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

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Chapter V. Fertilization

The complex processes of the formation, diflferentiation, and maturation of the germ cells, described in the two preceding chapters, are to be regarded as preliminaries to the process of fertilization. They can be understood only as preparatory steps leading to the final meeting of an ovum and a spermatozoon, and their fusion into a single cell. The cell thus formed is a "new organism, which immediately commences a long series of reactions, collectively termed development, leading finally to the establishment of a form resembling that of the individuals from which the fusing germ cells were themselves derived.

Among animals, with the probable exception of a few of the simplest, an almost invariable condition of the continued existence of any specific form of protoplasm is such a periodic mingling of the living substance of two individuals of the same species. The few exceptions among the Metazoa are found in the rare self-fertilizing hermaphroditic creatures, and even here the mingled plasms may be said to have had somewhat separate histories, although formed within the body of a single organism. Animals which reproduce parthenogenetically, or by such methods as budding or fission, sooner or later in their life history exhibit these processes of germ cell formation and fusion.

Among the plants, on the other hand, while the union of germ cells may be a frequent, and in some cases a necessary preliminary to the formation of a new organism, yet other tissues and living masses composed of several kinds of tissue, taken from almost any part of the organism, may give rise to a new individual. Many species of plants are thus normally propagated by cuttings of leaf, stem, or root, or by runners, buds, etc. This is particularly true of the Begonias, where even a few cells, from almost any part of the growing plant, may be removed and, under proper conditions, be made to form a new complete organism capable of producing typical germ cells.

The questions why fertilization should be necessary, and how fertilization actually accomplishes the results which obviously follow it, are not easy to answer. But the essential facts regarding the process of syngamic fusion, and the visible results of it, are clear. We shall, therefore, confine our attention first to these; then, having described the phenomena of fertilization we may consider briefly some of the more theoretical aspects of the process.

The word fertilization" is a general, inclusive term, used to denote all of the various phenomena concerned in the meeting and fusion of the germ cells (gametes) or germ plasms, and even some of the results of such a fusion. The simpler fact of the mere fusion of gametes is more precisely termed syngamy. In all of the Metazoa syngamy may be defined as the meeting of two completely specialized, unicellular gametes, an ovum and a spermatozoon, derived in most cases from two individuals, and their subsequent fusion, nucleus with nucleus, and cytoplasm with cytoplasm, into a single, uninucleate cell, the zygote. This definition is not completely applicable to the unicellular organisms, for in these the gametes are usually not completely specialized, sometimes iadeed not especially differentiated at all. Such forms may offer some suggestions as to the history and significance of the fertilization process, and we shall return to consider this subject later in this chapter.

We have seen in Chapter III some of the methods by which it is ensured that eggs and sperm shall be brought into the same general region or into fairly close proximity, but it remains to be seen how the ovum is actually encountered by the sperm cell. Taken altogether, the processes leading to this result often become very complicated and special, and in most species the probability is very high that practically every normal egg produced will be fertilized. The gametes are completely specialized cells, and if they cannot conjugate and develop, they soon perish, not able even to remain alive long, except in a few special instances. Spermatozoa discharged freely into the water, as in external fertilization, are usually able to remain active only a few minutes or hours. But when fertilization is internal and the spermatozoa are received into some reproductive cavity of the female, or into some storage cavity, they may remain alive and able to function under appropriate conditions for a much longer period: various observers give the following specific mstances: dog and rabbit, eight days; man, seven to twenty days; fowl, twenty days; the bats and some snakes, from autumn to the following spring; Salamandra maculosa, from summer to the following spring; snails, three years; bees, four to five years.

Whatever the particular circumstances connected with and ensuring the meeting of sperm and ovum, the medium in which it occurs is a fluid. In this the sperm cells are in active, though apparently random, movement, due to rapid vibration of the flagellum or tail. In many instances their movement follows a spiral path (Buller, Adolphi), either close or open, such as is common among flagellated unicellular organisms. In a few rare instances (some Crustacea) the spermatozoa are amoeboid.

Fertilization becomes more likely when direction is given to the active random movements of the sperm cells. In some instances where fertilization is internal, the movements of the sperm seem to be directed by ciliary currents of the oviduct or other passage. Spermatozoa tend to swim against such a current, and thus to ascend toward the eggs which are being carried down the passage toward the exterior (Lott). According to the interesting observations of Lott, human spermatozoa swim at the rate of 27 mm. (i.e., about 550 times their own length) in 7.5 minutes. At the corresponding rate of progression a man 5,8 feet m stature would walk a mile in 12.4 minutes.

