A textbook of general embryology (1913) 7

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Kellicott WE. A Textbook of General Embryology (1913) Henry Holt and Co., New York.

Kellicott (1913): 1 Ontogeny | 2 The cell and cell division | 3 The germ cells and their formation | 4 Maturation | 5 Fertilization | 6 Cleavage | 7 The germ cells and the processes of differentiation, heredity, and sex determination | 8 The blastxtla, gastrula, and germ layers. Morphogenetic processes

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Chapter VII. The Germ Cell and the Processes of Differentiation, Heredity, and Sex Determination

The problem of heredity is the problem of development. The student of heredity is concerned primarily with the comparison of the traits of adult organisms as they appear in successive generations, and with the methods of the distribution of distinctively individual parental characteristics among successive generations of offspring. The student of development is concerned primarily with the genesis of the traits of the individual, with that continuous and orderiy sequence of changes that gives to the single-celled zygote the final form of the fully matured animal.

It might seem, therefore, that any consideration of the problems of heredity is somewhat out of place in an account of the processes of development. At one time this might justly have been urged. But to-day the students of heredity and of embryology have in common much that is fundamental. Their interests meet in the ideas that the organism is a specific creature at every stage of its existence, from zygote to adult; that the qualities of each later stage are conditioned by those of an earlier; and so ultimately the structural and functional differentiations of the adult must be traced back to corresponding differentiations of the zygote, or even to pre-conjugation phases of the gametes. It is the common endeavor of the students of embryology and of genetics to answer the question why the egg of a star-fish develops into a star-fish, with the characteristics of its parents, rather than into a seaurchin, although it may develop in the same dish of water with other eggs that do develop into sea-urchins.

Heredity is the fact of resemblance between offspring and parents, not the resemblance of adult stages alone, but the likeness at all corresponding ages. That the individual ova of Asterias vulgaris are alike, that the cleavage processes, blastulas, gastrulas, larvse, and adolescent stages of all the individuals of this species are essentially alike, in structure and in behavior — all this is similarly the fact of heredity. A specific kind of protoplasm is never, whatever its form, anything other than that specific kind.

In other words, the interest of the embryologist in the problems as to why the ovum develops as it does, passes from one condition to another as it does, and finally produces the kind of adult that it does, is essentially an interest in the problem of heredity — the problem of organismal specificity. This is the central point of embryological study. Consequently we are fully justified in considering in this place, these general problems of the relation of the facts of cjrtology and embryology to the facts of parental and specific likeness of organisms. Failure to do so would mean the omission of the most vital topic around which much, perhaps it would not be going too far to say most, of recent embryological investigation has centered, and upon which it is to-day focussed.

The answers which the facts of embryology have to offer to these fundamental questions are still rather vague and uncertain. Most of them are stated in the potential mood and must still be framed as hypotheses. But although the facts here may be much clearer than their significance, we must attempt a statement of both ; and it goes almost without saying that our treatment of both must be as brief and as elementary as possible; this is not the place for extended consideration of hypothetical views, however great the importance of the central ideas.

We may address ourselves, therefore, to a survey and brief analysis of the answers which have been given to the question why the organism develops in the way it does. First, let us recall the idea, stated briefly in the introductory chapter, that development is a form of behavior — a series of reactions. In any organic reaction the two factors of external and internal conditions axe involved. The reactions of the ovuin in cleaving, of the blastula in gastrulating, and the like, are of a general nature, i.e., the external conditions involved may be considerably varied, within limits, and yet produce the same response on the part of the organism. Emphasis thus is placed upon the internal factors in the reactions of development, for on account of the definite character of habits of life and of spawning, the normal external conditions of development are sufficiently uniform to produce a series of reactions, a development, which is also uniform, i.e., normal. Of course it is easily possible to alter, artificially, the external as well as the internal conditions of development and the results of such alteration often lead, Bs we shall see, to unportant ideas regarding normal or characteristic embryonic behavior.

Development is, then, a series of reactions, one condition leading to the next; and the primary factor in determining the quality of each reaction is the internal condition or structure, both morphological and physiological, of the organism, whether it be ovum, zygote, blastula, larva, or adolescent individual.

Throughout the efforts to solve the problem of individual development, the attempt has always been to explain existing differentiations as being dependent upon some preexisting differentiation, related but of a different kind. Thus at one time the earliest differentiations that became visible in the developmg organism were the germ layers, and these were consequently regarded as the fundamental differentiations of the embryo, determining its subsequent history. Next, differentiations among the cleavage cells were noted and emphasized as primary. Then as technique improved, and the subject of cytology developed, attention became focussed upon the nucleus and its organs not only as the centers of cell life, but as the structures primarily concerned in the differentiations of the developing egg cell. And lately, chiefly as a result of experimental analysis of the processes of development, rather than as the consequence of observation alone, structural differentiations of the cytoplasmic portion of the ovum have occupied the center of interest as determining factors in development. This succession of views represents rather the order of their general acceptance as working bases, than of their discovery and individual promulgation.

In accordance with this historical succession of ideas regarding the nature of the underlying differentiations in development, we may outline briefly three general hypotheses of the causes of differentiation. We may omit, as being now of historical interest chiefly, any further reference to the germ layers as the primary determiners of development, and may begin with the idea of Pfluger, that the egg and the blastomere group are homogeneous or isotropic throughout, and that the early developmental processes of cleavage are nothing more than indifferent multiplications of similar units, resulting in the formation of blocks out of which later differentiating structures may be built. During the early '90's this view was quite prevalent, and especially favored by such embryologists as Oscar Hertwig and Driesch, who developed it somewhat farther into what has been termed the "cell interaction" hypothesis. According to this hypothesis, while the cells of the blastomere group are essentially similar and equivalent in their potentiaUties (prospective potency, Driesch), differentiation exists among them by virtue of their relative position in the cell group — not through any actual, individual, intracellular differentiation. Stated in the words of Wilson (The Cell, etc,, page 415), two sentences of Driesch sunmaarize this view as follows : The blastomeres of the sea-urchin 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." 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 ["prospective significance," Driesch] is a function of its position."

In itself, then, the cell interaction hypothesis offers no explanation of differentiation or development, for it throws back upon some unknown factor the real cause of differentiation through position. Later investigation, chiefly experimental, has shown clearly that cell interactions alone play but a small part indeed in the process of differentiation, and has led to the search for that underlying factor or group of factors. And while Driesch himself concludes that no explanation is possible in known terms of matter or energy, and reUes upon an unknown, and therefore metaphysical, factor, the great majority of embryologists believe that the question is still susceptible of further scientific analysis. We find two general hypotheses regarding the nature of the causes or conditions of differentiation, and since differentiations are always specific we may speak of these as also hypotheses as to the causes of those resemblances among generations of organisms which we call in a word, heredity.

The first of these is the hypothesis of "germinal localization" or "germinal, organ-forming regions" associated primarily with the names of His, Lankester, and Whitman. The essentials of this hypothesis in its present form may be stated as follows. The cjrtoplasm of the ovum before development (i.e., cleavage) begins, has a definite structure or morphology of its own, such that particular regions or substances, by effecting specific developmental reactions, are seen to correspond with, or to lead to the formation of, particular tissues or structures of later stages and of the fully developed organism. The cytoplasm is conceived as a mosaic-work of physiological units which have not only a definite morphology but a definite promorphology, looking toward the structure of the mature individual. This immediately suggests the old idea of preformation" but it omits, of course, the naive crudities of this conception and rests upon the idea that certain structures and regions of the egg cytoplasm are in direct genetic relation with corresponding structures and regions differentiating later by a true process of development or epigenesis. These germinal structures have specific reference, but not resemblance, to the pao^ of the mature organism. This predetermination may be only general to begin with, but it becomes more complete and specific as one condition succeeds another, the cjrtoplasmic structure of the ovum representing in the beginning all there is of definite, specific, organismaJ structure. This cjrtoplasmic structure of the germ is regarded as continuous from one generation of ova to the next, through the germ within the organism, and it thus serves as the physical basis of heredity.

As more or less opposed to this conception we have a group of hypotheses which may collectively be termed the hypothesis of "nuclear analysis/' Here the nucleus alone is regarded as that part of the germ or zygote which bears a specific relation to later differentiations, and within the nucleus, the chromosomes are the elements chiefly concerned. Chromosomes are supposed to possess a structural predetermination that is j>^omorphological; they are imlike and individually behave specifically in determining the characteristics of developmental reactions. This hypothesis, in its various forms, is associated chiefly with the names of Nageli, Roux, Weismann, DeVries, and Oscar Hertwig. It will be recognized as also preformational in its essentials; specific configurations of chromatin represent potentially, corresponding embryonic and adult traits, which become actual by a truly epigenetic series of developmental reactions.

It has been pointed out frequently that this hypothesis really transfers the idea of germinal localization from the cytoplasm to the nucleus; the structure of the cytoplasm is regarded as real, but as secondary and dependent upon the primary structure of the nuclear elements. These nuclear organs, the chromosomes, are thought to maintain a specific physiological continuity from one generation of ova to the next and thus to constitute a real physical basis of heredity.

These two general hypotheses have in common the idea of a fixed promorphological structure within the germ which becomes expressed epigenetically. They differ as regards the particular part of the germ whose promorphology is to be regarded as primary, and yet it is quite possible that both hypotheses contain elements of truth; It remains now for us to review some of the more significant facts of development bearing directly upon these ideas in order to determine, perhaps which, perhaps how much of each, is justified. In doing this we shall not attempt to relate the evidence directly to either hypothesis, leaving that for the reader; and in conclusion we shall attempt a summary which may serve to bring the two hypotheses together.

We may first describe certain facts associated chiefly with the hypothesis of germinal cytoplasmic localization.

In the first place it is perfectly clear that the oviun does possess a marked structure and organization, indicated in several ways. Occasionally the ovum may show external differentiations of form; such an egg as that of the squid {Loligo, Fig. 113) or the fly {Muscay Fig. 47), is obviously not only bilaterally symmetrical but it exhibits definite anteroposterior and dorso-ventral differentiation. In a few instances the eggs of a species are dimorphic, and while apparently the nuclei of both kinds are identical in structure, the total volume of one form may be three times that of the other. One of the very important and highly significant factors in the organization of the ovum is that of polarity, aheady described (6.gf., Figs. 42, 93). In many cases the polarity of the ovum can be traced back into oogonial stages, where it is seen to correspond with the polarity of the cells of the germinal epithelium (Mark). Polarity pervades the whole structure of the mature ovum and is expressed in a variety of ways, by the eccentric position of the nucleus, by the point at which the polar bodies are formed, by the disposition of the deutoplasm, and by the arrangement and distribution of a variety of formed substances, such as pigments, granules, and vacuoles of many kinds.

Living protoplasm, as we have seen, consists of a fundamental matrix or ground substance of rather uncertain form and composition, and suspended within this, particles and granules of many sizes, forms, and materials, the nature of which gives character to a particular region of protoplasm. These different kinds of substance are not distributed at random through the ovum, but they are localized in certain regions, as zones or layers, either horizontal or concentric.

