die erste Theilung des Froscheies : Breslauer drtzl. Zeitg., 1885. — Id., '87. Bestimmung der medianebene des Froschembryo durch die Kopulationsrichtung des Eikernes und des Spermakernes : A.m. A., XXIX. — Id., '88. tiber das kunstliche Hervorbringen halber Embryonen durch Zerstorung einer der beiden ersten Furchungskugeln, etc. : Virchow*s Archiv, 114. — Id., '90. Die Entwickelungsmechanik der Organismen. IVien, 1890. — Id., 7 92, 1. Entwickelungsmechanik : Merkel and Bonnet* Erg., II. — Id., '92, 2. Uber das entwickelungsmechanische Vermogen jeder der beiden ersten Furchungszellen des Eies: Verh. Anal. Ges., VI. — Id., *93, 1. t*ber Mosaikarbeit und neuere Entvvickelungshypothesen : An. Hefte, Feb., 1893. — Id., '93, 2. Cber die Spezifikation der Furchungzellen, etc. : B. C, XIII., 19-22

— Id., '94, 1. t!ber den " Cytotropismus " der Furchungszellen des Grasfrosches : Arch. Entwm., I., i, 2. — Id., '94, 2. Aufgabe der Entwickelungsmechanik, etc.: Arch. Entwm., I., 1. Trans, in Biol. Lectures, Wood's Holl, 1894. — Ruckert. J., '91. Zur Befruchtung des Selachiereies : A. A., VI. — Id., '92,1. Zur Entwicklungsgeschichte des Ovarialeies bei Selachiern : A. A., VII. — Id., '92, 2. Uber die Verdoppelung der Chromosomen im Keimblaschen des Selachiereies: Ibid., VIII.

— Id., '93, 2. Die Chromatinreduktion der Chromosomenzahl im Entwicklungsgang der Organismen : Merkel and Bonnets Erg., III. — Id., '94. Zur Eireifung bei Copepoden : An. Hefte. — Id., '95, 1. Zur Kenntniss des Befruchtungsvorganges: Sitsb. Bayer. Akad. IViss., XXVI., 1. — Id., '95,2. Zur Befruchtung von Cyclops strenuus : A. A., X., 22. — Id., ? 95, 3. t'ber das Selbstandigbleiben der vaterlichen und mutterlichen Kernsubstanz wahrend der ersten Entwicklung des befruchteten Cyclops-Eies : A. m. A., XLV., 3. — Riige. G., '89. Vorgange am Eifollikel der \virbelthiere : M.J., XV. —Ryder, J. A., '83. The Microscopic Sexual Characteristics of the Oyster, etc., Bull. U. S. Fish. Comm., March 14, 1883. Also, Ann. Mag. iVat. Hist., XII., 1883.


GENERAL LITERATURE-LIST 465


', M., '97. Beitrage zur Kenntniss der Chromatinreduktion in der Ovogenesis von Ascaris: Bull. Soc. Nat., Moscow, 1. — Sab a tier, A., '90. De la Spermatog^nese chez les Locustides: Comptes Rend., CXI., '90. — Sachs, J., '82. Vorlesungen liber Pflanzen-physiologie : Leipzig. — Id. Uber die Anordnung der Zellen in jlingsten Pflanzentheile : Arb. Bot. Inst. Wiirzburg, II. — Id., ^92. Physiologische Notizen, II., Beitrage zur Zellentheorie : Flora, 1892, Heft I. — Id., '93. Stoffund Form der Pflanzen-organe : Gcsammelte Abhandlungen, II., 1893. — Id., 35. Physiologische Notizen, IX., weitere Betrachtungen liber Energiden und Zellen : Flora, LXXXI., 2. — Sala, L., '95. Experimentelle Untersuchungen iiber die Reifung und Befruchtung der Eier bei Ascaris megalocephala : A. m. A., XL. — Sargant, Ethel, '95. Some details of the first nuclear Division in the Pollen-mother-cells of folium mar/agon: Journ. Roy. Mic. Soc, 1895, P art *3« 

— Id., '96. The Formation of the Sexual Nuclei in Lilium, I., Oogenesis: Ann. Bot., X. — Id., W. Same title, II., Spermatogenesis : Ibid., XI. — SchaVfer, E. A.,

  • 91. General Anatomy or Histology: in Quain's Anatomy, I., 2, 10th ed., London.

— Schaffner, J. H., '97, 1. The Life-history of Sagittaria: Bot. Gaz., XXIII., 4. — Id., '97, 2. The Division of the Macrospore Nucleus (in Lilium) : Ibid., XXIII., 6. — Id., '98. Karyokinesis in Root-tips of Allium : Ibid., XXVI., 4. — Schaudinn, P., '95. Uber die Theilung von Amoeba binucleata Gruber : Sit z. -Ber. Ges. Naturforsch. Freunde, Berlin, Jahrg. 1895, No. 6. — Id., ^96, 1. Uber den Zeugungskreis von Paramosba Eilhardi: Sitz.-Ber. Akad. Wiss., Berlin, 1896, Jan. 16. — Id., '96, 2. Uber die Copulation von Actinophrys Sol: Ibid. — Id., "96, 3. Uber das Centralkorn der Heliozoen : Verh. D. Zo'dl. Ges. — Schewiakoff, W., , 88. U ber die karyokinetische Kerntheilung der Eusfypha alveolata: J/./., XIII.

— Id., '93. Uber einen neuen Bakterienahnlichen Organismus: Hab. Schrift, Heidelberg, Winter. — Schieffer decker and Kossel, '91. Die Gewebe des Menschlichen Korpers: Braunschweig. — Schimper, '85. Untersuchungen iiber die Chlorophyllkorper, etc. : Zeitsch. wiss. Bot., XVI. — Schleicher, W., '78. Die Knorpelzelltheilung. Ein Beitrag zur Lehre der Theilung von Gewebezellen : Centr. med. Wiss. Berlin, 1878. Also A. m. A., XVI., 1879. — Schleiden, M. J., '38. Beitrage zur Phytogenesis : Mullens Archiv, 1838. [Trans, in Sydenham Soc, XII.: London, 1847.] — Schloter, G., '94. Zur Morphologie der Zelle: A. m. A., XLIV., 2. — Schmitz, '84. Die Chromatophoren der Algen. — Schneider, A., *73. Untersuchungen iiber Plathelminthen : Jahrb. d. oberhess. Ges. f. Natur-Heilkunde, XIV., Giessen. — Schneider, C, '91. Untersuchungen liber die Zelle: Arb. Zo'dl. Inst. Wien, IX., 2. — SchottlMnder, J., m Uber Kern und Zelltheilungsvorgange in dem Endothel der entziindeten Hornhaut: A. m. A., XXXI. — Schottl&nder, P., '93. Beitrage zur Kenntniss des Zellkerns, etc: Cohn's Beitrage, VI. — Schultze, Max, '61. Uber Muskelkorperchen und das was man eine Zelle zu nennen hat: Arch. Anat. Phys., 1861. — Schultze, O., '87. Untersuchungen iiber die Reifung und Befruchtung des Amphibien-eies : Z. w. Z., XLV. — Id., '90. Uber Zelltheilung: Sitzb. phys. med. Ges. Wiirzburg. — Id.,

  • 94. Die klinstliche Erzeugung von Doppelbildungen bei Froschlarven, etc. : Arch.

Entwm., I., 2. — Schwann, Th., '39. Mikroscopische Untersuchungen iiber die Ubereinstimmung in der Structur und dem Wachsthum der Thiere und Pflanzen: Berlin. [Trans, in Sydenham Soc, XII.: London, 1847.] — Schwarz, Fr., '87. Die Morphologische und chemische Zusammensetzung des Protoplasmas : Brcslau.

— Schweigger-Seidel, O., '65. Uber die Samenkorperchen und ihre Entwickelung: A. m. A., I. — Sedgwick, A.. 'SS-'SS. The Development of the Cape Species of Peripatus, I.-VI. : Q. J., XXV.-XXVIII. — Id., '94. On the Inadequacy of the Cellular Theory of Development, etc: Ibid., XXXVII., 1. — Seeliger, O., '94. Giebt es geschlechtlicherzeugte Organismen ohne miitterliche Eigenschaften ? : A. Ent., I., 2. — Selenka, E., '83. Die Keimblatter der Echinodermen : Studien

2H


I


466 GENERAL LITERATURE-LIST

iiber Entwick., II., Wiesbaden, 1883. — Sertoli, B., '65. Dell* esistenza di particolari cellule ramificate dei canaliculi seminiferi del testicolo umano : 11 Margagni.

— Shaw, W. R., '98, 1. Uber die Blepharoplasten bei Onoclea und Marsilia: Ber. D. Bot. Ges., XVI., 7. — Id., '98, 2. The Fertilization of Onoclea : Ann. B<*^ XII., 47. — Siedlecki, M., '95. Uber die Struktur und Kerntheilungsvorgange bei den Leucocyten der Urodelen : Anz. Akad. IViss., Krakau, 1895. — Id., ' f 9S. £tude cytologique et cycle eVolutif de Adelea: Ann. Inst. Pasteur., XIII. — Sobotta, J., '95. Die Befruchtung und Furchung des Eies der Maus: A. m. A., XLV. — Id., '97. Die Reifung und Befruchtung des Eies von Amphioxus : Ibid., L. — Solger, B., '91. Die radiaren Strukturen der Zellkorper im Zustand der Ruhe und bei der Kerntheilung : Bert. Klin. Wochenschr., XX., 1891. — Spallanzani, 1786. Experiences pour servir a l'histoire de la gdn^ration des animaux et des plantes : Geneva. — Spitzer, '97. Die Bedeutung gewisser Nucleoproteide fur die oxvdative Leistung der Zelle: Arch. ges. Phys., LX VI I. — Stevens, W. C, *98. Ober Chromosomentheilung bei der Sporenbildung der Fame: Ber. D. Bot. Ges., XVI., 8. — Stevens, F. L., '99. The compound Oosphere of Albugo: Bot. Gas., XXVIII., 3,4.