There appear to be two chief methods by which spermatozoa are finally brought to the surface of the ovum. In some few forms the egg is said to give off a chemical substance to which the active sperm are attracted; when the random movements of the sperm bring them within the sphere of chemical influence of the egg, their movements immediately become directed toward the unfertilized egg. Among some of the lower plants it is known that weak solutions of malic acid and its compounds attract spermatozoids; in others, solutions of cane sugar act similarly (Pfeflfer). It is at present doubtful, however, whether in many animal eggs the control is also of a chemical nature (Buller). In some forms the stimulus is certainly not of a chemical sort, but is a contact stimulus. The sperm of many fishes, for example, swim at random until they touch some solid object, egg or other body, and from this they are apparently unable to escape. According to the observations of Drago the collection of the spermatozoa about an ovum is unusual, and when it occurs, it is the result of agglutination. It should be said that in most cases it is doubtful whether the movements of the sperm really are given direction toward the ovum, and what the nature of the stimulus may be, when such is the case. As we have seen, the contact between sperm and egg results chiefly from the large numbers of sperm produced, and from their general proximity to the egg resulting from special habits of spawning, copulation, etc.

When the ovum is naked, or surrounded by only a thin vitelline membrane, the sperm apparently may enter at almost any point on the surface of the egg. This is true of many forms among the Medusae, Turbellaria, Nemertea, Annelida, Echinodermata, Gasteropoda, Cephalochorda, Amphibia, and Mammalia. Entrance is usually said to be effected by the active swimming movements of the spermatozoon, which force the sharp acrosome, adapted to this purpose, through the limiting surface of the ovum, into its superficial cytoplasm. But here again extended evidence is lacking, and in many forms the egg is known not to be a wholly passive recipient of the sperm, but to take a considerable share in accomplishing its entrance. Thus in the sea-urchin, when a sperm head approaches the egg closely, the superficial cytoplasm, at the point nearest the sperm, is elevated into, a small cone or papilla called the attraction cone (Wilson). This not only rises to meet the spermatozoon, but seems to aid in drawing it into the egg (Fig. 85). In some instances (e.g., Julus) this attraction cone may be quite high and may contain a part of the chromatic substance of the egg nucleus. According to the observations of lillie, the spermatozoon of Nereis is clearly drawn into the egg through the activity of the latter, the sperm itself taking no active part in the process (Fig. 86).

Fig. 85. — Entrance of the spermatozodn into the egg. From Wilson, ** Cell," Ht after Metschnikofif; J, after Fol. A. Spermatozodn of Toxojmeu^tea, X 2000; a, the apical body; n, nucleus; m, middle-piece; /, flagellum. B. Contact with the egg-periphery. C,D, Entrance of the head, formation of the entrance-cone and of the vitelline membrane (v), leaving the tail outside. In some other Echinoderms, the tail may enter the ovum. E,F. Later stages. O. Appearance of the sperm-aster (a) about three to five minutes after first contact; entrancecone breaking up. H, Entrance of the spermatozoon into a preformed depression. J. Approach of the spermatozodn, showing the attraction-cone.

When the egg is surrounded by membranes of some thickness or density, the spermatozoa are usually unable to penetrate them and the only path of entrance is then through the micropyle, the existence of which is an adaptation for this event. There is apparently no agent directing the sperm toward this perforation in the membranes; the finding of it is a matter of chance. The chance is not small, however, that some spermatozoon will enter the micropyle, for ordinarily the fluids around the egg are filled with a swarm of sperm cells. As already noted the micropyle is commonly at the animal pole of the egg, though at the vegetal pole in a few instances (some

Molluscs). Thus the region which is to receive the spermatozoon is already determined, and frequently the cytoplasm is considerably modified, in the region just beneath the micropyle, into a special substance concerned in the receipt of the sperm; this is known as the entrance disc (Fig. 86).

Fig. 88. — Entrance of the spenuBtoiodn in the fertilization of the Annulate, Nereie Un^Ma. After Lillia. A. Spermatozo6n. B. Perforatorium has penettfttad egg membrane; entrance cone well developed. Fifteen minuteB after insemination. C. Thicty-seven minutea after insemination. D. Entrance cone sinking in and drawing the head of the Bpermatozo6n after it. Forty-eight and one-half minutes after insemination. E. Head drawn in still further. Fortyeight and one-half minutes after insemination. F. Entrance completed. First maturation division in anaphase. Fifty-four minutes after insemination. The middle piece, as well as the tail, remains outside, c, head cap; e, entrance cone; A, head of spermatoio5n (nucleus): n. middle piece; p, perforatorium; v. vitelUna membrane; /, Qrst polar division Ggure.

With but comparatively few exceptions, only one sperm cell normally enters a single egg {monosfermy). This sperm is the first one to reach the egg or micropyle, and there are various methods of excluding additional sperm, and thus of preventing abnormal multiple fertilization. In some of the lower plants, after one sperm cell has entered, the egg gives oflf immediately a chemical substance which actually repels the other sperm congregated about the egg. A frequent method among animals is the secretion of an impenetrable membrane, or a layer of jelly, immediately upon the entrance of one spermatozoon. Or, if a membrane was previously present, its density may be suddenly increased, or an additional membrane formed

Fig. 87. — Polyspermy in the egg of the Elasmobranch, Torpedo oceUata* From Ziegler, after Ruckert. Germ disc with first cleavage spindle, /, and accessory sperm nuclei, ap.