There are at present two views as to the relation of these substances to the fundamental polarity and general organization of the egg. In the opinion of some these substances are truly to be regarded as the initial developmental differentiations of the ovum. Each of these substances has a specific function in development and leads to the formation of a certain tissue or organ alone. Consequently these materials are known as "organ-forming substances." Before cleavage they usually assume a very definite symmetry of distribution, closely related to that of the later developing organism, and during cleavage they become distributed among certain specific groups of cells whose lineage can be traced directly to the rudiments of certain organs.

Thus in the egg of the Ascidian, Cynthia (JStyela), fully described by ConkUn and one of the best examples of this type of structure, at the close of the first cleavage there are five regions of protoplasm, present in amounts roughly proportional to the size of the parts to which they later give rise, and distinguishable by the character of their granular contents (Fig. 92). At the animal pole is a superficial region of comparatively clear protoplasm, the ectoplasm, from which the ectoderm develops; in the vegetal pole there is a dark gray region, the endoplasm, rich in yolk and later forming the endoderm; the mesoderm is formed from a crescentic region, the mesopldsmj located just below the equator, on the posterior side; this is characterized by its content of yellow pigment, anij is divided mto lighter and darker areas, forming respectively the mesenchyme and the tail muscles (myoplasm); a light gray crescent around the anterior border forms later the neural plate and notochord (neuroplasm, chordaplasm).

These substances are arranged symmetrically with reference to the first cleavage plane and this corresponds also to the median plane of the larva and adult, and thus from the first separates right and left sides of the body. The second cleavage plane, at right angles to the first, separates the yellow mesoplasm from the light gray neuroplasm and chordaplasm, and the third cleavage, horizontal, separates the clear ectoplasm from the other substances. The later cell lineage of this form.

Fig. 123. — Changes in the atnicture ot the egg of the Annulate, ChaUyptem*, during and after fertiliiation. From Lillie. A. Aiial section through fully grown oftcyte, still within ovarian epithelium. Ectoplasm in upper two-thirds of egg only. B. Axial section through primary oCcyte, ten minutes after extrusion, three minutes after fertilisation. Ectoplasm has already flowed to the vegetal pole, leaving an exposed area of endoplasm at the animal pole. Part of the a is fully described by the same author and it is clear that each of these substances becomes contained within the cells forming the rudiment of the particular organ or tissue mentioned.

There are few other instances known where it is quite so easily possible to distinguish the specific formed substances in the egg. But in many ova, before cleavage it is possible to distinguish several kinds of substance; thus there are known in the eggs of certain Echinoderms four differentiated materials (Lyon), three in Hydatina (Whitney), Dentalium (Wilson), Physa, Lymncea (Conklin), four in Cumingia (Morgan), three in the frog (McClendon), etc. (Figs. 42, 45, 86, 91, 123, 126, 129).

The term "organ forming" as apphed to these materials does not rest aJone upon the observation of normal development, but upon experimental grounds as well. For an example of this kind of evidence we may return to the work of Conklin on the egg of Cynthia. If one, say the right, of the blastomeres of the two-cell stage is injured so as to prevent its further development, the remaining blastomere develops as it would normally, i.e., into the left half of an embryo and larva, containing approximately one-half the number of cells found in the normal organism at the corresponding stage (Fig. 124). Embryos derived from the two anterior cells of the four-cell stage never possess a tail or any muscle cells, while chorda cells and neural plate cells differentiate normally, and the endoderm and covering ectoderm are also typically formed (Fig. 125). Correspondingly, embryos derived from the two posterior cells have no chorda, nerve, or sensory cells, or gastral endoderm, but endoplasm has also passed to the vegetal pole. The germinal vesicle has broken down, and the maturation spindle is in the process of formation, between the two asters. The residual substance of the germinal vesicle is clearly seen.

C. Axial section through secondary o5cyte, thirty-two minutes after fertilization.

D. Longitudinal section, first cleavage; late anaphase. Posterior end toward the left, anterior toward right. The ectoplasm of the polar lobe has been separated from the remainder. E. Sagittal section through stage of about sixty-four cells. The small upper cells are the apical cells. The ectoplasmic defect will be noted in the posterior apical cell to the observer's right. A, animal pole; c, chromatin; C.V., chromosomal vesicles of daughter nuclei; E, ectoplasm; e.a., e.h., e.c, endoplasm a, &, and c. E.d.t ectoplasmic defect; En^ endoderm cells; m, mesoblast cell; n, nucleolus; p.l., polar lobe; r.«., residual substance of germinal vesicle; «.a., sperm aster; «.n., sperm nucleus; V, vegetal pole; X, derivatives of first somatoblast.

Fig. 124. — Tbe derelopmeiit of one of the bloBtomeres of the two-oell sttiEe of the Tunicate. Cynihia. From Conklin. The yellow material (see Figs. 91, 92) is stippled; the boundary between clear protoplasm and yolk is indicated by a crenated line.

A. Right hair of eight-cell stage; posterior view. B. Right half of thirty-cell stage; dorsal view. C. Right half of forty-eight-cell stage; dorsal view. D. Right half of sixty-four- to seventy-Bii-cell stage; doraal view. E. Right half of gaatrula of about 220 cells derived from four-cell stage. The neural plate, chorda, and mesoderm cells, are present only on the right side, and in their normal positions and numbers. F, RiRht half of young tadpole; dorsal view. Derived from four-cetl stage. The notochord consiatB of a small number of cells which are interdigitating; muscle-cells and mesenchyme lie on the right side of the chorda, but not on the left side, though the muscle cells have begun to grow around to the left side. The neural plate is normal in position but not in form. m'ch., mesenchyme; mi, muscle cells.

The cell nomenclature in this and the following figure, differs from that described in Chapter VI. The right and left halves of the embryo are designated consist of a mass of muscle and mesenchyme cells with a double row of caudal endoderm cells, as in the corresponding region of a normal larva. Equivalent results may be obtained by injuring one or three cells of the four-cell stage (Fig. 125).

The work of Roux, Fischel, Wilson, and many others has demonstrated similar localizations in the eggs of many forms — the frog, other Ascidians, several Molluscs, Annulates, and the Ctenophores, but we must limit ourselves to the mention of only a few interesting details of the experiments on these forms.

The MoUusca afford several very striking illustrations of the effects of the removal of parts of the egg or of blastomeres. The egg of Dentalium, as described by Wilson, has an upper clear area which normally forms the ectoderm, a middle reddish or brownish pigmented zone forming endoderm, and a lower clear area which during cleavage forms a peculiar yolk lobe or "polar lobe" (Fig. 126). When this yolk lobe is entirely removed from the segmenting egg the development of the remainder proceeds as if it were present, and a larva is formed which lacks the apical organ and the entire post-trochal region (for explanation of terms see Fig. 126), and which develops later into an organism lacking those structures which would normally have been formed from this part of the egg and larva, namely, the foot, mantle, shell glands and shell, pedal ganglion, and apparently also coelomic mesoblast. Other MoUusca give essentially similar results although of course not all possess a yolk lobe; but removal of blastomeres is always followed by absence of specific parts in later development [e.g., Wilson, Crampton, Conklin) (Fig. 126).

The blastomeres of several species of animals fall apart, or may be shaken apart easily, after a brief treatment with calciumfree sea water, a fact discovered by Herbst and applied by him and many others to an analysis of this problem of localization. The blastomeres of many E^hinoderms, Molluscs, etc., can be thus separated, and it is a renarkable fact that one of two, four, eight, or even one of sixteen cells, continues to develop for some time and forms those parts, and only those, which it Tould have formed, had development of the entire cell group by the same letters, those referring to the right side being underscored. A and B refer, respectively, to the anterior and posterior hemispheres. After the third cleavage, all cells Ijdng on the polar body side of that cleavage plane are xiesig-r nated by lower case letters, while those on the opposite side of that plane continue to be designated by capitals. The first exponent following a letter indicates the generation to wl^ich the cell belongs. The second exponent refers to the position of the cell relative to the vegetal pole.

Fig. 125. — The derelopmeDt of blastomeres of the four-cell stage of Cf/n/Aio* Prom Conkhn. A. Anterior half-embryo derived from two anterior blastsmerea. The yellow cteBcent remaJOB visible in the posterior, uninjured cells (S*). Seosa spots are present but the neural plate never forms a tube. The chorda cells lie in a heap at the left side. There is do trace of muscle substance or of a tail. B. Posterior half-embryo from the two posterior blsstomereB. Dorsal view, focuased deeply upon the double row of ventral endoderm cells in the mid-line, a mass of mesenchyme cells on each side. No neural or chorda cells. C. Left anterior quarter embryo from cell A; dorsal view. An invagination of the ectoderm cells has the appearance of a gastrula, but is probably the invagination of the neural plate. D. Lett anterior, and right posterior quarter-embryos, from cells A and B; dorsal view. The former shows thickened ectoderm cells, probably neural plate, around the endoderm cells; in the latter are eight muscle cells and three caudal endoderm cells, m'ck, mesenchyme; ms, muscle cells; n-y., neural plate; v.ertd., ventral endoderm.

Fig. 126. — Development of the Mollusc, DeiKoJium, after removal of the "polar lobe." From Wilsoo. A. Egg tiventy minuteB after eitrusion, and before maturation is completed, ahowinK regioaal difierentiation. B. Section through egg oiie hour after fert.iliiatioa, showing the beginniDg of the formation of the polar lobe. C. Normal eight-cell stage, viewed from lower pole. The polar lobe is the light part of cell D. D. Normal aiiteen-cell stage viewed from lower pole. The materials of the polar lobe are Dow contaioed in the cell marked X, B. Siiteeo-cell stage of egg from which the polar lobe was removed during the first cleavage period. F. Normal trochophore of twenty-four hours. O. Trochophore of twenty-four hours, developed from "lobeleas" egg. H. Normal larva of seventy-two hours, showing foot and shell. /. Seveuty-two-hour larva from " lobeless " egg. p, polar lobe.

Fig. 127.^Cleavage of iaoLated blaBtomeres in the egg of the MoIIuhc, Patella. rroin Wilson. A-D. x 167; G. H. x 342; others X 308, A. Normal eight-cell stage, viewed from upper pole. Fourth cleavBge !□ proBress. B. Normal thirtytwo cell Btsge, from side. C. The so-called "cteaophore stage" (aonual) vieved from upper pole. The primary troohoblasts are ciliated. D. Normal troihophore of thirty hours, from left aide. Body wall in section, prototrochal cells in surface view. E. Second cleavage of an isolated micromere of the first Quartet (one of eight cells). F. Entire quadrant — products of first and second quartet cells, being formed much as in the normal egg. G. Larva of tweuty-tour hours from one of eight cells (micromere). From aide, showing trochoblaHta below, apical cells above. H, Product of primary trochoblost isolated from siiteencell stage. /. First division of isolated first quartet. J. Division of isolated basal cell of eight^cell stage, showing typical arranKemect of these four cells aa in the normal group of thirty-two. K. Larva of twenty-four hours, developed from group like I, showing two secondary trochoblasts and two feebly ciliated cells (T pre-anal cells), m, primary mesoblaat cell; (.0, shell gland.

been occurring normally, although ultunately a normal larva may be formed (Fig. 127).