— Strasburger, B., '75. Zellbildung und Zelltheilung : 1st ed., Jena* 1875. — IcL '77. t v ber Befruchtung und Zelltheilung: /. Z., XL — Id., '80. Zellbildung und Zellteilung: 3d ed. — Id., '82. Cber den Theilungsvorgang der Zellkerne und das Verhaltniss der Kerntheilung zur Zelltheilung: A. m. A., XXI. — Id., '84, 1. Die Controversen der indirecten Zelltheilung: Ibid., XXIII. — Id., '84, 2. Neue Untersuchungen liber den Befruchtungsvorgang bei den Phanerogamen, als Grundlage fur eine Theorie der Zeugung : Jena, 1884. — Id., '88. tlber Kern- und Zellteilung im Pflanzenreich, nebst einem Anhang liber Befruchtung: Jena. — Id., '89. V ber das Wachsthum vegetabilischer Zellhaute: Hist. Bei., II. , Jena. — Id., '91. Das Protoplasma und die Reizbarkeit : Rektoratsrede, Bonn, Oct. 18, 1891. Jena, Fischer. — Id., '92. Histologische Beitrage, Heft IV. : Das Verhalten des Pollens und die Befruchtungsvorgange bei den Gymnospermen, Schwarmsporen, pnanzliche Spermatozoiden und das Wesen der Befruchtung: Fischer, Jena, 1892. — Id., *93, 1. Cber die Wirkungssphare der Kerne und die Zellengrosse : Hist. Beitr., V. — Id.. "93, 2. Zu dem jetzigen Stande der Kern- und Zelltheilungsfragen : A. A.., VIII., p. 177. — Id., '94. Cber periodische Reduktion der Chromosomenzahl im Entwicklungsgang der Organismen : B. C, XIV. — Id., '95. Karyokinetische Probleme : Jahrb. f. tuiss. Botanik, XXVI II., 1 . — Id., '97, 1. Kerntheilung und Befruchtung bei Fucus : Jahrb. iviss. Bot., XXX. —Id., 97, 2. fber Befruchtung: Ibid. — Id., ^97, 3. Cber Cytoplasmastrukturen, Kern- und Zelltheilung: Ibid. — Id., -98. Die Pflanzlichen Zellhaute: Ibid., XXXI. — Strasburger and Mottier, '97. Cber den zweiten Theilungsschritt in Pollenmutterzellen : Ber. D. Bot. Ges., XV., 6. — Van der Stricht, O., 92. Contribution a l'e'tude de la sphere attractive: A. /?., XII., 4. — Id., '95, 1. La maturation et la fe'condation de l"ceuf d\Amphioxus lanceolatus: Bull. Acad. Roy. Belgique, XXX.. 2. — Id., ? 95, 2. De l'origine de la figure achromatique de l'ovule en mitose chez le Thysanozoon Brocchi : Verhandl. d. anat. lersamml. in Strassburg, 1895, p. 223. — Id., '95, 3. Contributions a Te'tude de la forme, de la structure et de la division du noyau : Bull. Acad. Roy. Sc. Belgique, XXIX. — Id.. "98, 1. La formation des globules polaires, etc., chez Thysanozoon Arch. Biol., XV. — Id., '98, 2. Contribution a Te'tude du noyau vitellin de Balbiani \ T erh. An. Ges., XII. — Strieker, S., '71. Handbuch der Lehre von den Geweben Leipzig. — Stuhlmann, Fr., 86. Die Reifung des Arthropodeneies nach Beobach tungen an Insekten, Spinnen, Myriopoden und Peripatus : Ber. Naturf. Ges. Fretburg, I. — Suzuki, B., '98. Notiz liber die Entstehung des Mittelstlickes von Selachiern : A. A., XV., 8. — Swaen and Masquelin. '83. lStude sur la Spermatogdnese: A. B., IV. — Swingle, W. T., '97. Zur Kenntniss der Kern- und Zellteilungen bei den Sphacelariaceae : /. w. B., XXX.


GENERAL LITERATURE-LIST 467

THOMA, R„ , 9€. Text-book of General Pathology and Pathological Anatomy : Trans, by A. Bruce, London. — Thomson, Allen. Article 4i Generation " in Todd's Cyclopaedia. — Id. Article "Ovum" in Todd's Cyclopaedia. — Townsend, C. O.,

  • 97. Der Einfluss des Zellkerns auf die Bildung der Zellhaut : Jahrb. wiss. Bo/.,

XXX. — Treat, Mary, '73. Controlling Sex in Butterflies: Am. Nat., VII.— Trow, A. H., ^95. The Karyology of Saprolegnia: Ann. Bot., IX. — Tyson, James, '78. The Cell-doctrine : 2d ed., Philadelphia.

UNNA, P., '95. Vber die neueren Protoplasmatheorien, und das Spongioplasma: Deutsche Med. Zeit., 1895, 98-100. — Ussow, M., '81. Untersuchungen liber die Entwickelung der Cephalopoden : Arch. Biol., II.

VEJDOVSK^, F., '88. Entwickelungsgeschichtliche Untersuchungen, Heft I. : Reifung, Befruchtung und Furchung des Rhynchelmis-Eies : Prag, 1888. Vejdovsky and MrAzek, '98. Centrosom und Periplast : Sitzber. b'dhm. Ges. Wiss. — Verworn, M., '88. Biologische Protisten-studien : Z. w. Z., XLVI. — Id., '89. Psychophysiologische Protisten-studien : Jena. — Id., ^l. Die physiologische Bedeutung des Zellkerns : Pfliiger^s Arch.f. d. ges. Physiol., II. — Id., "95. Allgemeine Physiologie : Jena . — Virchow, R., '55. Cellular-Pathologie : Arch. Path. Anal. Phys., VIII., 1. — Id., '58. Die Cellularpathologie in ihrer Begriindung auf physiologische und pathologische Gewebelehre : Berlin, 1858. — De Vries, H., '89. Intracellulare Pangenesis : Jena.

WAGER, H., '96. On the Structure and Reproduction of Cystopus. Ann. Bot., X. — Waldeyer, W., '70. Eierstock und Ei : Leipzig.— Id., '87. Bau und Entwickelung der Samenfaden : Verh. An. Ges. Leipzig 1887. — Id., '88. tlber Karyokinese und ihre Beziehungen zu den Befruchtungsvorgangen : A. m. A.,XXXU. [Trans, in Q»J^\ — Id., '95. Die neueren Ansichten liber den Bau und das Wesen der Zelle: Deutsch. Med. Wochenschr., No. 43, ff., Oct. fF., 1895. — Warneck, N. A., '50. Uber die Bildung und Entwickelung des Embryos bei Gasteropoden : Bull. Soc. Imp. Nat. Moscou, XXIII., 1. — Watase\ S.. '91. Studies on Cephalopods; I., Cleavage of the Ovum: /. M., IV., 3. — Id., '92. On the Phenomena of Sex-difFerentiation : Ibid., VI., 2, 1892. — Id., ^3, 1. On the Nature of Cellorganization: IVood's Holl Biol. Lectures, 1893. — Id., ^3, 2. Homology of the Centrosome : /. M., VIII., 2. — Id., '94. Origin of the Centrosome : Biological Leetures, Wood*s Holl, 1894. Webber, H. J., '97. 1. Peculiar Structures occurring in the Pollen-tube of Zamia: Bot. Gazette. XXIII., 6. — Id., '97, 2. The Development of the Antherozoids of Zamia: Ibid., XXIV., 1. — Id., '97, 3. Notes on the Fecundation of Zamia and the Pollen-tube Apparatus of Gingko: Ibid., XXIV., 4. — Weismann, A., '83. Ober Vererbung: Jena. — Id., '85. Die Kontinuitat des Keimplasmas als Grundlage einer Theorie der Vererbung: Jena. — Id., '86, 1. Richtungskorper bei parthenogenetischen Eiern : Zool. Anz., No. 233. — Id., '86, 2. Die Bedeutung der sexuellen Fortpflanzung fur die Selektionstheorie : Jena. — Id.. '87. Cber die Zahl der Richtungskorper und liber ihre Bedeutung fur die Vererbung : Jena. — Id., '91, 1. Essays upon Heredity, First Series: Oxford. — Id., ^1, 2. Amphimixis, oder die Vermischung der Individuen: Jena, Fischer. — Id, 1 92. Essays upon Heredity, Second Series: Oxford, 1892. — Id., ^3. The Germ-plasm: New York. — Id., '94. Aussere Einflusse als Entwicklungsreize : Jena. — Id., "99. Regeneration: Nat. Sci., XIV., 6. [See also A. A., 1899.] "Wheeler, W. M., '89. The Embryology of Blalta Germanica and Doryphora decemlineata : J. M., III. — Id., "93. A Contribution to Insect-embryology: Ibid., VIII., 1. — Id., '95. The Behaviour of the Centrosomes in the Fertilized Egg of Myzostoma glabrum: Ibid., X. — Id., '96. The Sexual Phases of Myzostoma:


468 GENERAL LITERATURE-LIST

Mitth. ZooL St. Neapel, XII., 2. — Id., W. The Maturation, Fecundation, and early Cleavage in Myzostoma: Arch. Biol., XV. — "Whitman, C. O., '78. The Embryology of Clepsine: Q. J., XVIII. — Id., '87. The Kinetic Phenomena of the Egg during Maturation and Fecundation : /. M., I., 2. — Id., *88. The Seat of Formative and Regenerative Energy : Ibid., II. — Id., "93. The Inadequacy of the Cell-theory of Development : WoocTs H oil Biol. Lectures, 1893. — Id., 1*4. Evolution and Epigenesis : Ibid., 1894. — Wiesner, J., '92. Die Elementarstruktur nnd das Wachstum der lebenden Substanz: Wien. — "Wilcox, E. V., *95. Spermatogenesis of Caloptenus and Cicada : Bull, of the Museum of Comp. ZooL, Harvard College, Vol. XXVII., No. 1. — Id., '96. Further Studies on the Spermatogenesis of Caloptenus : Bull. Mus. Comp. ZooL, XXIX. —Will, L., '86. Die Entstehung des Eies von Colymbetes: Z. w. Z, XLIII. — Wilaon, Bdm. B., ^92. The Celllineage of Nereis: f. M., VI., 3. — Id., "93. Amphioxus and the Mosaic Theory of Development : Ibid., VIII., 3. — Id., '94. The Mosaic Theory of Development: Wood^s Holl Biol. Led.* 1894. — Id., '95, 1. Atlas of Fertilization and Karyokinesis: New York* Macmillan. — Id., '95, 2. Archoplasm, Centrosome, and Chromatin in the Sea-urchin Egg: /. M., XI. — Id., '96. On Cleavage and Mosaic-work. [Appendix to Crampton and Wilson, '96.] : A. Entivm.^ III.. 1.— Id., '97. Centrosome and Middle-piece in the Fertilization of the Egg. Science* Vol. V., No. 114. — Id., '98. Considerations on Cell-lineage and ancestral Reminiscence: Ann. N. Y. Acad. Sci., XI. See also Wood? s Holl Biol. Lectures, '99. — Id., '99. On protoplasmic Structure in the Eggs of Echinoderms and some other Animals: /. M., XV. Suppl. — Wilson and Mathews, ^5. Maturation, Fertilization, and Polarity in the Echinoderm Egg: /. M., X., 1. — "Wolff, Caspar Friedrich, 1759. Theoria Generationis. — Wolff, Gustav, ^94. Bemerkungen zum Darwinismus mit einem experimentellen Beitrag zur Physiologie der Ent wicklung: B, C, XIV., 17. — Id., '95. Die Regeneration der Urodelenlinse : Arch. Entwtn., 1., 3. — Wolters, M., '91. Die Conjugation und Sporenbildung bei Gregarinen : A. m. A., XXXVII. — Woltereck, R., '98. Zur Bildung und Entwicklung des Ostrakoden-Eies : Z. w. Z., LXIV.