(Amphioxus), or the micropyle may be closed by the rapid swelling of the egg membranes. Although this process of membrane formation may really have this effect of excluding supernumerary spermatozoa, the general significance of the process renders it doubtful whether this is to be regarded primarily as an event adapted toward this end.

Occasionally two or more spermatozoa succeed in gaining entrance into the ovum (polyspermy). This ordinarily results in an abnormal course of development, which does not proceed very far before the egg ceases to develop and dies. A few forms, however, are adapted to the receipt of more than one sperm and polyspermy occurs normally (physiological poly^ spermy). Such eggs (Fig. 87) are usually yolk-filled, for example those of some Insects, Petromyzon, Selachians, Urodeles, Reptiles, Birds, and perhaps the toad and some Teleosts. Sometimes a few, sometimes many, sperm thus enter the ovum, but in any case only one of them ever takes any real part in the actual processes of fertilization. The others, known as accessory spermatozoa, may either remain quite inactive and soon degenerate, or they may give rise to "vegetative nuclei, and perish after a brief period of activity. While active they seem chiefly to be concerned in the preparation of the yolk for ready absorption; they are then called merocytes. Rarely, if ever, do the nuclei derived from accessory spermatozoa contribute directly to the formation of any part of the embryo proper.

Apparently there is little specific adaptedness in the behavior of the germ cells such that an egg and a sperm of the same species tend to unite much more readily than do those of different species. With some eggs any spermotozoon that is morpho" logicaUy capable of gaining entrance, can do so, apparently about as readily as the specific sperm. The limitations here are frequently due to the size of the sperm head as compared with the micropyle, or to the necessity for special perforating mechanisms or powerful swimming movements in order to penetrate the egg membranes, or the performance of appropriate reactions upon the part of the egg itself. As a rule the eggs and sperm of a single species unite, because, as the result of the breeding or spawning habits, only the sperm and ova of a single species are associated in time and space in any considerable numbers. When eggs are placed in a mixture of equal quantities of two or more kinds of sperm, there seems to be no appreciable selective fertilization, provided, as said above, that both or all kinds of the sperm are able to enter the egg at all.

The ease with which "foreign" sperm may enter an egg is affected in many instances by chemical treatment of the eggs and sperm; treatment with alkalies or with specific salts often renders penetration of the sperm readily possible in cases where normally it is difiicult or impossible (Loeb, Godlewski).

And once within the ovum, a "foreign sperm seems to act almost as efficiently as the proper sperm in inaugurating development. After the entrance of a foreign sperm the two germ nuclei may not, usually do not, fuse, and other internal developmental processes may not be entirely normal, but the external processes of cleavage and differentiation may proceed normally for some time, even to the formation of a free-swimming larva, as in many species of Echinoderm eggs fertilized by the sperm of other species, genera, or even of other classes of Echinoderms (Baltzer), or of other phyla (MoDusca, Kupelwieser).

In many species the entire spermatozoon enters the cytoplasm of the ovum (some of the Turbellaria, Annelids, Insects, Molluscs, and many Vertebrates) while in others the tail piece separates from the remainder of the sperm cell and is left outside of, or embedded within, the vitelline membrane, so that only the head and middle piece actually share in the formation of the zygote. In some instances the middle piece, too, fails to enter the egg (e.g., Nereis^ Fig. 86). Once within the egg, the sperm continues its mward course for a short distance only. If the entire sperm cell has entered, one of the first events is a sharp bending or flexure between the tail and middle piece, often followed by a separation of the two, after which the tail piece is left behind as the remainder continues its migration (Fig. 88) .

The entrance of the spermatozoon within the egg cytoplasm is the event which inaugurates a whole series of fertilization processes culminating in the formation of a typical mitotic division figure within the zygote. The precise character of the stimuli which start this chain of actions is still in doubt, but it seems likely that in many instances it acts, first, by bringing about the formation of a permeable membrane over the surface of the egg, through which may occur rapid and extensive osmotic interchanges leading to marked oxidations; and second, by reducing the amount of fluid in the egg cytoplasm, either actually, by its loss through the permeable membrane, or relatively by the addition of the much denser substance of the spermatozoon itself (Loeb). At any rate, whether it be a primary or a secondary process, this loss of water following the entrance of the spennatozoon, ap[>ear8 aa one of the important aspects of fertilization.

Fig. 88.— Fertiliiation in the aea-urchin, Toxojmeuales. rrqin Wilson, "Call." A-F.X 1067;O. X 533rH,J, X 667, A. Specm-bead before entrance; n, mieleuB; m, middle-piece and part of the QaBellum. B.C. Immediately after eDtrance, showing entrance-cone, D. Eotatioc ot the Bperm-head, formation of the spcRn-aater about the middle-piece. B. Castinn off of middle-piece; centrosome at focus of rays, F,Q. Approach of the pronuclei; Erowtb of the astei, H, Union of pronuclei. I. Flatteiuiig of the Bperm pronucleus against the esg ptonueleus; division ot the aster.