The idea that these diflFerentiated materials of the cytoplasm really play the r6le of organ-forming substances in development, is opposed by some (Lillie, Morgan, and others). The opposed idea rests upon the experimental evidence that, briefly stated, the really primary and fundamental organization of the egg cytoplasm concerns the ground substance of the protoplasm; the arrangement of the various formed stuffs coincides with a similar and primary polarity and organization of this fundamental protoplasmic matrix. This structure is less manifest, but is really the factor which determines the arrangement of the suspended cytoplasmic and deutoplasmic granules and vacuoles. The correspondence between the arrangement of these stuffs and the organ-forming substances proper, is thus unessential, for the localization of the germ is primarily a localization of the ground substance. In other words, the varieties of material described by Conklin in Cynthia, for example, are only secondarily related to the later differentiation of particular organs or tissues, and their arrangement is dependent upon the same primary factor that determines the arrangement of the organs and tissues.

The evidence for this view is found chiefly in the results of certain experiments upon the eggs of ChcBtopterus (Lillie) and Arbacia (Morgan and Spooner). The granules which give character to the various regions of the cytoplasm differ in specific density, and consequently can be thrown, by centrifugal force, into abnormal regions of the ovum. When this is done normal cleavage and development may proceed, normal with respect to the original polarity of the ovum and not with respect to the new polarity as indicated by the altered arrangement of the plasmas.

To illustrate, the egg of the sea-urchin Arbacia^ contains four different kinds of substance; one of these is distinguished by the presence of bright orange or reddish pigment. In normal development this substance lies toward the lower pole and becomes localized in the lower quartet, so that when the micromeres form here, they axe composed of this material. The micromeres, which later form the mesenchyme, always appear at the pole opposite the micropyle (Fig. 109), which marks the point of attachment of the ovum in the ovarian germinal epithelium; this is also the point at which gastrulation commences. The centrifuge brings about a stratification of these .substances which is independent of the polarity of the ovum, pince the ovum may assume any position with reference to the axis of rotation of the machine. The pigmented protoplasm may be thrown to any part of the cell. But Morgan has found that the cleavage of eggs with abnormally distributed substances proceeds normally with reference to the original polarity of the ovum and not according to .the induced arrangement. The micromeres, for example, continue to form opposite the micropyle, and gastrulation occurs here, as usual, although the pigmented protoplasm may occupy some remote and unusual position in the cell (Fig. 128). Perfectly typical larvse develop from such eggs, normal save in the distribution of pigment. Development and differentiation thus seem to be quite independent of the so-called "formative stuffs," which are, in such instances, evidently not "organ forming."

Fig. 128. — Normal cleavage in the sea-urchin, Arhaciat following abnormal distribution of egg substances by centrifuging. From Morgan and Spooner. The figures are turned so that the pigment (dotted area) is downward. The location of the cleavage planes, and the position of the micromeres, which always mark the invaginating pole also, are independent of the induced stratification of the egg substances.

Several other forms are known to give similar results. One of the clearest instances is to be seen in the Lamellibranch, Cumingia, also described by Morgan. The egg of Cumingia contains, besides the clear protoplasm, three kinds of formed substance, yolk, pigment, and oil. With the centrifuge these can be thrown to any part of the cell whatever, and yet cleavage and development proceed normally with reference to the original polarity and not at all to the actual distribution of these substances (Fig. 129). It should be noted that, although this has not been definitely determined for Cumingia, the MoUusca in general are excellent examples of determinately cleaving eggs and the removal of parts of ova is followed by definitely corresponding defects in embryo and larva. Such experiments as these seem to indicate clearly that the detennining structure of the ovum is really that of the underlying protoplasmic ground substance, and that the arrange ment of the various formed substances coincides with this, is determined by it in the first place, but is not always or necessarily concerned directly in the later differentiations of the ovum. The defects following removal of parts of the unsegmented ovum, or of blastomeres result therefore from the loss of parts of this underlsdng structure, and not from the loss of the formed materials or "formative stuffs," which in such cases at least turn out not to be "formative/^

Fig. 129. — Normal development of the Pelecypod, Cumingia, following abnormal arrangement of the egg substances by centrifuging. After Morgan. The pigment is indicated by stipples, the oil by small circles. A. Two-cell stage with oil in small cell. B. Same with oil in large cell. C. Same with oil in both cells. D. Normal trochophore, showing usual distribution of pigment and oil. E. Trochophore with oil on oral side, and yet normal. F. Normal trochophore with oil aboral and interior.

Lillie has shown that carefully graduated centrifuging reveals the existence in the egg of Chostopterus, of certain regional differentiations of the ground substance, indicated by the differences in the ease with which the granides of various sizes, and other structures, such as parts of the mitotic figure, pass through it. These regions are not otherwise visible but Lillie suggests that since they are undoubtedly real they may represent or mark in some way the primary organization of the cytoplasm.

According to this view of organization the term "organforming substances" for the visibly differentiated substances is a misnomer. For if these axe removed to abnormal positions within the cell they are then not related to the formation of the same structures that they are in normal development.

At present it seems difficult, though not impossible as we shall see later, to reconcile these two views as to the real seat of the primary organization of the cytoplasm of the ovum; the balance of evidence appears to favor the conception of organization as a condition of the fundamental ground substance of protoplasm. But in any event it is perfectly clear that the cytoplasm is organized definitely.

We should call attention in passing to the fact that many of the results described above indicate that cleavage is not to be regarded always as a developmental process of primary importance. Conklin has called attention to the fact that in Cynthia the early cell boundaries do not always coincide with the limits of the various kinds of cytoplasm. The determination of the structure of the egg and the localization of these materials, precede cleavage and are independent of it. Certam pressure experiments to be mentioned shortly, illustrate the independence of cleavf^e and determination, and the centrifuging experiments similarly demonstrate the independence of cleav^e and the distribution of the cytoplasmic stuffs. Indeed Lillie describes the formation of a trochophore-like larva from the egg of Chastoplerus in which cleavage had been artificially prevented; this embryo formed external cilia and certain other differentiated structures in the complete absence of cell divisions (Fig. 130). The normal processes of development are varied and more or less independent of each other, while having common reference to some general underlying condition.

Fig. 130. — Development and differentiBtion in the absence of cell divirion, in Chatojierua. From Lillie. A, B, C. Ciliated, uninucleated unsesmeDted etie, about twenty-three hours old. The vaouolee are about in the position of the prototroch of the larva. D. Ciliated unsegmented egg about twenty-eisht hours old; most of the endoplaem haa been coniumed. e, endoplasm.

We must now consider the facts of development in a considerable group of eggs which do not show any such results as those described in the foregoing pages. Although these eggs possess more or less differentiated regions of cytoplasm, yet removal of parts or of blastomeres is not followed by any structural defect. For example, the blastomeres of many Coelenterates (Haeckel, Zoja, Maas, Wilson) may be separated when in the two-, four-, eight-, or even, in some cases, in the sixteen-cell sta^e, and from such isolated blastomeres typically formed embryos and even free-swimming larvse develop, normal in every respect save that of size, being respectively approximately one-half, one fourth, one-eighth, or one-sixteenth the normal size (Fig. 131). This is true to a certain extent also of some of the Teleosts and of Amphibians, the Nemerteans, and Echinoderma (Figs, 132, 133); in the last named forms even portions of the blastula or gastnila (Driesch) may give rise to normal but diminutive larvEB (Fig. 134). It seeme very apparent that if cj'toplasmic localization occurs at all in such cases, it must foe of a very different kind from that described above.

Fig. 131. — Normal development of oae of the blaatomerea of the two-cell stage of the Hydrojd, Clt/lia flavidula. After Zoia. A. Two-cella. B. Fourcellfl. C. Eight-cells. D. BlaBtula. E. Young polype.

This is an appropriate place to mention certain experiments of a different kind bearing upon this same problem. Eggs may be subjected, during their early cleavages, to deforming pressure so that the planes of cell division appear in abnormal relations to one another and to the egg as a whole (Hertwig, Bom). Thus in the sea-urchin (Driesch) the blastomeres of the eight-cell stage instead of forming a spheroidal group may be forced into the form of a flat plate (Fig. 135). When released from the pressure such eggs form perfectly typical larvae. And even in a form like the Annulate, Nereis, whose cleavage is determinate and whose blastomeres are highly differentiated, Wilson has found that when the egg, subjected to pressure, became divided by vertical planes into a flat plate of eight cells, each one contained substance normally found only in the macromeres of the lower piole; when released these eight cells divided into sixteen, eight micromeres and eight macromeres, instead of into the normal twelve and four respectively. And from these, normal larvse developed; the eight macromeres developed as the noi^nal four would have done, although under normal conditions four of the eight cells and nuclei would have formed the first quartet, giving rise to the apical nerve cells and anterior band of ciliated cells.

Fig. 132. — Gastruls and plutei from isolated blastomeres of the sea-urchins. Echinus (A-D), and Sphcerechintis (E-G). After Driesch. A. Gastrula from entire egg. B. Gastrula from one blastomere of the two-cell stage. C. Gastrula from one blastomere of the four-cell stage. D, Gastrula from one blastomere of the eight-cell stage. E, Normal pluteus. F, Pluteus from one blastomere of the two-cell stage. O, Pluteus from one blastomere of the four-cell stage.

Fig. 133. — Four normal but diminutive plutei from the ieolated bloatomer of the four-cell stage of the seB-urchin, StTonoi/loceniroltis. After Boveri. A, i in oral view. C, D, lateral view.

Furthermore, the experiment of bringing about the coalescence of parts of two eggs, or even of two complete eggs, has been accomplished with Ascaris (Sala, Zur Strassen) and with the searurchin (Driesch). The result is again the development of a normal larva, of very large size when two entire eggs are fused (Fig. 136). Even when two blastulaa coalesce the final result may be a single larva, though with some doubling of parts. One especially interesting point is that normal development may result even though the parts of the two blastulas may be of dififerent species, in somewhat different stages of development, and no micromeres included in the mass (Garbowsky).

Fig. 134. — Normal but diminutive larvsB of Echinoderms, derived from portions of gastrulse. From Jenkinson, after Driesch. a. Normal pluteus of SphcBrechinus, h. Pluteus of same from portion of gastrula. c,e. Normal bipennaria of Aateriaa glacicdia, dj, Bipennaria of same, from vegetative half of gastrula. g. Larva of Aaterias with typical three-parted gut, but no coelom, from vegetative half of gastrula, removed after development of the coelomic sacs.