TUNG, B.. '81. De Tinfluence de la nature des aliments sur la sexualite' : C. R., XCIII ; also Arch. Exp. ZooL, 2d, I., 1883.

ZACH ARIAS. O., '85. If ber die amoboiden Bewegungen der Spermatozoen von Polyphemus pediculus : Z. w. Z., XLI. — Zacharias. E.. "93, 1. Cber die chemische Beschaffenheit von Cytoplasma und Zellkern : Ber. deutsch. Bot. Ges.. II., 5. — Id., '93, 2. Cber Chromatophilie : Ibid., 1893. — Id., '95. t'ber dns Verhalten des Zellkerns in wachsenden Zellen : Flora, 81, 1895. — Id., '94. Tber Beziehungen des Zellenwachstums zur Beschaffenheit des Zellkerns : Bcrichte der deutschen bot an. Gesellschaft, XII., 5. — Id., '98. Cber Nachweis und Vorkommen von Nuclein: Bcr.d. Bot. Ges., XVI., 7. — Ziegler, E., '88. Die neuesten Arbeiten liber Vererbung und Abstammungslehre und ihre Bedeutung fur die Pathologie: Beitr. zur path. Anat., IV. — Id., , 89. t*ber die Ursachen der pathologischen Gewebsneubildungen : ////. Beitr. zur. wiss. Med. Festschrift, R. Virchmv, II.— Id., '92. Lehrbuch der allgemeinen pathologischen Anatomie und Pathogenese. 7th ed., Jena. — Ziegler, H. E., '87. Die Entstehung des Blutes bei Knochenfischenembryonen : A. m. A. — Id., '91. Die biologische Bedeutung der amitotischen Kerntheilung im Tierreich : B. C, XI. — Id., '94. t v ber das Verhalten der Kerne im Dotter der meroblastischen Wirbelthiere : Ber. Naturf Ges. Freiburg, 1894.— Id., '95. Untersuchungen liber die Zelltheilung : Vcrhandl. d. deutsch. ZooL Ges., 1895. — Id., '96. Einige Betrachtungen zur Entwicklungsgeschichte der Echinodermen : Verh. d. ZooL Ges. — Id., '98. Experimented Studien iiber die Zellthei


GENERAL LITERATURE-LIST 469

lung, I., II. : Arch. Entwm., VI., 2. — Ziegler and vom Rath. Die amitotischc Kerntheilung bei den Arthropoden : B. C, XI. — Zimmermann, A., *93, 1. Beitrage zur Morphologie und Physiologie der Pflanzenzelle : Tubingen. — Id., "94. Sammelreferate aus dem Gesammtgebiete der Zellenlehre: Bot. Centb. Beihefte* 1894. Zimmermanu, K. W., 93, 2. Studien liber Pigmentzellen, etc. : A. m. A , XLI. — Id, *98. Beitrage zur Kenntniss einiger Driisen und Epithelzellen : A. m. A.* LII. — Zoja, R., ^95, 1. Sullo sviluppo dei blastomeri isolati dalle uova di alcune meduse : A. Entwm., I., 4 ; II., 1 : II., IV. — Id., *95, 2. Sulla independenza della cromatina paterna e materna nel nucleo delle cellule embrionali: A. A.* XI., 10. Id., '97. Stato attuale degli Studii sulla Fecondazione : Boll. Sci. di Pavia, XVIII., XIX.— Zur Strassen, O., '98. Uber die Riesenbildung bei AscarisEiern: Arch. Entwm., VII., 4.


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INDEX OF AUTHORS


Albrecht, nuclei, 32.

Altmann, granule-theory, 25, 27, 290; nu clein, 332. Amici, pollen-tube, 218. Andrews, spinning activities, 61. Apathy, nerve-cells, 48. Aristotle, epigenesis, 8. Arnold, fibrillar theory of protoplasm, 23;

leucocytes, 117; nucleus and cytoplasm,

3<>3Atkinson, reduction, 269.

Auerbach, 6; double spermatozoa, 142;

staining-reactions, 176; fertilization, 181.

Von Baer, cleavage, 10; cell-division, 64; egg-axis, 378; development, 396.

Balbiani, scattered nuclei, 40 ; spiremenuclei, 36; mitosis in Infusoria, 88; chromatin-granules, 1 12; yolk-nucleus, 155156; regeneration in Infusoria, 343.

Balfour, polar bodies, 243; rate of division, 366; unequal division, 371.

Ballowitz, structure of spermatozoa, 139, 140; double spermatozoa, 142.

Van Bambeke, deutoplasm and yolk -nucleus, 156-160; elimination of chromatin, 155.

Barry, fertilization, 181.

De Bary, protoplasm, 4, 5, 20; conjugation, 181 ; cell-division and growth, 393.

Beale, cell-organization, 291.

Bechamp and Estor, microsome-theory, 290, 291.

Belajeff, spermatozoids, 172-175; reduction in plants, 267.

Benda, spermatogenesis, 163; Sertoli-cells, 284.

Van Beneden, cell-theory, I, 6, 7; protoplasm, 23; nuclear membrane, 38; centrosome and attraction-sphere, 51, 74, 77, 310, 323; cell-polarity, 55; cell-division, 64, 74; origin of mitotic figure, 74-77; theory of mitosis, 100; division of chromosomes, 112; fertilization of Ascaris, 7, 182; continuity of centrosomes, 75; germ


nuclei, 205; centrosome in fertilization, 208; theory of sex, 243 ; parthenogenesis, 281 ; nucleus and cytoplasm, 303; nuclear microsomes, 302; promorphology of cleavage, 381; germinal localization, 399.

Van Beneden and Julin, first cleavage- plane, 380.

Bergmann, cleavage, 10; cell, 17.

Bernard, Claude, nucleus and cytoplasm, 341 ; organic synthesis, 431.

Berthold, protoplasm, 42 ; cell-division,

37°Bickford, regeneration in coelenterates, 392,

429.

Biondi, Sertoli-cells, 284.

Biondi-Ehrlich, staining-fluid, 157.

Bischoff, cell, 17.

Bizzozero, cell-bridges, 60.

Blanc, fertilization of trout, 210.

Blochmann, insect-egg, 132; budding of nucleus, 155; polar bodies, 281 ; bilaterality of ovum, 383.

Bohm, fertilization in fishes, 192.

Bolsius, nephridial cells, 47.

Bonnet, theory of development, 8, 432.

Born, chromosomes in Triton-tgg t 338; gravitation-experiments, 386.

Boveri, centrosome, named, 51 ; a permanent organ, 51, 74; in fertilization, 192, 211, 215, 230; structure, 309; functions, 354; archoplasm, 69, 318; origin of mitotic figure, 74, 77, 319; varieties of A scar is, 87; theory of mitosis, 101, 108; division of chromosomes, 112; origin of germ-cells, 147; fertilization of Ascaris, 182; of Pterotrachca, 184; of Echinus, 192; theory of fertilization, 190, 21 1; of parthenogenesis, 281 ; partial fertilization, 190, 194; reduction, 233; maturation in Ascaris, 238; tetrads, 238; centriole, 309; attraction-sphere, 324; egg-fragments, 353.

Braem, cell-division, 377.

Brandt, symbiosis, 53; regeneration in Protozoa, 342.


47 ]


472


INDEX OF AUTHORS


Brauer, bivalent chromosomes, 82; mitosis in rhizopod, 96; fission of chromatin granules, 113; deutoplasm, 153; fertilization in Branchipus, 192; parthenogenesis in Ar/emia, 281 ; spermatogenesis in Ascaris, 255; intra-nuclear centrosome, 304.

Bra us, 81.

Brogniard, pollen-tube, 218.

Brooks, heredity, 12; variation, 179.

Brown, Robert, cell-nucleus, 18; pollentube, 218.

Brucke, cell-organization, 289.

Von Brunn, spermatozoon, 141.

Buhler, astral systems, 318.

Biitschli, 6; protoplasm, 25,36, 50; diffused nuclei, 40; artifacts, 42; asters, 48, 316; cell-membrane, 54; mitosis, 109, no; centrosome in diatoms, 51; rejuvenescence, 178; polar bodies, 238.

Calberla, micropyle, 200.

Calkins, nuclei of flagellates, 40; mitosis in Noctiluca, 92; yolk-nucleus, 157; origin of middle-piece, 165; reduction, 253, 257.

Campbell, fertilization in plants, 216.

Carnoy, nucleus, 40; muscle-fibre, 48; centrosome, no; amitosis, 1 15, 117; germnuclei, 184; asters, 305, 317.

Carnoy and Le Brun, nucleoli, 130; fertilization, 211; reduction, 263.

Castle, egg-axis, 379; fertilization, 193.

Chittenden, organic synthesis, 341.

Chmielewski, reduction in Spirogyra, 280.

Chun, amitosis, 117; partial development of ctenophores, 418.

Clapp, first cleavage-plane, 381.

Coe, fertilization, 194, 213; centrosome, 321.

Cohn, cell, 17.

Conklin, size of nuclei, 71; union of germnuclei, 204; centrosome in fertilization, 210; centrosome and sphere, 323; unequal division, 373; protoplasmic currents, 377; cell-size and body-size, 388; types of cleavage, 423.

Corda, pollen-tube, 218.

Crampton, yolk-nucleus, 158; reversal of cleavage, 368; experiments on snail, 419, 421; on tunicates, 419.

Crato, protoplasm, 50.

Darwin, evolution, 2, 5 ; inheritance, 12, 396; variation, n ; pangenesis, 12, 290 ; gemmules, 290.

Darwin, F., protoplasmic fragments, 346.

Dendy, cell-bridges, 60.


Dogiel, amitosis, 118.