Fig. 89. — Changes in the structure of the ovum in Nereis, upon tertiliiation. After LilUe. A. Unfertilized oScyte. I,aree germinal veside; cytoplasm contains oil drops and yolk apheres. and shows well marked cortical layer (eioplasm). B. Fifteen minutes after eosemination (the apermatozodn is not shown). The cortical layer haa chiefly gone to form a jelly-like layer outaide the ovum, and is not shown. C. Egg after centrifuging to show component substances, c, cortical layer (exoplasm); n. germinal vesicle or nucleus; no, nucleolus; o, oil drops; p, perivitelline space; v. vitelline membrane; v> yolk spheres; I. layer of oil drops; 2. hyaline cytoplasm with small baaophile granules; 3, yolk spheres; 4, hyaline cytoplasm with large basophile granules.

In the eggs of many species there is a peripheral layer of cytoplasm (exoplasm) which is comparatively clear, free from granules, and characterized by the presence of fluid vacuoles (Echinoderms, Nereis {Fig. 89), Amphioxus (Fig. 90), Teleosts; see Chapter III). Entrance of the spermatozoon leads to the breaking down of these vacuoles and the discharge of their substance from the surface of the ovum (Figs. 89, 90), This substance may be in part transformed into, or may cany before it a modified surface layer of material which appears then as a fertilization membrane; this may be the vitelline membrane or it may be an addition to a previously present vitelline membrane (Fig. 90). Then either by the shrinkage of the egg, or the expansion of the membrane, or both, or by the rapid absorption of water by the substance between the egg and the membrane, a space of widely varying dimensions in different species, is left between the egg and its membranes; this is the perivitelliTie space (Fig. 90). Within this space the e^ is free to move or rotate, although the superficial membrane may be fixed to some foreign body. Frequently this phenomenon of membrane formation is but one phase of a general physical and chemical reorganization of the whole substance of the egg, following the entrance of the spermatozoon. The egg may exhibit more or less amoeboid movement, or waves of contraction may pass over it. A frequent result is seen in the rapid streaming of differentiated cytoplasmic substances into certain regions, where these specific substances collect. Thus in many yolk-filled eggs like those of the Teleosts, the protoplasm, which before the entrance of the sperm is quite uniformly distributed over the surface of the egg as a very thin layer, now collects at the animal pole into a thick and fairly circumscribed disc called the germ disc (Fig. 48). The Ascidian egg, as described by Conklin, offers one of the most marked examples of this rapid transformation and redistribution of the substances of the egg cytoplasm (Figs. 91, 92). In the secondary oocyte of Cynthia (JStyela) the greater part of the cell is composed of a gray "endoplasm; superficially there is a thin but complete layer of yellowish "mesoplasm"; while the large nucleus or germinal vesicle contains a clear "ectoplasm." During maturation, which here precedes sperm entrance, the ectoplasm collects at the upper pole of the oocyte. Immediately upon entrance of the sperm the yellow mesoplasm streams from all directions toward the lower pole; this is followed by the clear ectoplasm which forms a stratum just above the mesoplasm, and leaves the upper half or more of the egg cytoplasm composed entirely of the gray endoplasm. Then this radial or rotatory symmetry gives place to a bilateral symmetry, for ihe mesoplasm and ectoplasm move up on one side (the posterior) of the egg, appearing on the surface in the form of a crescent just below the equator. Meanwhile the yellow mesoplasm and gray endoplasm have each become differentiated into two distinct substances, so that altogether five forms of protoplasm are distinguishable in the cjiioplasm of the zygote (Fig. 92).

Fig. 90. — Fertiliiation in the egg of Amphioiua. C, after CerfoatBiue, othecg after Sobotta. A. Ovarian egg showing cortical plaam. B. Cortical layer forming a membrane on the surface of the egg, within the vitelline membrane. C Egg membrane fully formed but still attached to surface of egg. D. Extruded, fertilized egg. Membrane fully formed and beginning to leave the surface of the egg. c. Cortical layer; e, endoplaam; m, eeg membrane; externally vitelline, internally a product of the exopUsm; p. perivitelline space; «, apermatoioon; t, vitelline membrane; I, first polar body; II, second polar spindle.

Fra. 91. — SeotioDS through the egg of the Tunicate, CynJAio partita. After Coaklin. X about 350. A. Ovarian egg fully formed. Germiaal vesicle ■urrouoded by yolk bodies; peripheral layer of protoplasm contaimng test cells and yellow Eraaules (small circles). B. After eitiusion of the test cells. Nuclear membraae still intact with chromoaoniea at periphery of nucleus (germinal vesicle). C. After laying (before fertilisation, the egg remains in this condition until fertilised). Chromosomes and granular substance, from which the spindle is formed, lie in the center of the karyoplasm, now free in the cell, e, eioplaBin or cortical layer; ir, granules of yellow pigment; n, egg nucleus or germinal vefflcle; t, nuclei of ingested teat cells or follicle ceUa; j/, yolk.