While such results as these are at first sight opposed to the hypothesis of germinal localization, yet it is quite possible to reconcile the differences between such extreme forms as the Echinoderms, where one of eight or sixteen cells finally forms a typical larva one-eighth or one-sixteenth normal size, and the Ascidian, where one of four, eight, or sixteen cells gives rise, not to a complete diminutive larva, but to a group of differentiated tissue cells of the same kind that would normally have been formed from the particular cell, had it remained in situ in the normal group.

Fig. 135. — Cleavage in the egg of the sea-urchin, Echinus micro-tuberculatua, under pressure. From O. Hertwig, after Ziegler. A, B. Eight- and sixteen-cell stages. C. Sixteen-cell stage preparing for division. D. Thirty-two-cell stage, in the form of a flat plate. E. Thirty-two-cells preparing for next division. Crosses mark cells in which the spindle is vertical or oblique, to the plane of the cell group.

The discordance of these results may have one of two meanings. First, it may mean that in such eggs as those of the Echinoderms and Amphioxus a process of regeneration or regulation goes on. And, just as many adult organisms are easily capable of restoring or regenerating lost parts, so the embryo or even the ovum may have the property of reforming parts artificially removed. This process has received the special term of posir^eneration (Roux). Such a possibility is indicated by the classic experiment of Roux upon the egg of the frog. Here, if one of the two blastomeres is destroyed the remaining one, if undisturbed, develops into a half -embryo ; but if the egg is inverted after the injury of one blastomere, then during the consequent rearrangement of the substances of the uninjured cell, through the action of gravity, the organization is restored to the normal and a small normal embryo subsequently develops. This shows that the uninjured half of the egg does possess the potentiality of developing as a com- . plete egg.

Fig. 136. — FuBioQ of Echinoderm I«rvie. A, B. SjAareckimiB. After Driesch. C, D. Paammechina* miliarii. After Garbowski. A. Normal gastrula. B. Single gastrula derived from the fudoa of tvo normal blastuls, showing single, larKB cut and doubled spicule. C. Normal stase of thirty-two-cells. D. Organigm formed by the coalescence of parts of two organisms in different Btages. The eells ruled obliquely were part of an eii[ht-cell stage, stained intravitally with neutral red. The remaining cells were part of a normal thirty-two-cell stage. In both C and D the stippling marks the cells derived from the vegetative half of the egg.

Or second, the contrast between the two extreme cases mentioned may mean that localization results from a progressive process of true development. Of course, in all organisms^ sooner or later, groups of cells become specifically differentiated as particular tissues and organs or parts of oi^ans. And similarly there comes a time in the history of any cell groupi when, once started on its course of differentiation, return or redifferentiation in another direction is impossible.

The formation of localized germinal areas of cytoplasm is to be regarded as a process of development, and in the eggs of different species this process may be carried forward at relatively different times with respect to fertilization, cleavage, and other early developmental phases. The most important steps in cytoplasmic localization of the germ may be completed while maturation and fertilization are going on, prior to the first cleavage (Ascidians) ; or localization may be accomplished during cleavage (Cerebratidus), or not until the gastrula or postgastrula stages (Echinoderms).

This idea is not essentially different from that of post-generation in certain respects, for regeneration and regulation are after all essentially processes of development, deferred development. The two differ however in that, according to the former view localization is really present throughout the early stages and disturbances are followed by an active process of regulation; according to the latter, localization is not determined during the earlier stages and when it does appear, the parts of the egg remaining after the removal or injury of parts, behave as a complete and normal unit, no regulation being necessary.

There is evidence for both of these views, and both may be true at the same time. The second appears to be the more widely applicable. Regulation seems more likely to occur during comparatively late phases of localization. Evidence of regulation following the separation of blastomeres is afforded by such eggs as those of the Echinoderms, where the isolated blastomere continues to segment for a time as if it were part of a normal cell group, but gradually its cell products assume the characters of a typical whole group and finally give rise to a normal embryo and larva. In other cases (Amphioxus) separated blastomeres develop from the beginning like whole eggs, and no regulation is necessary. The results of deformar tion by pressure also indicate that localization is subject to a regulatory process which may occur even in a comparatively late stage in cleavage.

That the "organization" of the cytoplasm results from a progressive developmental process is clearly evidenced by the experiments of Wilson, Yatsu, and Zeleny on the egg of Cerehratulus before cleavage. If portions of this egg are removed before maturation has begun, while the egg nucleus is still in the form of an intact germinal vesicle, no defects are seen in the resulting larva. Entrance of the spermatozoon is followed by maturation and a general rearrangement of the substances of the cytoplasm, one result of which is the formation of a cap of clear protoplasm at the animal pole. Removal of this substance prior to or during the first cleavage, often produces no later abnormality. The separated blastomeres of the two-cell stage, however, while for a time cleaving like halves, soon assume the character of wholes. Those of the four-cell stage continue longer to behave like parts, even through the blastula stage, although ultimately they may form typical free-swimming larvae. The degree of defect corresponds m ^ general way with the stage to which cleavage has progressed at the time of separation. Larvae developed from eggs without the upper quartet, which contains the clear protoplasm mentioned, have typically formed enteron, but lack the apical organ. Larvse from this upper quartet have the apical organ but are without enteron. And the same is true when, in the sixteen-cell stage, the upper and lower octets develop separately. Parts of the blastula continue to develop for a time and form only the restricted cell groups to which they give rise in normal development.

Such facts seem clearly to mean that cytoplasmic germinal localization may be complete in later stages, but incomplete or absent in the earlier, that it is truly a process or result of develr opment and not a primary determiner of the course of development, not a fixed thing persisting from generation to generation, which might be regarded as the physical basis of heredity.

The conception of cytoplasmic localization as a progressive process, i.e., as one factor or link in the chain of developmental events, immediately raises the question as to what condition then lies back of this, and determines the character of the progressive steps or reactions. This leads us directly to the second chief view as to the fundamental character of the specific organization of the ovmn, that is, to the hypothesis of ^^ nuclear analysis or niiclear determinationy and to this we may now give our attention. To state them again^ the essentials of this hypothesis are, that the real germinal localization of the ovum is to be sought in the nucleus, that the organization of the cytoplasm is preceded and its character determined primarily, by the organization of the nucleus, that this organization is continuous from one generation to the next and is so to be regarded as representing the physical basis of heredity. Polarity and other cytoplasmic differentiations, certainly exist in the ovum, even before fertilization or cleavage, but the only structural differentiation of the ovum which is invariably marked out at all stages of the organism's existence, is the differentiation between nucleus and cytoplasm. And while not alone development, but all the normal life processes of the cell are the results of interaction between nucleus and cytoplasm, both being essential, yet the action of the nucleus is primary and seems to determine the particularity of the cell actions.

This general subject of nuclear determination is enormously complex and has been the occasion for whole volumes; our account of it must perforce be brief and therefore more or less fragmentary and dogmatic.

The search for the imderlying causes of development is in part a search for elements or conditions that are comparatively fixed and that remain continuous from generation to generation through the individual waves of species life. Specificity is continuous; are there structural elements or conditions correspondingly fixed and constant, not having to develop anew in each individual ontogeny? Are there structures in the germ cells which determine the direction of development and thus represent (using this word in a very broad sense) the organs and parts of the developing embryo?

In the endeavor to answer these questions the nuclei of the germ cells at once compel attention as containing organs whose morphology appears to be constant and specific at all stages of the individual life history, and through successive generations • Thus the chromosomes at once become the foci of observation and discussion, and the hypothesis of nucleax determination becomes, to a considerable extent, the hypothesis of the specificity of the chromosomes.

This conception has already been outlined in Chapter II; the chromosomes are believed to be differentiated functionally, in a specific manner so that each chromosome of the nucleus represents a center of activity of a particular character. That is to say each chromosome, either individually or as a component of a unified group, determines a specific form of reaction with the cytoplasm, or rather influences in a particular way certain of the reactions constantly occurring between nucleus and cytoplasm. And the final result of these reactions is the production of certain structural and physiological characteristics of the embryo or mature organism. Thus, leaving out all the intermediate chain of processes or reactions, there is an actual correspondence between certain traits of the mature organism and certain chromosomal characters of the gametic nuclei. Of course the chromosomal characters determine only the first step in the development of the corresponding trait ; but this in turn determines the next, and so on. And since the quality of one step or reaction in development is determined by the preceding, we are correct in relating directly the character of the final steps in development with the factor that first determined the trend of reaction.

Emphasis is thus placed upon the physiological character of the relation between chromosome and later structure, and care must be exercised constantly, in the discussion of this subject, to guard against a conception of this relation which is too strictly morphological, and which might suggest too strongly the conception of development as preformational. A wrong interpretation of the modem view of the chromosome relation leads to a rather strict preformational view; but such an idea does not to-day represent the hypothesis fairly. What is formed, or preformed, in the germ is a certain arrangement or configuration of the chromatic substance, which in its reactions with the C3rtoplasm produces new and specific conditions, these lead to others, and so on through development.

The conception of the determinative character of the chromosomes must now be modified to include the idea that each chromosome is not a simple unit, homogeneous either morphologically or physiologically. Each chromosome is to be regarded as made up of a series or group of elements which singly are simple and homogeneous, and behave as physiological units or "determiners." These may or may not correspond with the chromioles, or granules of chromatin, of which the chromosome is composed; and while it is true that they have never been positively identified as units or determiners, some such bodies must be present in the chromosome according to this hypothesis. Such determiners, although apparently necessary hypothetical units cannot be described; they may prove not to be definite particles at all, but rather dynamic relations, or configurations of substance. So for practical and descriptive purposes we are nearly limited to the chromosomes.

It is quite likely that the chromosomes may not be the only factors in the determination of development, there may be a whole series of factors back of these, and we know that a whole series of factors follows after. But if they are proved to be necessary links in a chain of determining factors, then they are causes of differentiation, and if they are found to be the earliest visible differentiations with which later differentiations somehow correspond, then we may refer to them as the causes of specific differentiation. At some future time it may indeed be possible to push the analysis of the factors of differentiation still farther back ; such a possibility is in no wise excluded by the chromosome hypothesis as it stands to-day.

One of the obvious requirements of any hypothesis of differentiation and heredity is that it must readily allow interpretation, in cytological terms, of the enormously complex phenomena of alternative or Mendelian heredity. Most of the traits of an organism are the property of the species, common to all the individuals of a specific group. But there are other characters that are family possessions and may or may not be inherited by individuals. These individual characteristics are, in many cases, comparatively late developments. The early characters are those of the larger group; those of the species appear later, and finally the family and individual traits. The whole subject of Mendelism has developed into an extremely complicated system, in directions largely unforeseen. And yet it is hardly too much to say that the C3i.ology of the germ cells and their nuclei has on the whole fairly kept pace, and it is in most instances quite possible to parallel the facts of MendeUsm with the facts of chromoBome behavior. We shall return briefly to this subject in a more appropriate connection.