Driesch, dispermy, 198; fertilization of eggfragments, 200, 353; pressure-experiments, 375, 410; regeneration, 395; isolated blastomeres, 409; theory of development, 394, 415; experiments on ctenophores, 418; ferment -theory, 427.

Driiner, spindle-fibres, 79; central spindle, 105; aster, 321, 326.

Von Ebner, Sertoli-cells, 284.

Ehrlich, tar-colours, 335.

Eismond, structure of aster, 48.

Elssberg, plastidules, 291.

Endres, experiments on frog's egg, 399, 419.

Engelmann, ciliated cells, 44; rejuvenescence, 179.

Von Erlanger, asters, 48, 316; spindle, 81 ; elimination of chromatin, 155; Nebenkern, 163, 165; fertilization, 194, 212, 213; centroplasm, 324.

Eycleshymer, first cleavage-plane, 381.

Farmer, reduction in plants, 275.

Fick, fertilization of axolqtl, 192, 212.

Field, staining-reactions, 176.

Fischel, ctenophores, 419.

Fischer, nucleus, 40; artifacts, 42; stainingreactions, 335.

Flemming, protoplasm, 25, 27 ; chromatin, ^} ; centrosome, 51 ; cell-bridges, 60, 61 ; celldivision, 64, 70; splitting of chromosomes, 70; mitotic figure, 79 ; heterotypical mitosis, 86; leucocytes, 102; theory of mitosis, 106; division of chromatin, 113; amitosis, 117,285; nucleoli, 127; rotation of spernihead, 188; spermatogenesis, 259-262; astral rays, 317; germinal localization.

399. Floderus, follicle-cells, 150.

Fol, 1, 6, 64; amphiaster, 68; theory of mitosis, 108; sperm-centrosome, 191 ; polyspermy, 192; attraction -cone, 198; vitelline membrane, 199; asters, 316.

Foot, yolk-nucleus and polar rings, 156, 202; fertilization in earthworm, 187; entrancefunnel, 201; fertilization-centrosome, 212.

Foster, cell-organization, somacules, 291.

Francotte, polar bodies, 235; centrosome, 306; sphere, 312, 325.

Frommann, protoplasm, 23; nucleus and cytoplasm, 303.

Galeotti, pathological mitoses, 97. Gallardo. mitosis, 109.


INDEX OF AUTHORS


473


Galton, inheritance, 9.

Gardiner, cell-bridges, 59; chromatin-elimi nation, 276; sphere, 325. Garnault, fertilization in Arion, 207. Geddes and Thompson, theory of sex, 124. Van Gehuchten, spireme-nuclei, 36; nuclear

polarity, 36; muscle-fibre, 48. Giard, polar bodies, 235, 238. Gierke, staining-reactions, 335. Gilson, spireme-nuclei, 36. Godlewski, spermatogenesis, 168. Graf, nephridial cells, 47. Gregoire, reduction, 267. Griffin, fertilization, centrosomes in Thalas sema, 193, 194, 213; reduction, 259;

structure of centrosome, 314; aster-formation, 321. Grobben, spermatozoa, 141. Gruber, diffused nuclei, 40; regeneration in

Stentor, 342. Guignard, mitosis in plants, 82; fertilization

in plants, 218, 221; reduction, 263, 267.

Haberlandt, position of nuclei, 346.

Hack el, inheritance, 7; epithelium, 56; cellstate, 58.

Hacker, polar spindles, 276; bivalent chromosomes, 88; nucleolus, 125, 128; primordial germ-cells, 148; gerin-nuclei, 208, 299; reduction in copepods, 249.

Hallez, promorphology of ovum, 384.

Halliburton, proteids, 331 ; nuclein, 333.

Hamm, discovery of spermatozoon, 9, 181.

Hammar, cell-bridges, 60.

Hammarsten, proteids, 331.

Hansemann, pathological mitoses, 97.

Hanstein, metaplasm, 19.

Hardy, artifacts, 42.

Harper, mitosis, 82.

Hartsoeker, spermatozoon, 9.

Harvey, inheritance, 7; epigenesis, 8.

Hatschek, cell-polarity, 56; fertilization, 179.

Heidenhain, nucleus, 36; basichromatin and oxychromatin, 38, 337; cell-polarity, 55; position of centrosome, 57; leucocytes, 102; theory of mitosis, 105; amitosis, 116; staining-reactions, 337; nuclear microsomes, 303; microcentrum, 311; asters, 311, 317; origin of centrosome, 315; position of spindle, 377.

Heider, insect-egg, 132.

Heitzmann, cell-bridges, 59; nucleus and cytoplasm, 303.

Henking, fertilization, 187; insect-egg, 96; spermatogenesis, 165, 248, 253, 271.


Henle, granules, 289.

Henneguy, deuto plasm, 153; yolk-nucleus, 160; centrosome, 356.

Hensen, rejuvenescence, 179.

Herbst, development and environment, 428.

Herla, independence of chromosomes, 208, 299.

Hermann, central spindle, 78, 105; division of chromatin, 112; spermatozoon, 165, 166; staining-reactions, 176.

Hertwig, O., 1, 7, 9; bivalent chromosomes, 88; pathological mitoses, 97; rejuvenescence, 178; fertilization, 181 ; middlepiece, 187; polyspermy, 199; paths of germ-nuclei, 204; maturation, 241 ; polar bodies, 238; inheritance, 182; laws of cell-division, 364; theory of development,

415Hertwig, O. and R., 197; egg-fragments,

199; polyspermy, 199.

Hertwig, R., mitosis in Protozoa, 90; germcells in Sagitta, 146; amphiasters in unfertilized eggs, 306; conjugation, 222; reduction in Infusoria, 277; in ActinospJuzrium, 278; origin of centrosome, 315; cell-division, 391. ,

Hill, fertilization, 187, 193.

Hirase, spermatozoids, 144; fertilization, 218.

His, germinal localization, 398.

Hofer, regeneration in Am<xba } 343.

Hoffman, micropyle. 200.

Hofmeister, cell-division and growth, 393.

Holmes, cleavage, 368.

Hooke, R., cell, 17.

Hoyer, amitosis, 115.

Huie, Drosera, 350.

Huxley, protoplasm, 5; germ, 7, 396; fertilization, 178, 231; evolution and epigenesis, 432.

Ikeno, cell-bridges, 150; blcpharoplasts, 173; fertilization, 221.

Ishikawa, Noctiiuca, mitosis, 92; conjugation, 227; reduction, 267; flagellum, 171.

Jennings, cleavage, 377.

Jordan, deutoplasm and yolk-nucleus, 153,

156; first cleavage-plane, 381. Julin, fertilization in Styltopsis, 192.

Keuten, mitosis in Eugtena, 91. Klebahn, conjugation and reduction in desmids and diatoms, 280.


474


INDEX OF AUTHORS


Klebs, pathological mitosis, 97, 98; cellmembrane, 346.

Klein, nuclear membrane, 38; theory of mitosis, 100; amitosis, 118; nucleus and cytoplasm, 303; asters, 316.

Klinckowstrdm, fertilization, 213; reduction,

259. Von Kolliker, I, 6, 9, 10, 27; epithelium, 56;

cell-division, 63; spermatozoon, 9, 134;

inheritance, 182; development, 413.

Korff, spermatogenesis, 163, 168, 173.

Korschelt, nucleus, 37; amitosis, 115; movements and position of nuclei, 125, 349, 387; nurse-cells, 151; fertilization, 193; tetrads in Ophryotrocha, 258; physiology of nucleus, 348; polarity of egg, 387.

Kossel, chromatin, 336; nuclein, 334; organic synthesis, 340.

Kostanecki, fertilization, 193 ; astral rays, 318.

Kostanecki and Wierzejski, fertilization of Physa, 193, 210, 212; continuity of centrosomes, 211.

Kupffer, energids, 30; cytoplasm, 41.

Lamarck, inheritance, 12.

I*amarle, minimal contact-areas, 361.

Lankester, germinal localization, 398.

Lauterborn, mitosis in diatoms, 95; origin of centrosome, 315.

Leeuwenhoek, spermatozoon, 8; fertilization, 181.

Von Lenhossek, nerve-cell, 21, 47; spermatogenesis, 169, 315; centrosome, 314, 356.

Leydig, cell, 19; protoplasm, 20; cell-membrane, 54; spermatozoa, 142; elimination of chromatin, 159.

Lilienfeld, staining-reactions of nucleins, 336.

Lillie, fertilization, 196, 213; centrosome and aster, 312, 326, 327; regeneration in Stentor, 343 ; cleavage, 360, 369, 377.

Loeb, chemical fertilization, 215, 392; regeneration in ccclentcrates, 392; theory of development, 427; environment and development, 430.

Lustig and Galeotti, pathological mitoses, 98; centrosome, 51.

Maggi, granules, 290.

Malfatti, staining-reactions of nucleins, 335.

Mark, germ-nuclei, 204; polar bodies, 235;

polarity of ovum, 387. Mathews, pancreas-cell, 44; aster-formation,

no; fertilization of echinoderms, 192,212;

origin of centrosome, 125; nucleic acid,

334; staining-reactions, 337.


Maupas, sex in Rotifers, 145; rejuvenescence, 179; conjugation of Infusoria, 223.

Mayer, staining, 335.

McClung, spermatogenesis, 271.

MacFarland, spindle, 79; fertilization, 213, 214; centrosome and sphere, 312, 314,

321. McGregor, spermatogenesis, 167; reduction,

261. McMurrich, gasteropod development, 152;

metamerism in isopods, 390. Mead, fertilization of Chatopierus, 192, 194,

215; sperm-centrosome, 215; centrosomes

de novo, 212, 306; cell-division, 391. Merkel, Sertoli-cells, 284. Mertens, yolk-nucleus and attraction-sphere,

I5 6 » *59Metschnikoff, insect-egg, 383.

Meves, amitosis, 119, 285; spermatogenesis, 167, 169; reduction, 260; cilia, 357.

Meyer, energids, 30; cell-bridges, 60.

Miescher, nuclein, 332.

Mikosch, protoplasm, 44.

Minot, rejuvenescence, 179; cyclical division, 222; theory of sex, 243; Sertolicells, 284; parthenogen sis, 280.

Von Mohl, cell-division, 9; protoplasm, 17.

Montgomery, nucleolus, 34; spermatogenesis, 257, 271.

Moore, spermatozoon, 167, 171 ; reduction, 263.

Morgan, centrosomes, 307; fertilization of egg-fragments, 353; cell-division, 391; effect of fertilization, 201 ; numerical relations of cells, 389; regeneration, 393, 394; isolated blastomeres, 410; polarity, 417; experiments on ctenophores, 418; on frog's egg, 422.