Very few eggs exhibit such marked differentiation as this, but the corresponding phenomena are of widespread occurrence, and it is quite likely that they have frequently been overlooked because the various substances are not often marked by

Fig. 92. — Total views of the egg of the Tunicate Cynthia partita, showing the ■changes in the arrangement of the materials of the egg Hubaeqlieot to fertilisation. After Conkl in. X 200. A. Unfertilized egg, before the fading out of the germinal vesicle. Centrally is the mass of gray yolk; peripherally is the protoplasmic layer with yellow pigment, and aurrounding the egg, the teat cells and chorion- B. About five minutes aft«r fertiUzation. showing the streaming of the superficial layer of protoplasm toward the lower pole, where the spermatoiodn enters, and the consequent exposure of the gray yolk of the upper hemisphere. The teat cella are also carried toward the lower pole, C. Side view of egg showing the yellow protj^plasm at the lower pole; at the upper pole a small clear resion where the polar bodies are forming. The location of the sperm pronucleus is also indicated. D. Side view of egg shortly before the first cleavage, showing the posterior collection of the pigmented protoplasm (yellow crescent) and the clearer «rea above it. E. Posterior view of egg during the first cleavage, showing its characters so easily observed in the living egg. It is also noteworthy that frequently a radial or rotatory symmetry of the egg is changed to a bilateral symmetry by the entrance of the spermatozoon, and that usually the position of the plane of bilateral synmietry is determined by the point at which the sperm enters or by the path which the sperm takes through the cytoplasm. And further this new plane of symmetry of the zygote coincides closely with the plane of the first division of the zygote and with the median plane of the embryo and adult.

These aspects of organization and reorganization of the egg are among the highly important aspects of development, and largely determine many of the phenomena of subsequent differentiation. They also illustrate the statement made in • the introductory chapter, that some of the most important aspects of development are in^ro-cellular processes. We shall return to this subject in a later chapter (Chapter VII).

After the entrance of the spermatozoon and the consequent redisposition of the substances of the cytoplasm, the course of the immediately subsequent events is determined largely by the state of the egg nucleus as regards its maturation process. For in most cases maturation proceeds only to a certain point, varying greatly in different forms, when it pauses, and is completed only after receiving the stimulus caused by the entering sperm. Ordinarily the state of the egg is such that sperm can gain admission only when this pause has been reached.

In some cases, such as Nereis (Fig. 86), Ascaris (Fig. 94), Cerebratulus (Fig. 95), and many Molluscs, it is true that the sperm does enter the ovum before the maturation process has even begun — while the egg nucleus (germinal vesicle) is still in the final resting stage preceding the first oocyte division. I

In other cases — most Molluscs, Thalassema, Sagitta, Teleosts, \

the first polar spindle has formed and the first maturation |

division may have reached the metaphase or anaphase, when

^ i

relation to the symmetry of the egg. a, anterior; c, clear protoplasm; cr, yellow I

crescent; e, exoplasm or cortical layer, with yellow pigment; g.v, germinal vesicle j

, chorion; p, posterior; p.b, polar bodies; t, test cells; j/, yolk (central gray mate- | rial); ]/.^, yellow hemisphere; cJ^, sperm pronucleus.

it pauses to await the sperm entrance. In Sycandra, Lepas, Amphioxns (Fig. 90), and many Amphibia and Mammalia the first polar division is completed and the second polar spindle formed before the pause. And finally in some Coelenterata and most Echinodermata and Ascidians, the maturation of the egg is entirely completed before the entrance of the spermatozoon. In such cases as these, contact with sea water seems

Fig. 93. — Diagrams of the two most frequent relations between the events of maturation and fertilization. From Wilson, "Cell." J. Polar bodies formed after the entrance of the spermatozo5n (Annulates, Molluscs, Turbellaria). //. Polar bodies formed before the entrance of the spermatozoon (Echinoderms). A. Sperm pronucleus and centrosome at <5*, first polar body forming at 9 . B. Polar bodies formed; approach of the pronuclei. C. Union of the pronuclei. D, Approach of the pronuclei. E. Union of pronuclei. F. Cleavage nucleus.

to furnish the stimulus to complete the maturation process, which may be begun before the eggs are produced.

We may distinguish two general types of behavior on the part of the germ nuclei according to whether maturation has or has not been completed at the entrance of the sperm (Fig. 93). We shall consider first, and at greater length, those cases in which maturation is not yet completed, for. this would seem the more usual course of events. Otherwise maturation occurs precociously, apparently before the necessity for it has arisen.