Fig. 137. — The atruoture of chromosomes. A, after K. C. Bchoeider, others after Bonnevie. A. Nucleus from epidermis of Salamander larva, in telophase. B. Prophase of first eleavags of Ascaria megaiocephala bivaUn*. C. Nucleus from cleavage Htane of same. D. Interlunesia in Amphitima.

Let us now repeat, from this particular point of view the general ideas regarding the chromosomes mentioned in Chapter II, emphaaizmg obtain topics and adding a few details which bear directly upon the relation of the chromosomes to the processes of development and heredity.

The chromosomes are not structurally homogeneous masses, but are built up of certain granules which often have a definite arrangement giving the chromosome as a whole a general structure. This structure has been variously described (Fig. 137); in some cases it seems to be a cylinder of chromatin granules with a core of differentiated substance, probably linin; in other cases the granules are arranged in a linear series wound in a definite spiral; but in still other cases the granules seem to have little if any regular disposition. These granules are very close to the limit of vision, indeed often they are invisible, and in most cases it is difficult to make confident assertions regarding their arrangement. Of course their invisibility on account of minuteness does not prove that they are not present. Indeed many, following Weismann, postulate an elaborate series of representative particles within the nucleus: the chromosomes or "idants," are divided into "ids" or chromatin granules; the ids are then assumed to be formed of groups of "determinants," and these in turn are thought to be composed of the really elementary, self-propagating protoplasmic units, the "biophores." Of thisseries, theidants andidsare visibly known; the determinants and biophores are invisible hypothetical bodies, postulated to aid in relating many of the complicated facts of heredity to certain c3rtological facts. The assumption of a final determining unit that is, and seemingly must remain, invisible has proved fortunate as affording a convenient shelter against criticism, for such an assumption partly removes the question from scientific treatment. We shall lose little and gain much by considering here merely those elements of the nucleus that can be identified and whose behavior can be traced to some extent, i.e., the chromosomes and chromatin granules.

Our ideas in this field are, to a remarkable degree, the outgrowth of the pioneer work of Weismann. Although based upon the properties of hypothetical units whose behavior was outlined upon purely hypothetical grounds, his conceptions of the relation between chromosome behavior and the facts of development and heredity, formulated more than thirty years ago, before the science of cytology was established, have a distinctly modem aspect. The remarkable convergence of the facts of heredity, of development, and of cytology, which have become known subsequently to the formulation of Weismann's hypotheses, constitutes splendid evidence of the keenness of this great embryologist.

We may here suggest again the character of the evidence for regarding the chromosomes of the germ nuclei of the very greatest importance as factors controlling or directing the process of development (i.e., the process of heredity), facts of such constancy and universality that they must have some meaning.

First of all comes the fact of the very high degree of morphological constancy of these organs throughout the tissues of the species, not merely of the individual. This constancy, always considering corresponding ages of course, concerns their number, size, and form, and proves them to be specific organs in a real sense. They are, with a few easily explained exceptions, present in pairs of similar elements, whose history can be traced back to their derivation in groups of unpaired elements in the male and female gametes. During mitosis the distribution of the chromosomes to the daughter cells is never a haphazard process, but the whole process of mitosis appears to be an adaptation toward securing their equal division, and the distribution to the daughter nuclei of groups of similar morphological composition.

It is unnecessary to repeat here any of the evidence outlined in Chapter II for the idea of the genetic continuity of the chromosomes from cell to cell. We saw there that it is the chromatin granules which may be regarded as actually morphologically continuous, and that these may seem to become similarly associated every time that the chromosomes visibly appear. What determines their constant association in the reforming chromosomes of each cell generation is to be answered only hypothetically if at all. But in spite of the contrary belief held by some, the chromosomes may be regarded as genetically continuous individual elements, although the details of their composition may vary slightly from generation to generation. And after all it may be that the most important continuity is that of the chromatin granules, supposing them to be qualitatively unlike and to play the r6le of specific determiners.

The validity of the chromosome hypothesis has always been strongly indicated by the essential facts of syngamy, namely, the formation of the zygotic nucleus from equal contributions of chromosomes from the male and female parents. This is the only portion of the new organism which is derived equally from both parents, and while it is true that a small amount of cytoplasm accompanies the sperm nucleus in its entrance into the ovum, this varies considerably in amount in different forms. This latter fact together with the general fact of the primary importance of the nucleus in all aspects of cell activity, combine to enhance the significance of the equal derivation of the chromosomes (Fig. 106), especially in view of the further fact that on the whole the family and individual traits of organisms are inherited with equal likelihood from either parent.

The behavior of the chromosomes throughout the maturation process affords many highly interesting and significant parallels between chromosome behavior and the facts of heredity. Interest centers here in the phenomena of synapsis and the "reducing" divisions.

Any precise interpretation of these two phenomena seems impossible until more is known with certainty regarding the behavior here of the chromatin granules, but the phenomena themselves are readily interpretable in the light of the facts of alternative, or Mendelian heredity.

In synapsis we see the final union of pairs of chromosomes introduced into a single nucleus at the time of fertilization, but remaining distinct throughout the life of the hybrid generation, until the time when the hybrid organism forms its gametes. Synapsis is not a haphazard junction of chromosomes, but an orderly union of elements of paternal and maternal origin, similar in size, in details of form, and probably also in function. The bivalent chromosomes thus formed are, in consequence of their derivation from two individuals, not quite homogeneous throughout. Following synapsis come two divisions of each chromosome, and in most organisms one of these apparently divides the chromosome equally, into two similar parts (equation division), while the other divides each of the daughter chromosomes dissimilarly (reducing division), the dissimilarity resulting from the relation of the plane of division to the plan of arrangement of its dissimilar component granules. The relation of the reducing division to the chromosome depends upon the character of the synapsis, whether telosynapsis or parasynapsis, and also upon the behavior of the chromatin granules in all these events, and it is difficult to be certain of this. It is safe to say, however, that in most cases each bivalent chromosome, composed in equal parts of substance from each parent, clearly separates into foiu* elements, two having one composition, two another. These elements are then distributed to separate gametes, so that with respect to the composition of each separate chromosome, the gametes produced by an organism are of two kinds, approximately equal numerically. This accords perfectly with the facts of Mendelian heredity, upon the supposition that there is a correspondence between chromatic elements and organismal traits. This may be made somewhat clearer with the aid of a diagram : see Fig. 80.

In the process of maturation, therefore, it is easily possible to find a mechanism which permits the segregation of characteristics in the germ cells and their distribution to separate organisms in regular Mendelian ratios. One important correspondence should not be overlooked. In Mendelian heredity the individual qualities of the parents may not appear separately until the first generation after the hybrids. This is possibly related to the fact that the parental chromosomes undergo synapsis and subsequent redistribution first in the germ cells formed by the hybrid, and the segregated elements are, therefore, distributed separately first in the organisms formed from these hybrids, i.e., in the F^ generation.

The conclusion resulting from the study of Mendelian heredity, that the organism is a sum of "unit characters" which in the organism interact with one another, so as to produce a physiological whole, but which in heredity are more or less clearly separable units, affords strong evidence for the general hypothesis of the representative particle composition of the germ nuclei. Chromosomes might thus represent groups of such "units" or in occasional instances perhaps, single units, although this must be the case only rarely, for the total number of unit characters is far in excess of the number of chromosomes.

That the chromosome of the Metazoan is really made up of a group of unit determiners, is also indicated by the behavior of the Protozoan nucleus in maturation. In most of the simpler Protozoa where the maturation phenomena appear, there is no indication of definite elements like chromosomes in the nucleus. But in many of the Ciliates, in which vegetative chromatin and reproductive chromatin become sharply separated, the latter, or idiochromatin, is seen to be formed into definite bodies. Thus in ParamcBcium, as observed by Calkins and Cull (Fig. 82), the micronucleus (idiochromatin) becomes resolved into a large number — more than 200 — chromatin granules (idiochromidia) whose definite behavior can be traced. Their behavior is complex, but the result is that each idiochromidium is divided longitudinally and transversely, and the resulting daughterbodies may, therefore, be dissimilar. After fertilization the division of the zygotic nucleus brings about the division of each chromidium and the distribution of the halves to the two daughter cells.

The very large number of separate elements in these "gametic nuclei may indicate that each corresponds with a single character, or with a smaller group of characters than in the Metazoan, and that therefore the chromosome of the Metazoan must be an enormously complex affair. All of this lends weight to the idea that " chromosomes, the characteristic structures of the nucleus in mitosis, have had an evolution no less surely than has the nervous system, digestive system, or supporting; system of the higher animals, and that the chromosomes of the protozoa have the same relation to the chromosomes of the metazoa that the organization of the protozoan body has to that of the metazoan, i.e., a unit structure." (Calkins, "Protozoology," page 171). Admitting the representative particle composition of the chromosomes, it must of course follow that their evolution in the Metazoa, parallels the evolution of adult form and structure.

If this is true, then the chromosomes of the Metazoan germ cells must each represent a congeries of determiners, the form of association of which might differ in different species, as widely as the groups of characteristics of the adults differ.

The question as to just how the chromatic determiners (assuming their existence) really do affect the quality of the reactions of the developing organism, is stiU practically un. touched. To some it seems necessary to postulate the asymmetrical distribution of the chromatin granules through successive mitoses, so that certain kinds of granules or determiners become distributed to certain cells and regions, directly effecting there specific reactions. No such form of distribution has been observed, though indeed it has not been sought in a thorough fashion. In tissues whose differentiation is fairly advanced there are certainly characteristic and specific nuclear appearances which indicate that the nuclei as well as the cytoplasms have undergone a real differentiation, but whether this is related to chromosome or granule structure remains undemonstrated.

If any such sortmg out of determmers occurs it must be at widely divergent stages of development in the various groups, on account of the variety of the results in the way of specific embryonic defect following the removal and pressure experiments described above. Indeed the results of the pressure experiments referred to, become highly significant from this point of view, for it will be remembered that the presence of a completely "foreign" nucleus may in some cases not influence the particular form of differentiation of the cytoplasm. To say, in such cases as these and in the removal experiments, that reSneration may occur and th. proper detomin^ be retoed, does not offer much that is helpful in the way of a solution of this particular problem, for it would necessitate the assumption of some mechanism back of the "determining particles, by which they themselves are formed and determined.

The fact that parts, even small bits, of a fully developed and differentiated organism may finally, through a process of regulation, give rise to a complete organism again, or that in many plants, buds, bits of stem or leaf, may similarly give rise to a completely formed organism capable of developing typical germ cells, renders extremely unlikely any strictly morphological conception of the relation between strictly diflferentiated germinal determiners and the formation of certain tissues or organs. The ideas cannot be overemphasized or repeated too often that, while the thing or the relation that we call a determiner may sometimes have a morphological expression in the germ, essentially the relation is physiological — functional — probably chemical or energetic (djnaamic), and that the reactions or interactions of whole groups and masses of particles or systems are involved in determining the intermediate and final results of development.