Mottier, mitosis, 83; fertilization, 221; reduction, 266; asters, 305.

Munson, yolk -nucleus, 156.

Nageli, development, I; cell-organization, micella?, 289, 291 ; polioplasm, 41 ; idioplasm-theory, 401.

Nawaschin, fertilization, 218.

Nemec, mitosis, 82; yolk-nucleus, 159.

Newport, fertilization, 181; first cleavageplane, 380.

Nissl, chromophilic granules, 48.

Nussbaum, germ-cells, 122; sex, 145; regeneration in Infusoria, 342; nucleus, 426.

Obst, nucleoli, 130; follicle-cells, 151. Osterhout, spindle, 82; tetrads, 253.


INDEX OF AUTHORS


475


Overton, germ-cells of Vohox, 134; conjugation of Spirogyra, 229; reduction, 274,

275Owen, germ-cells, 122.

Paladino, cell-bridges, 60.

Paul mier, spermatozoon, 165; reduction, 252, 271.

Peremeschko, leucocytes, 117.

Peter, cilia, 357.

Pfeffer, hyaloplasm, 41 ; amitosis, 119; chemotaxis of germ-cells, 197.

PHtzner, cell-bridges, 60; chromatin- granules, 112.

Pfluger, position of spindle, 375 ; first cleavage-plane, 380; gravitation-experiments, 386; isotropy, 378.

Plateau, minimal contact-areas, 366.

Platner, mitosis, no; egg-centrosome, 125; formation of spermatozoon, 163; fertilization of Arion, 20J; maturation, 241.

Pouchet and Chabry, development and environment, 428.

Prenant, spermatozoon, 162; archoplasm, 322.

Preusse, amitosis, 119.

Prevost and Dumas, cleavage, 10.

Pringsheim, Hautschicht, 41; fertilization, 181.

Purkinje, protoplasm, 17.

Rabl, nuclear polarity, 36; cell-polarity, 56; centrosome in fertilization, 210; individuality of chromosomes, 294; astral systems,

3<7Ranvier, blood-corpuscles, 54.

Vom Rath, bivalent chromosomes, 88; amitosis, 118, 225; early germ-cells, 149; reduction, 249.

Rauber, cell-division and growth, 393.

Rawitz, amitosis, 116; staining-reactions,

335Redi, genetic continuity, 290.

Reichert, cleavage, 10, 64.

Reinke, pseudo-alveolar structure, 50; nucleuses, 303; cedematin, 36; asters, 305; nucleus and cytoplasm, 303.

Remak, cleavage, 1, 10, 361; cell-division, 64; egg-axis, 378.

Retzius, muscle-fibre, 48; cell-bridges, 60; end-piece, 140.

Rhumbler, 105.

Robin, germinal vesicle, 64.

Rosen, staining-reactions, 220.

Roux, 245, 301, 351 ; meaning of mitosis, 244,




301 , 351, 405; position of spindle, 377; first cleavage-plane, 380; frog-experiments, mosaic theory, 399; theory of development, 405 ; post-generation, 408.

Ruckert, pseudo- reduction, 248; fertilization of Cyclops, 193; independence of germnuclei, 208, 209; reduction in copepods, 249» 251 ; early history of germ-nuclei, 273; reduction in selachians, 257; history of germinal vesicle, 338.

Riige, amitosis, 117.

Ryder, staining-reactions, 175.

SabaschnikofT, tetrads, 256.

Sabatier, amitosis, 116.

Sachs, energid, 19, 30; laws of cell-division, 362; cell-division and growth, 393; development, 427.

St. George, I-a Valette, spermatozoon, 10, 134; spermatogenesis (terminology), 161.

Sala, polyspermy, 199.

Sargant, reduction in plants, 267.

Schafcr, protoplasm, 29.

Scharff, budding of nucleus, 155.

Schaudinn, mitosis in Protozoa, 92, 94, 102; polar bodies, 278.

Schewiakoff, mitosis in Euglypha, 91.

Schimper, plastids, 290.

Schleicher, karyokinesis, 64.

Schleiden, cell-theory, I; cell-division, 9; nature of cells, 17; fertilization, 218.

Schloter, granules, 38, 303.

Schmitz, plastids, 290; conjugation, 216.

Schneider, discovery of mitosis, 64.

Schottlander, multipolar mitosis, 99.

Schultze, M., cells, 1, 19; protoplasm, 20.

Schultze, O., mitosis, 318; gravitation-experiments, 422; double embryos, 422.

Schwann, cell-theory, I; the egg a cell, 8; origin of cells, 9; nature of cells, 17; organization's; adaptation, 433.

Schwarz, protoplasm, 42 ; linin, 33; chemistry of nucleus, 41 ; nuclei of growing cells,

340. Schweigger-Seidel, spermatozoon, 9, 134. Sedgwick, cell- bridges, 60. Seeliger, egg-fragments, 353; egg-axis, 379. Selenka, double spermatozoa, 142. Shaw, spermatozoids, 175. Siedlecki, polar bodies, 280. Sobotta, fertilization, 185, 211. Solger, pigment-cells, 102; attraction-sphere,

5 1 Spallanzani, spermatozoa, 9; regeneration,

393


476


INDEX OF AUTHORS


Spencer, physiological units, 289; development, tyi.

Stauffacher, egg-cent rosome, 125.

Stevens, fertilization, 217.

Strasburger, 1, 7; cytoplasm, 20; Kdrnerplasma, 41; centrosphere, 68, 356, 324; membranes, 55; origin of amphiaster, 82; multipolar mitoses, 99; theory of mitosis, 105, no; spermatozoids, 173; kinoplasm, 27, 82, 322; staining-reactions of germnuclei, 220; fertilization in plants, 216, 219, 221; reduction, 265, 269; theory of maturation, 275; organization, 289; inheritance, 7, 182, 35 1; action of nucleus, 426.

Zur Strassen, giant-embryos, 296; germcells, 148.

Van der Stricht, spindle, 79; amitosis, 116; fertilization, 210; reduction, 259; centrosome and sphere, 312, 325.

Strobe, multipolar mitoses, 99.

Stuhlmann, yolk-nucleus, 156.

Suzuki, spermatogenesis, 168.

Swingle, mitosis, 82.

Tangl, cell-bridges, 59.

Thiersch and Boll, theory of growth, 392.

Townsend, cell-bridges, 61, 346.

Treat, sex, 145.

Treviranus, variation, 179.

Unna, protoplasm, 27.

Ussow, micropyle, 133; deutoplasm, 153.

Yejdovsky, centrosome, 76; fertilization in Rhynchelmis % 192, 194; metamerism in annelids, 390.

Yerworn, cell-physiology, 6; regeneration in Protozoa , 344; inheritance, 359, 431.

Yirchow, I; cell-division, 10, 63; protoplasm, 25: cell-state, 5S.


m t


De Vries, organization, pangens, 291, 327, 406; tonoplasts, 53; plasties, 229; chromatin, 431 ; development, 404.

Waldeyer, nucleus, 3S; cytoplasm, 41 ; cellmembrane, 54. Walter, frog-e\periments, 419. Watase, theory of mitosis, 106; stain ing


J


reactions of germ-nuclei, 176; nucleus and cytoplasm, 292; asters, 305; theory of cent rosome, 315; astral rays, 321 ; cleavage of squid, 381 ; promorphology of ovum, 3S3.386.

Webber, spermatozoids, 144, 173; fertilization, 221.

Weismann, inheritance, 12; cell -organization, biophores, 291 ; somatic and germ cells, 122; amphimixis, 179; maturation, 243-246; constitution of the germ-plasm. 245; parthenogenesis, 281 ; theory of development, 404, 407, 432.

Went, vacuoles, 53.

Wheeler, amitosis, 115; insect-egg, 132; egg of Myzostoma, 151 ; fertilization in Mytostoma^ 208; bflateralitv of ovum, 3S5.

Whitman, on Harvey, 7; polar rings, 202: cell-division and growth, 393; polarity, 384; theory of development, 400, 416.

Wiesner, cell-organization, 290, 291.

Wilcox, sperm-centrosome, 165; reduction*

257. Will, chromatin-elimination, 135.

Wilson, protoplasm, 27, 44; mitosis, 106: fertilization in sea-urchin, 187, 212; paths of germ-nuclei, 202; origin of linin, 303: astral rays, 28; centrosphere and centr**some, 314; dispermy, 355; rudimentary cells, 372; pressure-experiments, 41 1: experiments on Amphioxus? 410; theory of development, 415.

Von Wittich, yolk -nucleus, 155.

Wolff, C. F., epigenesis, 8.

Wolff, G., regeneration of lens, 433.

Wolters, polar bodies in gregarines, 27S.

Yung, sex, 144.

Zacharias, E., nucleoli, 34; of meristem. 37; staining-reactions, 176; nuclein in growing-cells, 340.

Zacharias, O., amoeboid spermatozoa. 142.

Ziegler, artificial mitotic figure, 10S; amitosis, 117; sphere, 324.

Zimmerman, pigment-cells, 102; centrosome,

356.

Zoja, independence of chromosomes, 209; isolated blastoiueres, 410.


INDEX OF SUBJECTS


AcanthoeysttSy 94, 304, 306.

Achromatic figure (see Amphiaster), 69;

varieties of, 78; nature, 316. Achromatium, 39. ActinophrySy 92, 278. Actinosph&rium, mitosis, 90, 94; reduction,

278; regeneration, 342. s&quorta, metanucleus, 128. Albugo, 217. Albumin, 331. Allium, 83, 253, 267. Allolobophora, teloblasts, 374. Alveoli, 25. Amitosis, 114; biological significance, 116;

in sex-cells, 285. Amaba, 5; mitosis, 91 ; experiments on,

343Amphiaster, 68; asymmetry of, 70, 373;

origin, 72, 74, 316; in amitosis, 116; in fertilization, 187, 213; nature, 316; position, 375.

Amphibia, spermatozoa, 140; sex, 145.

Ampkioxus, fertilization, 210; polar body, 236, 277; cleavage, 370; dwarf larvae, 389, 410; double embryos, 410.

Amphipyrenin, 41.

Amphiuma, 167, 261.

Amyloplasts, 53; in plant-ovum, 133.

Anaphases, 70; in sea-urchin egg, 106.

Anasa % sperm-formation, 165, 271; reduction, 272.

Ancylus, 368.

Aniloera, gland-cells, nuclei, 36; amitosis, 116.

Anodonla, ciliated cells, 43, 357.

Antipodal cone, 101.