We left the sperm head and middle piece lying a short distance below the surface of the egg. We may disregard the tail piece now, for even in those cases in which it enters the egg it is left behind the head and takes no active part in subsequent processes. The head and middle piece now move more slowly^ along a path which is, for a short distance at any rate, a radius of the ovum. Then they separate slightly, and the two rotate through approximately 180*^, so that the middle piece is placed in advance of the head (Figs. 94, 95). There ensues a considerable metamorphosis of these elements. The sperm head loses its sharp outline and gradually enlarges; its outline soon becomes very irregular and indistinct, and vacuoles appear. Soon it has expanded into an organ of considerable size and has acquired a typical nuclear structure with linin network, chromatin granules, and nuclear membrane. In the meantime the middle piece has undergone an even more extensive transformation (Fig. 94). Before the sperm halts in its inward progress, even before the rotation in some cases, the middle piece has begun to dissolve and in connection with it appears a centrosome, surrounding which a small aster appears. During the pause of the sperm nucleus, the centrosome and aster each divide into two, and the daughters diverge slightly while the asters grow somewhat larger.

It will be remembered that during the metamorphosis of the spermatid into the spermatozoon, one or both of the centrosomes of the former either passed into the middle piece, or actually formed the larger part of it, although frequently no centrosome is actually visible in the middle piece of the fully formed spermatozoon. When the centrosome appears in the egg, in connection with the middle piece of the entering spermatozoon, it is possible, though not likely, that this is really the same centrosome that was present in the spermatid, and that there is consequently a genetic continuity of centrosomes, from generation to generation, as well as of nuclear components. Such a continuity has, however, not been definitely observed. On the other hand, it may be that the middle piece forms, either from its own substance, or from that of the. egg, a new centrosome. The aster doubtless is formed out of the egg cytoplasm, by the influence of the centrosome or centrosome-forming substance of the spermatozoon; and it is quite possible that the centrosome itself may be similarly formed from the cytoplasm through the action of some chemical substance introduced by the sperm. For it is known that the cytoplasm of the egg does possess the property of forming asters with typical centrosomes, under the influence of appropriate "artificial" stimulus (Yatsu). And recently Lillie has shown, in Nereis, where the middle piece does not enter the egg at all, that the centrosome forms in association with the sperm nucleus, even when only a small portion of this is allowed to enter the egg. This indicates that the centrosome, as well as the aster, results from the redisposition of substances of the egg cytoplasm following the entrance of the spermatozoon. A conclusion as to whether or not the law of genetic continuity applies to the centrosome in fertilization is less important than recognition of the uniformity of its chemical and physical actions, in either case. And although the centrosome as an organized body may disappear in the spermatozoon, this still contains kinoplasmic substance of an equivalent function. Here, as elsewhere, the essential continuity may be chemical rather than morphological, but for that reason it is not to be regarded as any less actual or important.

Fig. 94. — Fertilization in A icons megaUicepkaUi bivalena. From Wilson, "Cell," after Boveri. (Later stages are shown in Fig. 36.) A. The spermatozObn has entered the egg. its Ducleus is shown at 6'; beside it lies the granular mass of "archoplaam" (attraclion-apliere) ; above are the closing phases in the formation of the second polar body (two chromosomes in each nucleua). B. Germ-nuclei (9 , (?1 in the reticular stage; the attraction-sphere (o) containfl the dividing oentrosome. C ChromoBomea forming in the gerro-nuolei; the eentroBome divided. Z>. Each germ-nucleus resolved into two chromoeomeB ; attraction-sphere (a) double. E. Mitotic Bgure forminR for the first cleavage; the chromosomes (e) already split. F. First cleavage in progrBsa, showing divergencB of the daughtet-chromosomes toward the spindl«-poles (only three chromosomea shown).

While the spermatic structures have been thus active, the egg nucleus has been completing its maturation, at the conclusion of which the egg centrosomes and asters have disappeared (Figs. 94, 95). The egg nucleus is left near the surface of the animal pole, either near the sperm nucleus or at some distance from it. There are now present in the ovum all of the chief elements which are to take part in the essentials of fertilization and development. These are (1) the egg nucleus

with its chromosomes, either distinct or formed into a characteristic nuclear reticulum, and with or without a nuclear membrane; (2) the sperm nucleus, also known to contain ^

Fig. 96. — Fertiliiation ia the Nemertean, Certbratului. After Coe, C, D,

X 376, others x 250. A. Primary oocyte. Part of the chrorontin has been CDndenaed into chromosomes, only five of which are shown (the number present is aiiteen). The reicamdeF of the chromatin ia thrown out into the cytoplasm. The centrosomes, each with a small aster, are diverging, and the nuclear mem' brane is commencing to disappear. B, First polar spindle fully formed and rotated into radial position. Chromosomes in equatorial plate. C, First oocyte division; anaphase. D. First polar body nearly separated. E. First polar body completely cut oS; second polar spindle formed and rotating into radial position. Spermatozoon within the egg. F. Second polar body completely separated. Egg pronucleus forming, surrounded by large aster. Sperm pronucleus, also with a large aster, enlarged and approaching the egg pronucleus. O. Approach of the two pronuclei. Egg aster reduced, sperm aster greatly enlarged and centroBome divided. H, Fusion of pronuclei; divergence of the sperm centroBomea. /. First cleavage figure in early anaphase. Chromosomes divided and beginning to diverge; centrospheres enlarged, c. chromosomes; o. nucleolus, vacuolated and commencing to disappear; s, spermatozo5n just within the egg; T, germinal vesicle; re. contents (eitra-chromosomal) of germinal vesicle; /, II, first and second polar bodies; if, sperm pronucleus; Q , egg pronucleus.