One further group of observations must be considered in connection with the possibility of the primary character of the nuclear control of development and heredity. During the process of development there occurs a constant giving off of substance from the nucleus to the cytoplasm (Figs. 138, 32). At every mitosis only a part of the chromatic substance is formed into chromosomes, while the remainder passes into the cytoplasm and dissolves, and of course the whole fluid content of the nucleus, the nuclear sap, is discharged into the cytoplasm. Herbst has emphasized the importance of the nuclear sap as an important determining factor in development and heredity.

In many instances, substances discharged from the nucleus into the cytoplasm of the oogonia, especially during their growth period, are directly concerned in the formation of specific materials and bodies of the mature ovum. And later in development Conklin has observed that the cilia of the superficial cells of Crepidula develop only when certain chromatic granules reach that region, a single cilium then differentiating opposite each granule. We have already mentioned the fact, described by Wilson, that in Cerebratulus the effects of the removal of parts of the egg cytoplasm before the germinal vesicle has broken down, are very different from the effects of the removal of similar portions after the contents of the vesicle have been discharged into the surrounding cytoplasm during the initial stages of maturation.

Fig. 138. — A. Chromatin eitrusion from the nucleua into the cytoplasm ia the oOcyte of the Medusa, Pdagia nactiliKa. After Schaxel. B. Eitrusion of chromatin into the cytoplasm during the maturation of the odcyte of Prottu*. anoutneiu. After Jdrgensen. X 1080.

But the evidence for the hypothesis of nuclear determination is not altogether purely observational; there is some experimental evidence as well, although largdy indirect and possibly none is positively conclusive.

For example, Boveri has described the results of dispermy in the sea-urchin egg. When two spermatozoa enter the ovum the result is frequently the formation of three or four centrosomes and asters connected with one another by spindles, upon which the chromosome groups are usually drawn in abnormal combinations, so that when such an ovum cleaves it separates into three or four cells containing nuclei whose composition is therefore abnormal. In many such cases Boveri finds that each of the three or four cells forms a group of cell descendants which can be identified by the presence or absence of certain characters or by unusual combinations of characters (Fig. 139), so that the entire embryo may be said to consist of three or four regions, each with certain distinctive characteristics. Furthermore, in some instances, the cells of the various fractions may be characterized by unusually large or small nuclei, indicating the presence of larger or smaller amounts of chromatin (numbers of chromosomes) than usual; the microscopic examination of these multipolar spindles shows that the chromosomes may be distributed with great irregularity in the first division.

Fig. 139. — Larva of SphtBttchinua, derived from a, dispennic egg, showiiiK differeacea ia nuclear size, distribution of pigment, etc. The dashed line marks the separation of the two portions of the larva. After Boveri (recouHtrucled from two GgureB). n. small nuclei; N, large nuclei; 2

More striking are the results following the separation of the blastomeres of such dispermic eggs. The isolated cells of a four-cell stage resulting from normal fertilization, develop normally, producing four similar but small normal larvae (Fig. 133). But the isolated cells of one of these three-cell stages develop dissimilarly, each with certain defects; and just as any possible combination of chromosomes may have occurred in each of the three original cells, so all possible combinations of characters are foimd in the larvae developii^ from such cells when isolated. Boveri believes that this warrants the conclusion that, while the presence or absence of certain chromosomes may not result in the presence or absence of specific traits, yet a certain conibincUion of chromosomes is essential for normal development, a fact which would mean only the physiological specificity of the individual chromosomes.

Perhaps the most striking experimental results are those obtained by fertilizing the eggs of one species, with 'the sperm of another species, genus, or even phylum. In the first place, Boveri in 1889 reported that non-nucleated egg fragments of Sphcerechinus (one of the sea-urchins), fertilized with the sperm of Echinus, developed into larvae exhibiting only paternal characters. This appeared to afford strong evidence that the characteristics of the nucleus rather than of the cytoplasm determine the course of development. Later attempts (Seeliger, Morgan, Boveri) to confirm these facts led to inconclusive results. Indeed exactly opposed results were obtained by several investigators (Driesch, Loeb, Godlewski, Hagedoom). Eggs of one species of Echinoderm fertilized with the sperm of another species, genus or class, of Echinoderm, or even with . MoUuscan sperm, resulted in the development of larvae possessing wholly or largely the maternal characters. These results indicated just as strongly that the nuclear composition is of the lesser importance in determining the development of specific traits, and of course seriously affected the validity of the hypothesis of nuclear determination.

Fig. 140. — History of the paternal chromatin during the first deavilEe in the pseudohybrid sea-urchin, Spkiereckinui 9 X StTOngylocenlrotut ^. After Herbat. A. First cleavage figure. Sperm and egg pronuclei associated, but not fused. ChromoBomes beginning to form in the egg pronucleus. B. ChromoBomes in both pronuclei; in separate groups with separate spindles. C. Anaphase of first cleavage. Maternal chromosomes reaching the poles. Paternal chromatia (chromosomes no longer) forming an irregular mass, spun out on the spindle between the maternal chromosome groups. D. Division completed. Daughter nuclei reconstructed and consisting entirely of maternal chromatin. One of the cells contains a smalt vesicle consisting of the paternal chromatin, which takes no further share in cleavage. ^, chromatin of the sperm pronucleus.

These experiments, however, curiously turned out to afford very strong evidence in support of this hypothesis. For other workers (Herbst, Kupelwieser, Bataillon) showed that in many of these, and in other instances, the nucleus of the spermatozoon did not actually fuse with the egg nucleus, but remained either partly or wholly mactive, taking little or no share in the formation of the mitotic figures of the first and subsequent cleavages (Fig. 140). The resulting larvae therefore were not truly hybrids; the spermatozoon had merely stimulated the egg to develop, as in artificial parthenogenesis, but itself took no part in the formation of the nuclear structures of the larva. In the absence of microscopic examination of the embryo, therefore, it is impossible to place any emphasis upon the development of purely maternal or paternal characters under such conditions.

Fortunately such cytological evidence is now provided extensively through the work of Baltzer, who has traced the nuclear history of many forms of Echinoderm hybrids. It appears that part or all of the paternal chromatin, never the maternal, may be thrown out of the nuclei of such "hybrids" (pseudohybrids). Such an elimination of paternal chromatin may occur during the very first cleavage, or it may be delayed until the blastula or even early gastrula stage (Fig. 141). The examination of a long series of hybrids, showing all degrees of purity of the maternal characters, leads Baltzer to the conclusion that the degree to which paternal characters appear in the resulting hybrids, is closely parallel to the relative amount of paternal chromatin which is retained within the nuclei of the organism. Where the fusion of the sperm and egg nuclei remains complete, the hybrids have intermediate characters; where little or no chromatin from the spermatozoon is retained in the nuclei, there appear, chiefly or alone, maternal characters. Only in the case of the fertilization of the eggs of a sea-urchin (Strongylocentrotus) with the sperm of a Crinoid (Antedon) has it been shown that the fusion of the germ nuclei really occurred (Godlewski, Baltzer) while the larvse resulting from this cross exhibited certain characters which were piirely maternal; but this result is wholly inconclusive as evidence opposing the hypothesis of nuclear control.

Fig. 141. — History or the patem&l chromatiD in the paeudohybrids of the sealuchins. Slrtmoj/locentrottie $ X Sphareckitius <?. After Baltier. A. Cleavage cell showing paternal chromatia (iJ) outside the division GKure. B. Early blastula. C. Late blastula, Bhowine the elimination of the paternal chromatin in the irregular cells and spaces within the blaatoccel (for normal biKstula seo Fig. 109).

For it has been found, in the first place, that external conditions often determine whether certain characters shaJl be paternal or maternal in their qualities (Vernon, Tennent). Under certain conditions of temperature, alkalinity, etc., the larva may exhibit paternal resemblances, while under other conditions maternal resemblances may appear in the same cross. And in the second place, the phenomenon of dominance" appears even in these early stages of development, and a hybrid may show certain cleariy maternal characters and yet in other respects closely resemble the paternal type (Steinbruck, Driesch, Boveri, Loeb and Moore). Great variability is often the rule and frequently it is impossible to say whether either parental trait really appears purely. It should be pointed out, first, that it frequently happens in Mendelian inheritance that true hybrids are either purely maternal or purely paternal with respect to single traits, and second, that only after synapsis, which occurs in the germ cells of the mature hybrid organism, are the paternal and maternal chromosomes really brought into complete relation.

On the whole, then, while there are some few results difficult of favorable interpretation, we may say that the evidence from hybridization, though at first distinctly opposed to the hypothesis of nuclear determination, at present affords the strongest support of this hypothesis, and indicates that normally the characters of a hybrid are determined by both of the germ nuclei, and that when nuclear material from only one parent is functional the characters of the so-called hybrid are determined thereby.

We might mention one further possible interpretation of some of the results opposed to this conclusion, and emphasized by Conklin and others, namely, that in some cases, at least, the fundamental or general traits of an organism may be determined immediately by the cytoplasmic structure of the ovum alone or chiefly, while the nuclei are equally concerned in the determination of the more particular specific or individual traits, often appearing relatively late in development. Such a possibility seems to be indicated by many of the facts of germinal localization already described, and it may be that some of the results indicated above, non-conformable with the hypothesis of nuclear determination, point in the same direction. Conklin writes (Science, XXVII, 89-99) : " At the time of fertilization the hereditary potencies of the two germ cells are not equal, all the early development, including the polarity, symmetry, type of cleavage, and the relative positions and properties of future organs being predetermined in the cytoplasm of the egg cell, while only the differentiations of later development are influenced by the sperm. In short, the egg cytoplasm fixes the type of development and the sperm and egg nuclei supply only the details." And yet we should not overlook this fact, of basic importance, that these fundamental cytoplasmic differentiations have resulted from interactions between the cytoplasm and the nucleus of the oogonial cell, and that the nucleus of the oogonial cell, and egg cell, is itself origmaQy derived in equal parts, from paternal and maternal ancestry. And further many of the conditions of polarity, symmetry, and the like, may m some cases be determined or altered by the entrance and subsequent activity of the spermatozoon within the ovum.

Probably altogether the most striking evidence in support of the hypothesis under consideration is to be found in some of the recent work upon the association of sex with chromosome characters. The nature of this association is so particular and significant that certain chromosomes are actually regarded by many as representing sex "determiners." This relation, besides affording striking evidence in this connection, is of very great importance in itself, and we may therefore consider it at somewhat greater length than this connection alone would justify.

During recent years many instances have come to light, of a variation in the number of chromosomes in different individuals of a single species. With but very few exceptions these numerical differences are associated with difference in sex, and when any such difference exists it is usually found that the cells of the female contain one or more chromosomes in excess of the number found in the male. In some species then, the chromosome number may be uneven in one sex, and therefore not all the chromosomes are paired structures. In other cases the equivalent diversity of the chromosome groups is indicated by size differences between the members of a certain pair.