A plus, 281.

Arbaa'a, 192, 21 5, 307.

Archoplasm, 69; in developing spermatozoa, 171; nature of, 318.

Archosome, 52.

Argonautiiy micropyle, 133.

Aricia, rudimentary cells, 372.

Arion, spindle, 81 ; germ-nuclei, 207.

Arisama, 269.


Artemia, chromosomes, 89; parthenogenetic maturation, 281.

Artifacts, in protoplasm, 42.

Ascaris, chromosomes, 87, 301 ; mitosis, 80, ioi; primordial germ-cells, 146; fertilization, 182, 211; polyspermy, 199; polar bodies, 238; spermatogenesis, 241, 253; individuality of chromosomes, 295; intranuclear centrosome, 304; centrosome, 311; attraction-sphere, 323; supernumerary centrosome, 355.

Aster, 68; asymmetry, 70; structure and functions, 101; in amitosis, 116; in fertilization, 187, 213; nature of, 316; finer structure, 326; relative size, 70, 373.

Asttrias, spermatozoa, 176; sperm-aster, 187; fertilization, 192, 210.

Astrocentre, 324.

Astrosphere, 324.

Attraction-cone, 198.

Attraction-sphere, 51, 72; in amitosis, 115; of the ovum, 125; of the spermatid, 163; in resting cells, 323; nature of, 323.

Axial filament, 136; origin of, 165.

Axis, of the cell, 55 ; of the nucleus, 36, 294; of the ovum, 378, 386.

Axolotl, fertilization, 192.

Bacteria, nuclei, 31, 39.

Basichromatin, 38; staining-reactions, 338.

Bioblast, 290.

Biogen, 291.

Biophore, 245, 291.

Birds, blood-cells, 57; spermatozoa, 138; young ova, 155.

Blastomeres, displacement of, 366; individual history, 378; prospective value, 415; rhythm of division, 366, 389; development of single, 409, 418; in normal development, 423.

BUnnius, pigment-cells, 103.

Blepharoplastoids, 175.

Blepharoplasts, 173, 221.

Branchipusy yolk, 153; sperm-aster, 192; reduction, 256.


477


478


INDEX OF SUBJECTS


Calanus, tetrads, 25a

CalopUnus, 165, 257.

Cambium, 376.

Cancer-cells, mitosis, 98.

CanthocamptuSj reduction, 251 ; ovarian eggs, 273.

Cell, in general, 4; origin, 9; name, 17; general sketch, 19; polarity of, 55; as a structural unit, 58; structural basis, 23, 293; physiology and chemistry, 330; size and numerical relations, 389; in inheritance, 9, 430; differentiation of, 413, 426; independence of, 427.

Cell-bridges, 59.

Cell-division (see Mitosis, Amitosis), general significance, 10, 63; general account, 65; types, 64; Remak's scheme, 63; indirect, 65; direct, 114; cyclical character, 178, 223; equal and reducing or qualitative, 405; relation to development, 388, 405, 410, 427; Sachs's laws, 362; rhythm, 366, 389; unequal, 370; of teloblasts, 371 ; energy of, 388; relation to metamerism, 390; causes, 391; relation to growth, 388; and differentiation, 427.

Cell-membrane, 53.

Cell-organization, 289.

Cell-organs, 52; nature of, 291 ; temporary and permanent, 292.

Cell-plate, 71.

Cell-state, 58.

Cell-theory, general sketch, I-14.

Central spindle, 70, 78.

Centrodesmus, 79, 315.

Centrodeutoplasm, 163, 324.

Centroplasm, 324.

Centrosome, 22; general sketch, 50, 304; position, 55; in mitosis, 74; a permanent organ, 74; dynamic centre, 76; historical origin, 315; functions, 101, 354; in amitosis, 115; of the ovum, 125; of the spermatozoon, 137, 165-170; in fertiliza- ' tion, 100, 208; degeneration of, 186, 213; | continuity, 74, 77, 194, 214, 321 ; nature, 1 304; intra-nuclear, 304; supernumerary,

355Centrosphere, 68, 85; nature of, 324.

CeratiutHy 91.

Ct-ra/ozd/f/Kjy reduction, 275.

Cerebratidus, 193, 194,213, 306,307,321,325.

CerianthttSy regeneration in, 392.

Chictopttrus, spindle, 81, 84; fertilization,

192; sperm-centrosome, 213; centrosomes

de novo, 306; cell-division, 391. Chara, spermatozoids, 143.


Chilomonas, 32, 40, 192.

Chironomusj spireme-nuclei, 36.

Chorion, 132.

Chromatic figure, 69; origin, 72; varieties, 86; in fertilization, 181, 204.

Chromatin, 33; in meristem, 37; in mitosis, 65, 86; in cancer-cells, 98; of the eggnucleus, 126; elimination of, in cleavage, 147,426; in oogenesis, 233, 276; staining . reactions, 334-340; morphological organization, 37, 245, 294; chemical nature, 332, 404; relations to linin, 302 ; physiological changes, 338; as the idioplasm, 352; in development, 405, 425, 431.

Chromatin-granules, 37; in mitosis, 112; in reduction, 248; general significance, 301304; relations to linin, 302.

Chromatophore, 53; in the ovum, 133; in fertilization, 229.

Chromiole, 302.

Chromomere (see Chromatin-granule), 37, 301.

Chromoplast, 53.

Chromosomes, 67, 70, 86, 112; number of, 67, 206; bivalent and plurivalent, S7; division, 112; of the primordial germcell, 148; in fertilization, 182, 204; independence in fertilization, 204; reduction, 238, 243, 248; in early germ-nuclei, 273; conjugation of, 257; in parthenogenesis, 281 ; individuality of, 294; composition of, 301; chemistry, 334, 336; history in germinal vesicle, 338 ; in dwarf larvae, 296.

Ciliated cells, 44, 57.

Ciona, egg-axis, 379.

Clavelina, cleavage, 369, 381.

Cleavage, in general, 10 ; geometrical relations, 362; Sachs's rules, 362; Hertwig's rules, 364; modifications of, 366; spiral, 368; reversal of, 368; unequal, 370; under pressure, 375,41 1 ; promorphology of, 37S: bilateral, 381 ; rhythm, 366, 388; mosaic theory, 399, 423; half cleavage, 410.

Cleavage-nucleus, 204.

Cleavage-planes, 362; axial relations, 378.

Ciepsiiie, nephridial cell, 45; polar rings, 202 : cleavage, 370.

Ciosterinm, conjugation and reduction, 280.

Cockroach, amitosis, 115; orientation of egg,

384. Ccrlenterates, germ-cells, 146; regeneration,

392, 393. 430Conjugation, in unicellular animals, 222; unicellular plants, 228, 280; physiological meaning, 178, 223.


INDEX OF SUBJECTS


479


Contractility, theory of mitosis, ioo; inadequacy, 106.

Copepods, reduction, 251.

Corixa, ovum, 383.

Corpuscule central, 310, 314.

Crcpidula, fertilization, 210; dwarfs and giants, 389; cleavage, 323, 423.

Cross-furrow, 368. Crustacea, spermatozoa, 142.

Ctenophores, experiments on eggs, 418.

Cucurbita, 346.

Cuticular, 54.

Cyanophyceae, nucleus, 31, 39.

Cycads, spermatozoids, 144, 173; fertilization, 218, 221.

tyc/ops, o\'3i, 128; primordial germ-cells, 148; fertilization, 188; reduction, 251; attraction-sphere, 325; axial relations, 385.

Cytoplasm, 21, 41, 293, 303; of the ovum, 130; of the spermatozoon, 134; morphological relations to nucleus, 302; to archoplasm, 316, 319; chemical relations to nucleus, 333-341; physiological relations to nucleus, 341 ; in inheritance, 352-354, 359; in development, 398,421 ; origin, 431.

Cytosome, 322.

Dendrobana, metamerism, 390.

Determinants, 245.

Deutoplasm, 131 ; deposit, 153; effect on cleavage, 366, 371; rearrangement by gravity, 422.

Development, I-12; and cell-division, 388; mosaic theory, 399, 421 ; theory of Nagcli, 402; Roux-Weismann theory, 404; of single blastomeres, 399, 409, 418; of eggfragments, 296, 353, 419; De Vries's theory, 413; Hertwig's theory, 415, 432; Driesch's theory, 394, 415; partial, 409, 419; half and whole, 419; nature of, 413; external conditions, 428; and metabolism, 430; unknown factor, 431 ; rhythm, 432; adaptive character, 433.

Diaptomus, 250.

Diatoms, mitosis, 92; centrosome, 51.

Diaulula, 79, 314.

Diemyctylus, yolk, 153; yolk-nuclei, 156.

Differentiation, 361 ; theory of De Vries, 404; of Weismann, 405; nature and causes, 413; of the nuclear substance, 425 ; and cell-division, 427.

Dipsacus, 346.

Dispermy, 355.

Double embryos, 410, 422.

Drosera, 350.


Dwarfs, formation of, 353, 410, 422; size of cells, 389.

Dyads (Zweiergruppen), 239, 24 1; in parthenogenesis, 284.

Dyaster, 70.

Dycyemids, centrosome, 51.

Dytiscus, ovarian eggs, 153, 349.

Earthworm, ova, 152; spermatozoon, 165; yolk-nucleus, 154; polar rings, 156, 202; spermatogenesis, 257; teloblasts, 374.

Echinoderms, protoplasm, 28, 44, 293; spermatozoa, 137; fertilization, 188, 212; polyspermy, 194, 198; dwarf larvae, 353, 410; half cleavage, 410; eggs under pressure, 41 1 ; modified larvae, 428.

Echinus, fertilization, 210; centrosome, 314 ; dwarf larvae, 353; number of cells, 389.

Ectosphere, 324.

Egg-axis, 378; promorphological significance, 379; determination, 386; alteration, of, 422.

Egg- fragments, fertilization, 194; development, 352.

Elasmobranchs, spermatozoon, 140, 167, 169; germinal vesicle, 245, 273; reduction, 257.

Embryo-sac, 218, 263.

Enchylema, 23.

End-knob, 136.

Endoplasm, 41.

End-piece, 140.

End-plate, 91.

Energid, 19, 30.

Entosphere, 324.

Envelopes, of the egg, 132.

Epigenesis, 8, 432.

Equatorial plate, 68.

Equisetum, mitosis, 85.

Ergasto plasm, 322.

Erysiphe, mitosis, 82.

Euchictciy tetrads, 250.

Euglena, mitosis, 91, 315.

Euglypha % mitosis, 89, 95.