chromosomes; (3) the centrosomes and asters derived in some way, either directly or indirectly, from the spermatozoon. In syngamy, therefore, the ovum supplies the great bulk of the cytoplasmic basis of the zygote, together with one-half the nuclear material, while the spermatozoon furnishes the other half of the nuclear substance, and produces the centrosomes, which here as elsewhere are to be regarded as the dynamic centers for division. . It should not be overlooked that a small amount of cytoplasm, including certain mitochondrial structures, does accompany the sperm, particularly when the tail piece enters the egg; it is by no means impossible, though not at all demonstrated, that this cytoplasm from the sperm may contain substances of great importance in later development and dififerentiation. In brief, however, it is true that in this union of gametes the ovum is the material factor, the Spermatozoon the dynamic, and each contributes equally to the nuclear or controlling mechanism.

But these structures are as yet distributed in different parts of the cell. The association of the scattered elements into a typical mitotic figure now follows and constitutes the final step in fertilization and the formation of the new organism. Maturation completed and the sperm nucleus dissolved, the two germ nuclei commence to approach one another, the sperm nucleus following the centrosomes and asters. The paths of their approach are seldom directly toward one another, as they are in some of the Nematodes, but are more or less curved (Fig. 96), and seem in a way determined by some factors other than mere mutual attraction, though this is doubtless the essential factor in their movement (Wilson).

The entrance path of the spermatozoon is frequently marked by cytoplasmic modifications, often of a very pronounced character, giving evidence of intense metabolic (katabolic) activity; thus we might note the frequency of the accompanying formation of pigment, which is usually regarded as a by-product of protoplasmic decomposition.

We have already seen that the path of the sperm nucleus may be an important factor, either in determining or in making evident, the position of the plane of symmetry of the zygote, and hence of the embryo. In a few cases the nuclei are somewhat amoeboid, in others they seem to be carried by protoplasmic currents, and in still others' they seem directed by the location of the asters. These have grown to a considerable size now, and seem mechanically forced into that position where the tensions between the cytoplasm and the more or less rigid asters reach an equilibrium. Most frequently this is the center of the cytoplasmic mass, so that ultimately the two nuclei approach and meet in this region. As the nuclei come into contact the two asters diverge in such a way as to he at opposite ends of a tangent drawn through the point of nuclear contact (Figs. 94, 95). The nuclear walls then dissolve; a spireme forms in each nucleus and segments, in each, into ^ chromosomes, and these, as a typical spindle forms from the egg cytoplasm, become arranged at its equator. The result is the formation of a typical mitotic figure with s chromosomes. This is the first cleavage figure, and here, for the first time in the existence of the new organism, substances of paternal and maternal origin are associated, on equal terms, in a common structure.

Fig. 96. — Diagrams showing the paths of the germ-nuclei in four different eggs of the sea-urchin Toxopneustes, From camera drawings of the transparent living eggs. From Wilson, "Cell." In all the figures the original position of the egg-nucleus (reticulated) is shown at 9 ; the point at which the spermatozoon enters at E (entrance-cone). Arrows indicate the paths traversed by the nuclei. At the meeting-point (M) the egg-nucleus is dotted. The cleavage-nucleus in its final position is ruled in parallel lines, and through it is drawn the axis of the resulting cleavage-figure. The axis of the egg is indicated by an arrow, the point of which is turned away from the micromere-pole. Plane of first cleavage, passing near the entrance-point, shown by the curved dotted line.

Before we mention any of the further details in the history of this cleavage figure, we must return to consider briefly the course of events in the fertilization of those eggs which are already fully mature when the sperm cell enters (Echinoderms, Ascidians). In such cases (Figs. 88, 93) the chief divergences from the account just given result from the absence of any pause of the sperm nucleus and middle piece after their entrance. The egg nucleus is ready for fusion, and immediately upon the entrance of the sperm the two nuclei proceed toward one another as described above. The sperm nucleus thus does not have time to be dissolved to any considerable extent, so that when the two nuclei meet they are by no means of equal size (Fig. 88), for the egg nucleus nearly always returns to its "resting" state after its maturation is completed. Nevertheless it is known from their history that the two nuclei are equivalent in chromosomal composition. Frequently, too, the centrosome does not divide until just as the two nuclei meet, or even after they have begun to fuse. The two centrosomes accompanied by asters then move to opposite poles of the combined nuclei and there establish the mitotic figure. The sperm nucleus in these cases does not become resolved into a typical nuclear condition until after its fusion with the egg nucleus. It often results from this that the two nuclear substances seem to mingle quite completely before the spindle is formed and it is not so easy, as in the cases previously described, to distinguish at this time between the elements derived from the egg and sperm nuclei. When this duplex nucleus forms its spireme, this segments into the somatic number of chromosomes immediately, and the mitotic figure for the first cleavage forms typically.