Differences of these kinds are now known in many scores of species pf many groups, from the lower worms to man. It is clearly impossible to include here any extended history or survey of this fascinating subject and we can do little more than describe a typical instance or two, and then mention some. comparisons which may throw some light, from this point of view, upon the general interpretation of the chromosome problem.

Since the larger number of the known instances of this relation are found among the Arthropoda, particularly the Insecta, we may select our first illustration from this group. The number of chromosomes in the somatic cells of the conmaon squash bug, Anasa tristis (Henking, Paulmief), is twenty-two in the female, and twenty-one in the male. How does this difference come about?

For an answer to this question we must observe the behavior of the chromosomes during the process of maturation of the germ cells. In the process of oogenesis, preparatory to the first oocyte division, synapsis occurs normally, and eleven bivalent chromosomes are formed. The succeeding steps in oogenesis are not unusual and the result is the formation, in each ovum, of a group of eleven univalent chromosomes representing every pair of the original somatic or oogonial group.

The events of spermatogenesis do not run quite parallel, however. In the somatic and spermatogonial cells, twenty-one chromosomes are present, i.e., ten pairs plus one, and in the division of these cells every chromosome divides in the usual way (Fig. 142) . In synapsis the paired elements fuse, forming ten bivalent chromosomes, but the odd chromosome, or Xchromosome remains free, and usually quite apart from the other chromosomes. This X-element, or idiochromosome^ may be distinct, even throughout the growth period of the spermatogonia, and during the two spermatocyte divisions it can be identified in many species as a nucleolus-like body, indeed formerly it was described as a chromatin nucleolus. The behavior of this body during the maturation divisions is entirely unusual. During the first spermatocyte division the idiochromosome, although univalent, divides just as the ten bivalent elements do, and eleven chromosomes consequently pass into the nuclei of the secondary spermatocytes. But during the mitosis of these secondary spermatocytes the idiochromosome fails to divide and passes as a whole to one pole of the spindle (Fig. 142, F, G, H). The result is that the nuclei of one-half of the spermatids, and therefore of one-half of the spermatozoa contain eleven chromosomes, while the other half contain but ten, lacking the idiochromosome. Since the nuclei of all the ova contain eleven chromosomes there are but two possibilities in fertilization. The egg with its eleven chromosomes may be fertilized by a sperm with ten, giving a somatic group of twenty-one; or the egg with its eleven chromosomes may be fertilized by a sperm with eleven, giving a somatic group of twenty-two. And since there are equal numbers of ten- and eleven-chromosome spermatozoa, there will be approximately equal numbers of zygotes with twenty-one and twenty-two chromosomes. These relations are shown in diagrammatic form in Figs. 143, 144.

Fig. 142. — Maturation durins the BpermatogenesiB of the squoah-buE, Aniua trialit, Bhowing the behavior of the X-chromoBOnie or idiochromosoine. A, after Wilson, othera after Paulmier. A. Spennatogoaium. Polar view of equatorial plate showing twenty-one chromosomes (ten pairs, plus one). The X-chromoBome ia not diBtinttuiBhable at this time. B. Primary speTmat^cyte. Tetrads formed. C. Equatorial plate of first spermatocyte divimon. X-chromosome divided. D. Anaphase of same division. The daughter X-chiomoaomes have also diverged. E, Equatorial plate of second spermatocyte division. F. Metaphase of same division. The X-chromosome lies, undivided, between the two groups of daughter chromosomes. O. Anaphase of same division. The undivided X-chromosome has passed to the upper pole, lagging behind the others. H. Telophase of same division. X-chromosome still distinct.

Fig. 143. — Diagram illustrating the behavior of the X-chromosome during maturation. The X-element is shown in solid black. The essentials are the same in cases where X is a multiple element, or where it is paired with a Y-element (see text).

Since this numerical difference between the somatic chromosome groups is constantly associated with sex-difference, males possessing twenty-one, females twenty-two chromosomes, it may be said that the presence of the idiochromosome is in Mature ova. All Spermatozoa. Zygotes. Sex of of one cUfls Two classes Two classes adults some way connected with the determination of the female, or the lack of it the male, sex.

Fig. 144. — Diagram of the relations of chromosome number and sex during fertilization in Anasa, The essentials are the same in cases where X is a multiple element, or where it is paired with a Y-element.

Since Paulmier's description of these events in Anasa, in 1899, a great many similar instances have come to light, and more recently quite a variety of conditions related to this but more or less dissimilar in details, have become known. The X-chromosome or idiochromosome has been described under many different names such as accessory chromosome, allosome, heterotropic chromosome, heterochromosome, monosome, etc. Such an unequal distribution of the chromosomes was first observed by Henking in 1890, and in 1902 McClung described similar processes in several of the locusts and grasshoppers (Orthoptera), and first suggested the possible relation between this "accessory chromosome" and the determination of sex. The work of Wilson, Stevens, Montgomery, Payne, Guyer, Morgan, and many others, has made known the presence of these elements in a whole host of Insects, including most of the orders, in Myriopods, Arachnids, and Copepods. Among the lower forms. Nematodes (Boveri, Edwards, Boring, Gulick), Sagitta (Stevens), and Echinoderms (Baltzer) are now known to possess idiochromosomes. And more recently some of the Chordates have been added to the ever increasing list, for idiochromosomes have been described in the common fowl, guinea f owl^ and rat (Guyer), the guinea pig (Stevens) and even in man^ where Guyer has reported two idiochromosomes, half the sperm containing twelve, and half ten, chromosomes; the number of chromosomes in the human somatic cells is, therefore, twentytwo in the male, and twenty-four in the female.

Not all of the forms included in the above list exhibit this phenomenon as simply as it occurs in the case of Anasa; we may mention two general modifications of this typical condition* In many Coleoptera, Diptera, and Hemiptera, the idiochromosome is not strictly an unpaired element for during the spermatocjrte divisions, and in the spermatids, it is paired with a very small chromosome called the Y-element; together with X this makes up an XY bivalent chromosome which behaves like any bivalent chromosome in the preliminaries to the first spermatocyte division (Fig. 145). In the spermatozoa, therefore, half the nuclei contain the large idiochromosome (X), and half the small one (Y). The relation to sex is what might be expected, namely, the females contain the large X, the males the small Y. In Metapodius, one of the Orthoptera, this small Y-element may be either present or absent apparently, although it is possible that it may be present and fused with another chromosome when it is said to be absent.

In Ascaris megalocephala it seems clear that a small Xelement, no Y-element being present, may thus appear either as. a separate body, or fused with one of the other chromosomes (Boring, Boveri, Edwards). Such an attachment of the idiochromosome to a certain one of the ordinary chromosomes is well known as the constant relation in some Insects (Sin^ty, McClung), and among these forms various degrees of the intimacy of the association occur (Fig. 146).

Fig. 145. — Diagrams illustrating some of the variations in the X- and Ychromosomes. From Wilson. X, either as a simple or as a multiple element, may or may not be paired with a Y-chromosome.

Fig. 146. — Compound chromosome-groups, formed by the union of the X* chromosome with other chromosomes, in the Orthoptera. From Wilson, a, h, after de Sin^ty, others after McClung. a. Triad group, first spermatocyte division of Leptynia, metaphase. 6. Division of similar triad in Dixippua, c. Triad group formed by union of the X-chromosome with one of the bivalent chromosomes, first spermatocyte prophase, Heaperotettix, d. The same element from a metaphase group, e. The same element in the ensuing interkinesis. /. The compound element of Mermiria, from a first spermatocyte prophase. g. The same element in the metaphase (noWt according to McClung, united to a second bivalent chromosome, to form a pentad element), h. The same element after its division, in the ensuing telophase.

Several stages can be found in the gradual increase in the relative size of the Y-element, until in such forms as Nezara hilaris, one of the Hemiptera-Heteroptera described by Wilson, X and Y are nearly equal (Fig. 145), and finally in some of the Lepidoptera and other forms, X and Y are quite equal and indistinguishable from one another, although the XY pair may be distinguished from the other chromosomes by staining properties and behavior.

We thus reach through gradual transitions a condition where the spermatozoa are no longer dimorphic with respect to chromosome content. The conditions of such a series suggest, however, the possibility that spermatozoa that visibly appear morphologically alike, may after all be physiologically dimorphic as regards chromosome characters; such an assumption must of course be made with respect to traits other than sex, which are inherited in an alternative fashion.

Another series of modifications of the Artdsa-type is illustrated by various genera of Hemiptera, where the X-chromosome is represented by more than a single element. Such a series has been described by Payne, and is readily derived from the condition in such a form as Euschistus (Fig. 145), with unequal X- and Y-elements. Thus in Fitchia and several others, Y is a fairly large chromosome while X is represented by two somewhat smaller chromosomes; in PrionidiLS, Sinea, etc. there are three X-elements to one Y; in Gelastocoris there are four X and one Y, and in Acholla five X and one Y. In still another series (Fig. 145) Y is entirely absent and X is represented by several chromosomes — two in Phylloxera (Morgan), Syromastes (Wilson), Agalena (Wallace), and man (Guyer), five in Ascaris lumbricoides according to Edwards.

In all of these cases where X is a multiple element, different species show greatly varjdng relations among the members of the X-group; they may be approximately equal in size or very unequal, but it is important that the size relations, whatever they are, are constant within a given species.

One further general condition of the idiochromosomes must be noted. In all of the instances mentioned above the spermatozoa are the dimorphic gametes, i.e., the males are "digametic" (Wilson's term). In a few species, however, it is the female that is digametic, and while the spermatozoa are all alike, the ova are of two classes with respect to certain modified chromosomes which may properly be regarded as homologous with the X- and Y-elements of the spermatozoa. Thus in two of the sea-urchins, Strongylocentroius, and Echinus, Baltzer finds the chromosome groups of the spermatids all alike, with eighteen components uniformly differentiated; the nuclei of some of the ova, however, are characterized by the presence of a single modified component which is absent from the remainder (Fig. 147). This obviously corresponds with the X-element of the dimorphic sperm. There is also very good reason for believing the female digametic in some of the Lepidoptera; this is of considerable interest in view of the fact that repeated investigation has thus far failed to disclose idiochromosomes in the spermatocytes of these Insects.

Fig. 147. — Chromosomes in the sea-urchin, Strongylocentrotua lividus. After Baltzer. X 2610. A, First cleavage spindle (reconstructed from two drawings). No small hooks present. B, First cleavage spindle with small hooks. The modified chromosomes, both long and short hooks, are shown in solid black.

The idea that the relation between the chromosomes and sex characters parallels that between the chromosomes and any other traits involves the conclusion that sex is a character, or group of characters, .inherited in the same way that other bodily traits are. And this conclusion may how be accepted. Indeed there are in the field several hypotheses as to the precise statement of a Mendelian formula according to which sex is inherited, and while no one of them has a preponderance of evidence in its favor, the fundamental fact of sex heredity is clear.