Evolution (preformation), 8, 399, 432.

Evolution, theory of, 2, 8.

Exoplasm, 41.

Fertilization, general aspect, 9; physiological meaning, 180; general sketch, 180; Ascaris, 182; mouse, 185; sea-urchin, 188; Nereis, 188; Cyclops, 188; Thalassema, Chatopterus, 193, 195; pathological, 198; partial, 190, 194; of Afyzostoma, 196, 208; in plants, 215; egg- fragments, 194; Boveri's theory, 192, 211.


480


INDEX OF SUBJECTS


Fishes, pigment-cells, 102; periblast-nuclei, 117; spermatozoa, 137; young ova, 116; single blastomeres, 410.

Flagellates, diffused nuclei, 39.

Follicle, of the egg, 150.

Forficula, nurse-cells, 15 1.

Fragmentation, 64.

Fritillaria, spireme, 112; fertilization, 219.

Frog, tetrads, 259; egg-axis, 378; first cleavage-plane, 380; Roux's puncture experiment, 399; post-generation, 409; pressureexperiments, 410; effect of gravity on the egg, 422; development of single blastomeres, 399, 408, 422; double embryos, 422.

Fucus, 143, 217, 221.

Ganglion-cell, 48; centrosome in, 51, 314.

Gemmae, 291.

Gemmules, 12, 291.

Genoblasts, 243.

Gtophilus* deutoplasm, 154, 158; yolk-nucleus, 156.

Germ, 7, 396.

Germ-cells, general, 8, 9; detailed account, 122; of plants, 133, 142; origin, 144;. growth and differentiation, 150; union, 196; results of union, 200; maturation, 233; early history of nuclei, 272.

Germinal localization, theory of, 397.

Germinal spot, 124.

Germinal vesicle, 124, 125; early history, 273; movements, 349; position, 387.

Germ-nuclei, of the ovum, 125; of the spermatozoon, 135; of plants, 216; staining-reactions, 175; in fertilization, 182, 188; equivalence, 182, 205; paths, 202; movements, 204; union, 204; independence, 204, 299; in Infusoria, 224; early history, 272.

Giant-cells, 31; microcentrum, 314.

Gingko, 173.

Globulin, 331, 333.

Granules (see Microsomes), of Altmann, 290; nuclear, 37, 303; chromophilic, 23, 48; in general, 289.

Gravity, effect on the egg, 1 31, 422.

Gregarines, mitosis, 89; polar body, 278.

Ground-substance, of protoplasm, 23; of nucleus, 36.

Growth, and cell-division, 58, 388.

Gryllotalpa, reduction, 249.

Guinea-pig, spermatogenesis, 170; maturation, 277.


Heliozoa, 92, 103.

Helix, 163, 168, 259.

HcmcrocalliSy 306.

Hcterocope, tetrads, 250.

Heterokinesis, 406.

Histon, 334, 336.

Homceokinesis, 406.

Hydrophilusy orientation of egg, 384.

Id, in reduction, 245; in inheritance, 406.

Idant, 245.

Idioblast, 291.

Idioplasm, theory of, 401 ; as chromatin, 403; action of, 406, 414, 431, 432.

Idiosome, 291.

Idiozome, 163, 165, 324.

Ilyanassa, partial development, 419.

Infusoria, nuclei, 31, 224; mitosis, 90; conjugation, 223; reduction, 277.

Inheritance, of acquired characters, 12, 433 J Weismann's theory, 12; through the nucleus, 351-354 ; and metabolism,

430Inotagmata, 291.

Insect-eggs, 132, 386.

Interzonal fibres, 70.

Iris, 267.

Isopods, metamerism, 390.

Isotropy, of the egg, 384, 417.

Karyokinesis (see Mitosis), 64. Karyokinetic figure (see Mitotic Figure),

69. Karyolymph, 36. Karyoplasm, 21. Karyosome, 34. Kinoplasm (archoplasm), 54, 77, 82, 173,

322.

Lanthanin, 38.

Lepidoptera, sex, 144.

Leucocytes, structure, 102; division, 117; centrosome, 309; attraction-sphere, 326.

Leucoplasts, of plant-ovum, 133.

Liliuitiy mitosis, 83; spireme, 1 12; fertilization, 219; reduction, 265-271.

Lima.Xy germ-nuclei, 204.

Li m 14 1 us y 158.

Linin, 32; relations to cytoreticulum and chromatin, 302.

Li pa r is t 281.

Locus/ay orientation of egg, 384.

LoligOy spindle, 81 ; cleavage, 381.

LumbricttSy yolk-nucleus, 157; reduction,

2 57


INDEX OF SUBJECTS


481


Macrobdella, 305.

Macrogamete, 226.

Macromeres, 371.

Mammals, spermatozoa, 139, 169; young ova, 155.

Mantle-fibres, 78, 105.

Marsilia, 175.

Maturation (see Reduction), 234; theoretical significance, 243; of parthenogenetic eggs, 280; nucleus in, 353.

Medusae, dwarf embryos, 410.

Meristem, nuclei of, 340.

Metamerism, 390.

Metanucleus, 128.

Metaphase, 69.

Metaplasm, 19.

Micellae, 291.

Microcentrum, 31 1, 315, 324.

Microgamete, 226.

Micromeres, 371.

Micropyle, 124, 133.

Microsomes, 23; of the egg-cytoplasm, 131 ; nature of, 289, 290, 293 ; of the astral systems, 318, 326; of the nucleus, 301, 303; relation to centrosome, 315; stainingreactions, 337.

Microsphere, 324.

Microzyma, 291.

Mid- body, 71, 78.

Middle-piece, 135, 139; origin, 1 61, 165170; in fertilization, 187, 212.

Mitosis, 64; general outline, 65; modifications of, 77; heterotypical, 86; in unicellular forms, 87; pathological, 88; multipolar, 97; mechanism of, 100; physiological significance, 351 ; Roux-Weismann conception of, 245, 406.

Mitosome, 165.

Mitotic figure (see Mitosis, Spindle), 69; origin, 72; varieties, 78.

Molgula, 158.

Mouse, fertilization, 185, 193.

iVusca, ovum, 142.

Myriapods, spermatozoa, 142; yolk-nucleus, 156.

Afyzostoma, fertilization, 196, 208.

A r aias, 266.

Nebenkern, pancreas-cells, 44; of spermatid,

163, 165. Nebenkorper, 164, 165. Necturus, pancreas-cells, 44. Nematodes, germ-nuclei, 184. Nereis, asters, 49; perivitelline layer, 131 ;

ovum, 129; deutoplasm, 131; fertilization,

2 I


191; attraction-sphere and centrosome, 325; cleavage, 366, 369; pressure-experiments on, 411.

Nerve-cell, 48.

Net-knot, 34.

Noctiluai, mitosis, 93; flagellum, 171 ; conjugation, 227; sphere, 319.

Nuclear stains, 335.

Nuclein, 33, 332; staining-reactions, 334; physiological significance, 340.

Nuclein-bases, 331.

Nucleinic acid, 33, 332-334; staining-reactions, 334 ; physiological significance,

340.

Nucleo-albumin, 331, 334.

Nucleo-proteid, 331, 334.

Nucleolus, 33; in mitosis, 67; of the ovum, 126; physiological meaning, 128.

Nucleoplasm, 21.

Nucleus, general structure and functions, 31; finer structure, 37; polarity, 36, 294; chemistry, 41; in mitosis, 65; of the ovum, 125; of the spermatozoon, 135, 137; relation to cytoplasm, 302; morphological composition, 294; in organic synthesis, 340, 430; physiology, 341; position and • movements, 346; in fertilization, 181,352; in maturation, 353; in later development, 425; in metabolism and inheritance, 430; in inheritance and development, 341, 358, 405, 425, 431 ; control of the cell, 426.

Nurse-cells, 151.

(Edigonium, fertilization, 181; membrane,

340.

Onoclea, 175.

Oocyte, 236.

Oogenesis, 234, 236.

Oogonium, 236.

Oosphere, 133.

Ophryotrocha, amitosis, 1 15; nurse-cells, 151; fertilization, 189, 193; tetrads, 258.

Opossum, spermatozoa, 142.

Organization, 289, 291 ; of the nucleus, 294, 301; of the egg, 397, 433.

Origin of species, 3.

Osmunda, reduction, 275.

Ovary, 123; of Canthocamptus, 273.

Ovum, in general, 8, 9; detailed account, 124; nucleus, 125; cytoplasm, 130; envelopes, 132; of plants, 133; origin and growth, 150; fertilization, 178; effects of spermatozoon upon, 201 ; maturation, 236; parthenogenetic, 280; promorphology* 378; hilaterality, 382.


482


INDEX OF SUBJECTS


Oxychromatin, 38, 303; staining-reactions,

337Oxydation-ferments, 351.

Oxytricha, 342.

Oyster, germ-nuclei, staining-reactions, 175.

Pallavicinia, reduction, 275.

Paludina, dimorphic spermatozoa, 141.

Pangenesis, 12, 290, 431.

Pangens, 291.

Parachromatin, 41.

Paralinin, 41.

Paramaba, mitosis, 94, 315.

Parametrium, mitosis, 91 ; conjugation, 224; reduction, 277.

Paranucleus, 163.

Parthenogenesis, theories of, 281; polar bodies in, 280.

Pellicle, 54.

Pentatoma, 271.

Pctromyzon, fertilization, 192, 212.

Phallus ia, fertilization, 193, 212.

Physa % fertilization, 193, 210, 212; reversed cleavage, 368.

Physiological units, 289.

PiertSy spinning-gland, 37.

Pigment-cells, 102.

Pilularia, fertilization, 216.

Pinus, reduction, 275.

Planaria, regeneration, 394.

Plant-cells, plastids, 52; membranes, 54; mitosis, 82; cleavage-planes, 363.

Plasma-stains, 335.

Plasmocyte, 52.

Plasmosome, 34.

Plasome, 291.

Plastids, 52; of the ovum, 133; of the spermatozoid, 143; conjugation of, 229.

Plastidule, 291.

Plastin, 41, 331.

Pleurophyllidia, 78, 94.

Podophyllum, 267.

Polar bodies, 181; nature and mode of formation, 235-240; division, 236; in parthenogenesis, 281.

Polar rings, 1 56, 202.

Polarity, of the nucleus, 36; of the cell, 55; of the ovum, 378; determination of, 382.

Pole-plates, 91.

Pollen-grains, formation, 263-265.

Pollen-tube, 218.

Polyclades, cleavage, 416.

PolychceruSy 276, 325.

Polygordim* cleavage, 368.