There are extant scores of hypotheses regarding the factors and processes involved in sex determination, depending upon the action of conditions outside of the germ itself. These must be abandoned when the facts now known to be true of the germinal structure of a comparatively limited number of species, gain a wider applicability. For the sex of an organism, as well as other fundamental characters, appears to be already determined in the zygote, and all that external conditions can do toward determining sex is to alter sex ratios by affecting differentially (selectively) the gametes or immature organisms of a certain sex.

There is some evidence of other kinds that sex is determined in the gamete and not by external conditions. In certain cases a single egg indirectly gives rise to a number of embryos or larvae (multiple embryo formation) which are all of one sex, either male or female. Silvestri describes such a case in the development of a Hymenopter, LitomastiXy parasitic in the larva of a Lepidopter, Plusia, where as many as one thousand embryos, all of one sex, are thus formed. And there is good reason for believing that the embryos of one of the armadillos, described by Newman and Patterson, are all derived from a single ovum, and these are always of one sex only, either male or female. There is also the familiar example of the bees (Dzierzon), where unfertilized eggs develop parthenogenetically into males (drones), while the fertilized ova produce females (queen and workers) ; the same thing is apparently true of most ants. And in one of the rotifers, Hydatina, a certain kind of female produces eggs which if fertilized produce females, if unfertilized produce males.

One further point is to be mentioned in connection with the general hypothesis of nuclear determination. This is in connection with that curious form of Mendelian heredity known as "sex-limited" heredity, where certain characters are exhibited by the individuals of one sex only, although transmitted by the individuals of the other sex without being exhibited by them. Such a form of heredity can readily be explained upon the chromosome hypothesis, upon the simple assumption of a close relation between the "determiner" for the sex-limited character and the sex-determining element.

Any further consideration of the problems of sex determination and heredity would lead us too far afield; more extended treatment must be had from other sources. In conclusion then, it is hardly necessary to point out that the constancy of the form and of the complicated behavior of these idiochromosomes affords very striking confirmation of the hypothesis of the specificity and genetic continuity of the chromosomes. While it is possible that their form and behavior are determined by underlying conditions, such conditions cannot be directly observed and can only be postulated. Taken in connection with the facts mentioned in Chapter II, and with the results drawn from the development of dispermic eggs, and from hybridization, they amount to practical demonstration of some form of chromosomal specificity in development.

As to the question whether the idiochromosomes are in particular the sex determinants, several views may be held in the absence of conclusive experimental demonstration of the precise relation. It has been held in some quarters that sex is determined by the relative amount of chromatin received into the nucleus of the zygote, irrespective of its content in certain chromosomal elements. This is hardly tenable however in view of many contradictory conditions. Others have suggested that the dimorphism of the gametes is merely associated with other more fundamental diversities, and that sex differentiation and gamete differentiation are related only be cause both are related to some primary differentiation. Still others hold to the idea that the idiochromosomes actually determine by their presence or absence, the nature of the reactions of development, so that finally organisms with female or male characteristics are formed. The most adequately justified and most conservative view seems to be that the nature of the interrelations of the components of the whole chromosome group, among themselves and to the cjrtoplasm, is modified by the presence or absence of certain elements so that in one case the primary and secondary female characters develop, in the other case the male characters.

Returning now to the general subject of this chapter, namely, the factors determining the course of development and the process of heredity, we come to another extremely important subject. We have thus far emphasized the importance of the internal factors of development. But we have defined development as a series of reactions between internal and external factors. The omission hitherto of specific reference to the external conditions of development is not because these are of lesser general importance. Alterations in the conditions of gravity, pressure, temperature, light, moisture, and chemical composition of the surrounding medium may, each or all affect the course of development, either in a general or in a specific way. A great deal is known of the results of modifjdng such conditions and a rather full discussion of these effects would be in order, were the results susceptible of more definite and more uniformly applicable statement. For normal development, normal environing conditions are necessary. However, slight variations in external conditions rarely produce effects comparable with those following slight variations in the internal conditions. That is to say, slight variations in external conditions are "normal." When the modification of external conditions is sufficiently marked to produce visible effects upon development, these are frequently so marked as to be regarded as distinct abnormalities, and the organism so affected is rarely able to complete its development to maturity.

The natural environment ordinarily varies within rather narrow limits, frequently on account of the ovipositing habits of the adult, and changes within these limits rarely aflfect the course of development, so that for the subjects of heredity and differentiation we should inquire here only into the question to what extent external conditions are necessary factors in carrying on the life of the organism as it exists in the form of an egg or embryo. The particulars of development and heredity are referable to internal characteristics which determine the specific or individual quality of the reactions between organism and environment.

We may proceed, therefore, to mention a few illustrations of the effects of alterations in external conditions of development, not attempting to do more than to suggest the nature of the work accomplished in this field; an adequate survey falls outside the scope of such a text as this. (For a convenient summary, see, e.g., Jenkinson, "Experimental Embryology," Oxford, 1909.)

More is known regarding the effects upon development, of chemical substances than of other conditions. While a few forms, such as the minnow, Fundvlus, are able to develop normally in media so widely unlike, physically and chemically, as sea water and distilled water, this and other forms show specific effects of the presence or absence of certain salts aJone. Thus in Fundvlus Stockard has shown that the presence of certain amounts of magnesium salts brings about the fusion of the optic vesicle regions, so that one-eyed monsters develop, apparently normal in other respects (Fig. 148). The eggs and embryos of the Echinoderms offer many striking facts in this connection. We have already noted that the alkalinity of the sea water may determine the appearance of paternal or maternal characters in hybrid Echinoderm larvae. Herbst and others have shown that the absence of potassium salts is fatal or very harmful to Echinoderm larvae, apparently on account of the resulting diminution in the pit)cess of water absorption; the absence of calcium causes a tendency for the blastomeres to fall apart; magnesium and the sulphates are necessary for the normal differentiation of the alimentary tract; the production of ciliary movement depends upon the presence of magnesium, and an excess of calcium results in the hypertrophy of the cilia; sulphates are necessary also for the establishment of the fundamental structure of the embryo and for the formation of pigment; magnesium, sulphates, and calcium carbonate are necessary for the development of a normal skeleton (Fig. 149).

Fig. 148. — Effects of magnesium chloride upon the development of the Teleost, Fundulus, From Stockard. A. Normal fish, eight days after hatching. M, mouth. By C. Two views of fish, showing the fusion of the optic vesicles as the result of treatment with MgCls.

Certain optima exist for moisture, density, pressure, light and temperature; in development as in later life, deviations from the optimum condition, in either direction, affect the rate of development rather than its character. The direction of' gravity takes an essential part in determining normal develop^ ment in a few cases, but ordinarily development is independent of this factor.

In general all of these conditions are involved not so much inthe regulation of development in specific and particular directions, as in determining whether it shall proceed at all or not. Modifications of development produced by effective variations in these conditions are often so extreme that the phenomena of heredity are scarcely apparent and usually the modified organism does not come to maturity.

We may now attempt to summarize a conservative conception of the relation of the structure of the germ cells to the processes of development and heredity. The zygote is an organism, morphologically and physiologically specific. It possesses polarity, symmetry, various forms of differentiated substance, even organs, composed of subsidiary elements and capable of performing definite and highly varied and specialized functions. This organism in its parts and as a whole does certain things, makes certain reactions, in a word, develops. The quality of the developmental reactions is determined primarily by the conditions within the organism itself, and as it reacts, as the organism develops step by step, these internal conditions rapidly change. These reactions on the part of the organism fall into two groups. (1) Reactions between the organism (i.e., cytoplasm and nucleus, whether the organism consists of one cell or many) and its environing stimuli. (2) Reactions between the nucleus and cytoplasm of each cell. The idea of reaction must involve two factors, but while equally necessary for reaction, they are not necessarily of equal value in determining or controlling the quality of the reaction. A great many organisms react to light; but the quality of the reaction is determined primarily by the organism.

Fig. 140. — Ejects of chemical alteration of the aurrouading medium, upoD the development of the geSi-urcbia. From Jenkinaon. a. Without OH; ciliated solid blaatula of Spharechinui. b. KOH has been added, c. Normal blastula of Sphareckinui. d. Blastula in a K-free medium, e. Reared in E-free medium and replaced ia normal sea-water (SpAm-ecAinus). /■ Spharediinta larva from a medium devoid of Mg. g. Echinua pluteus with three-parted eut, mouth, and ccelomic sacs, but neither skeleton nor arms; reared without CaCOi or CaSO^. h. Normal pluteus of Echimtt.

The whole structure of the cytoplasm may play a large part in determining the quality of the reactions of the egg, but this cytoplasmic structure is itself the result of a series of interactions between cytoplasm and nucleus, and the action of the latter is of primary importance in affecting the quality of the result. Going one step further, what the nucleus does is determined by its structure, and this is also the result of interactions of its parts with one another, and with the cytoplasm, which is its environment; and here again certain elements of the nucleus, namely, the chromosomes, seem to be of primary importance in determining the quality of the interaction. The most important of the concrete, visible organs of the nucleus are the chromosomes. And when we attempt to analyze the behavior of these components we are met by the same problem — what determines the structure and behavior of these? Two answers have been offered. First, that here we reach the limit of analysis, that the chromosomes are autonomic, self-perpetuating, self-regulating bodies, whose morphology and behavior are the determining factors in all that happens in the life of the organism. Second, that the chromosomes are themselves made up of still more fundamental units, the chromatin granules; that these are the autonomic, self-perpetuating, finally determinative units in development, and chromosome structure is the result of the primary activity of these bodies.

Logically there is no reason why we must stop with the chromatin granules. And history, enumerating germ layers, cleavage cells, cytoplasmic organ-forming substances, chromosomes, and chromioles (chromatin granules), warns us against the idea that we must seek or hope to find ultimate particles, concrete, definable, and representatively determinative in function. In fact a few students of the problem frankly declare their belief that the idea of any sort of representative particle mechanism is futile, that the regulation of the processes of development and heredity depends upon interrelations which are not susceptible of interpretation in terms of any material basis.

Scientifically, however, we can to-day go no further back than the chromosomes, for here we find the most fundamental units whose actual behavior can be correlated with the facts of development. To say scientifically, that the chromosomes are (to-day known to be) the determining elements in development and heredity, is not to deny the existence of other bodies or conditions which may determine the existence and qualities of the chromosomes. Granules we know, but of their behavior we know little, and this little cannot at present be correlated with the facts of development. Of the real existence of elements underlying the granules we know nothing whatever. Assumption of the reality of such bodies or conditions may be a logical necessity, but to-day it carries us beyond the boundaries of. observed fact.

To repeat a statement made on an earlier page, if the existence and activity of the chromosomes can be shown to be a necessary link in the processes of development and heredity, and if these can be shown to be the simplest and most nearly primary factors whose behavior can be correlated with these processes, then we shall be justified in saying that the chromosomes are to-day the determining factors in development and heredity.

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