Polyspermy, 198; prevention of, 199.


Pofystomella % regeneration, 344.

Polyzonium, 159.

Porcellio, amitosis, 116.

Predelineation, 398.

Preformation (see Evolution).

Pressure, experiments, 375, 410.

Principal cone, 101.

Pristiurus, 338.

Promorphology (see Cleavage, Ovum).

Pronuclei, 202.

Prophase, 65.

Prostheceraus, 213, 235, 256, 259, 306.

Prostkiostomum, 212.

Protamin, 334.

Proteids, 331.

Prothallium, 264; chromosomes in, 275.

Protoplasm, 4, 5, 17, 19; structure, 23,42,

293; chemistry, 331. Protoplast (see Plastid). Pseudo-alveolar structure, 50. Pseudo-reduction, 248. Pteris, 253.

PUrotrachea, germ-nuclei, 186, 205. Ptychoptera, spireme-nuclei, 35. Pyg<rra t 165. Pyrenin, 41. Pyrenoid, 133. Pyrrhocoris, 165, 248.

Quadrille of centres, 210.

Rat, spermatogenesis, 170.

Reduction, general outline, 234; parallel between the two sexes, 241 ; theoretical significance, 243; detailed account. 246; in plants, 263; Strasburger's theory of, 275; in unicellular forms, 277; by conjugation, 257; modes contrasted, 247.

Regeneration, Weismann's theory, 406; in frog-embryo, 409; nature of, 425, 427; in coelenterates, 430; of lens, 433.

Rejuvenescence, 179, 224.

Renilla, ovum, 132.

Rhabdonema, amitosis, 1 1 5.

l\hynchelmis> fertilization, 192, 193, 212; cleavage, 370.

Rotifers, sex, 145.

Sagitta, number of chromosomes, 184; primordial germ-cells, 146; germ-nuclei, 184; spermaster, 191.

Salamander, epidermis, 3; spermatogonia, 20; mitosis in, 71, 78; pathological mitosis, 98; leucocytes, 1 02; spermatozoa, 140 ; maturation, 259.


INDEX OF SUBJECTS


483


Sargus, pigment-cells, 103. Scyllium, 263.

Segmentation (see Cleavage). Selaginetla, spermatozoids, 197. Senescence, 179. Sepia, spindle, 81. Sertoli-cells, 284.

Sex, 9; determination of, 144; Minot's theory of, 243. Siphonophores, amitosis, 117. Soma, 13. Somacule, 291. Somatic cells, 122; number of chromosomes,

233Spermary, 123.

Spermatid, 161, 1 63; development into spermatozoon, 164.

Spermatocyte, 161, 241.

Spermatogenesis (see Reduction), 234; general outline, parallel with oogenesis, 241.

Spermatogonium, 161, 241.

Spermatozeugma, 142.

Spermatozoid, structure and origin, 142, 172; in fertilization, 217, 221.

Spermatozoon, discovery, 9; structure, 134; essential parts, 135; giant, 141 ; double, 142; unusual forms, 142; of plants, 142; formation, 160; in fertilization, 1 81, 192; entrance into ovum, 197.

Sperm-centrosome, 135, 1 64-1 71; in fertilization, 192, 211-215, 22I>

Sperm-nucleus, 135; origin, 164-171; in fertilization, 182, 190; rotation, 188; path in the egg, 202; in inheritance, 353; chemistry, 334.

Sphtrrechinus, fertilization, 193, 210; number of cells, 389; hybrids, 353; regcnera• tion, 393.

Spindle (see Amphiaster, Central Spindle) , 68 ; origin, 72, 79,82; in Protozoa, 90; conjugation of, 227; nature of, 316; position, 375.

Spireme, 65.

Spirochona, mitosis, 90.

Spirogyra, nucleolus, 67 ; amitosis, 1 19 ; conjugation, 229; reduction, 280.

Spongioplasin, 25.

Spontaneous generation, 7.

Stem-cells, 148.

Stentor, regeneration, 342.

Stylonychia, senescence, 224.

Stypocaulon, mitosis, 82.

Surirella, 94.

Symbiosis, 53, 292.

Synapta, cleavage, 364.

Syncytium, 59.


Teloblasts, 371, 390.

Telophase, 71.

Tetrads (Vierergnippen), 238; origin, 246; in A scar is, 241, 253; in arthropods, 248; ring-shaped, 248; in amphibia, 259; origin by conjugation, 257; formulas for,

247. Teiramitus, 40, 92.

Thalassema, spindle, 81 ; fertilization, 193, 194, 213; reduction, 259, 263; centrosome, 321 ; attraction-sphere, 325.

Thalassicolla, experiments on, 344.

Thysanozodn, 212, 259, 326.

Tonoplast, 53.

Toxopneustes, cleavage, 10; mitosis, 107; ovum, 126; spermatozoon, 134; fertilization, 188; paths of germ-nuclei, 202; polar bodies, 114; double cleavage, 355.

Trachelocerca, diffused nuclei, 40.

Trilli unit 269.

Triton, 1 40, 212, 263, 277.

Trophoplasm, 322, 401.

Tubularia, regeneration, 430.

Tunicates, egg-axis, 379; cleavage, 381.

Unicellular organisms, 5; mitosis, 88; conjugation, 222; reduction, 277; experiments on, 342.

Unio, ceiitrosome and aster, 314; cleavage.

381. Urostyla, 40.

Vacuole, 50, 53.

Vanessa, ovarian e^, 153.

Variations, 1 1 ; origin of, 433.

Vaucheria, membrane, 348.

Vitalism, 394, 417.

Vitelline membrane, 132; of egg-fragments,

132; formation of, 198; function, 199. Volvox, germ-cells, 133. Vorticella, conjugation, 226.

Xiphidium, 271.

Yellow cells (of Radiolaria), 53. Yolk (see Deutoplasm), 152. Yolk -nucleus, 155. Yolk -plates, 131.

Zamia, 173, 221. Zirpfura, 259, 263. Zwischenkorper (mid-body), 71. Zygnema, membrane, 346. Zygospore, 228.


it


)


Columbia University Biological Series.

EDITED BT

HENRY FAIRFIELD OSBORN,

Da Costa Professor of Zoology in Columbia Cnirtrsity.


This series is founded upon a course of popular University lectures given during the winter of 1892-3, in connection with the opening of the new department of Biology in Columbia College. The lectures are in a measure consecutive in character, illustrating phases in the discovery and application of the theory of Evolution. Thus the first course outlined the development of the Descent theory; the second, the application of this theory to the problem of the ancestry of the Vertebrates, largely based upon embryological data; the third, the application of the Descent theory to the interpretation of the structure and phylogeny of the Fishes or lowest Vertebrates, chiefly based upon comparative anatomy ; the fourth, upon the problems of individual development and Inheritance, chiefly based upon the structure and functions of the cell.

Since their original delivery the lectures have been carefully rewritten and illustrated so as to adapt them to the use of College and University students and of general readers. The volumes as at present arranged for include:

I. From the Greeks to Darwin. By Henry Fairfield

OSBORN.

II. Ampliioxus and the Ancestry of the Vertebrates.

Bv Arthur Willey. III. Fishes, Living and Fossil. By Bashford Dean. IV. The Cell in Development and Inheritance. By

Edmund B. Wilson.

V. The Foundations of Zoology. By William Keith Brooks.

THE MACMILLAN COMPANY,

66 FIFTH AVENUE, NEW YORK.


I. FROM THE GREEKS TO DARWIN.

THE DEVELOPMENT OF THE EVOLUTION IDEA.

BY

HENRY FAIRFIELD OSBORN, Sc.D., Princeton,

Da Costa Prufe*nor of Zoology in Columbia University. 8vo. Cloth. $2.00, net.


This opening volume, " From the Greeks to Darwin," is an outline of the development from the earliest times of the idea of the origin of life by evolution. It brings together in a continuous treatment the progress of this idea from the Greek philosopher Thales (640 B.C.) to Darwin and Wallace. It is based partly upon critical studies of the original authorities, partly upon the studies of Zeller, Perrier, Quatrefages, Martin, and other writers less known to English readers.

This history differs from the outlines which have been previously published, in attempting to establish a complete continuity of thought in the growth of the various elements in the Evolution idea, and especially in the more critical and exact study of the pre-Darwinian writers, such as Buffon, Goethe, Erasmus Darwin, Treviranus, Lamarck, and St. Hilaire, about whose actual share in the establishment of the Evolution theory vague ideas are still current.

TABLE OF CONTENTS

I. The Anticipation and Interpretation of Nature. II. Among the Greeks.

III. The Theologians and Natural Philosophers.

IV. The Evolutionists of the Eighteenth Century. V. From Lamarck to 8t. Hilaire.

VI. The First Half-century and Darwin.

In the opening chapter the elements and environment of the Evolution idea are discussed, and in the second chapter the remarkable parallelism between the growth of this idea in Greece and in modern times is pointed out. In the succeeding chapters the various periods of European thought on the subject are covered, concluding with the first half of the present century, especially with the development of the Evolution idea in the mind of Darwin.


II. AMPHIOXUS AND THE ANCESTRY OF THE VERTEBRATES.


BT


ARTHUR WILLEY, B.Sc. LOND.,

Tutor in Biology, Columbia Vnirer»ity ; Balfour Student of the,

Unirertity of Cambridge.

8vo. Cloth. $2.50, net.


The purpose of this volume is to consider the problem of the ancestry of the Vertebrates from the standpoint of the anatomy and development of Amphioxus and other members of the group Protochordata. The work opens with an Introduction, in which is given a brief historical sketch of the speculations of the celebrated anatomists and embryologists, from Etienne Geoffroy St. Hilaire down to our own aay, upon this problem. The remainder of the first and the whole of tne second chapter is devoted to a detailed account of the anatomy of Amphioxus as compared with that of higher Vertebrates. The third chapter deals with the embryonic and larval development of Amphioxus, while the fourth deals more briefly with the anatomy, embryology, and relationships of the Ascidians; then the other allied forms, Balanoglossus, Cephalodiscus, are described.

The work concludes with a series of discussions touching the problem proposed in the Introduction, in which it is attempted to define certain general principles of Evolution by which the descent of the Vertebrates from Invertebrate ancestors may be supposed to have taken place.

The work contains an extensive bibliography, full notes, and 135 illustrations.

TABLE OF CONTENTS.

Introduction.

Chapter I. Anatomy of Amphioxus. II. Ditto.

III. Development of Amphioxus.

IV. The Ascidians.

V. The Protochordata in their Relation to the Problem of Vertebrate Descent.