In the formation of the first cleavage figure we see the net result of all the complex processes of the formation and maturation of the germ cells, and the union of the two gametes. In a word, what has been accomplished is the reestablishment of a single typical cell with specific organismal characteristics. But this cell now has a nucleus derived in equal parts from two separate individuals of the species. Into this nucleus the events of maturation have made it possible that there should have been brought a complete and equivalent series of chromosomes from each parent, for the haploid group is composed of one of each pair of chromosomes of the diploid or somatic series. And from this nucleus are derived all of the nuclei of the developing organism; hence every cell of the adult body may, probably does, contain substance derived from both its parents.

We may regard the organization of these two haploid series into a single nucleus as the culmination of the whole process of fertilization. Or, on the other hand as previously suggested, we may consider the final step in fertilization as not occurring until these pairs of chromosomes actually fuse in synapsis during the maturation of the succeeding generation of germ cells. From this point of view the actual union of maternal and paternal structures and substance never occurs in the somatic • cells, for in these synapsis is not known. There is involved in this process of fertilization much more than these simple morphological facts express, and to this subject we shall return presently.

We shall find it profitable to consider now, as briefly as may be, the phenomena of fertilization and accompan3dng gamete formation in a series of unicellular organisms of increasing complexity and resemblance to the Metazoa. This subject has been in part postponed from Chapters I and III, and it should be stated again that the series to be described is not supposed to represent a phyletic relation. It is now too late to state with any considerable degree of probability the course of evolution of the germ cells and the process of syngamy. Apart from this, however, the consideration of such a series as this brings out many important and interesting facts regarding the general process of fertilization, and emphasizes the idea that this complicated process as we see it in the higher organisms to-day, is all the product of an evolutionary history.

As a preliminary distinction of a general and underlying character we should note that among the unicellular forms the cells which meet or fuse may be of the same race or family, that is, closely related by descent from a comparatively recent common ancestor; or they may be only distantly related, so distantly as to be regarded as unrelated, coming from races or families that have long been distinct, and that have had different histories. The former condition is termed endogamy j the latter exogamy. These two relations may be distinguished in all forms of syngamy or conjugation; exogamy is much the more usual, and involves the more comphcated reproductive processes among the Protozoa, but no such relation seems really necessary, for conjugation may occur with equal facility between any two different individual protoplasms, whether closely related or not.

In a general way we may arrange the varied phenomena of conjugation and syngamy in three classes with reference to the nature and extent of the fusion which occurs. In its simplest form this fusion is not morphological, but is expressed by the congregation of cells in groups; this is to be regarded as a form of cytotropy. Occasionally large collections of cells result and the elements come into close and extensive contact without really fusing or losing cell limits. Whatever exchange of substance there may be occurs through an osmotic process. After a temporary association of this kind the cells scatter and resume vegetative and reproductive processes. Such a process of cytotropy has been observed in Am^ha (Rhumbler) .

The simplest form in which a real fusion of plasmas occurs id that known as plastogamy. Here two or sometimes more (2-30 in Actinophrys) vegetative cells meet and flow together so that the cytoplasms mingle completely; the cell nuclei remain separate, though osmotically they may affect one another and the fused cytoplasms. The result of this is the formation of a physiologically bi- or multinucleate celL Plastogamy may be only temporary; in such a case the cells come into relation only through comparatively limited contact surfaces and tho original cell outlines are not lost. Then after a brief period, during which chemical interchanges may occur, the cells separate again.^ More frequently, however, plastogamy is permanent, and the fusion of the cells is so complete that the original cell outlines are completely lost*. Then, following plastogamy, the nuclei of the combined cells usually divide eever&I times forming a coiuiderable number of smaller nuclei; finally the cytoplasm divides correspondingly, producing thus a group of zoospores (brood formation). Such processes as these occur most typically in the Mycetozoa (Myxomycetes) and also in such fonns as Arcella, Aclinophrys, and some Foraminifera (Fig. 97).

Finally we come to a third general form of conjugation known as karyogamy, which as the word indicates involves primarily a process of nuclear fusion of the conjugating cells, although accompanied, of course, by cytoplasmic fusion which may be of hardly secondary importance. For instance, in some of the Infusoria the achromatic spindles fuse, as well as the nuclei and uudiSerentiated parts of the cytoplasm; m soma of the lower plants, even the plastids of the gametes, perhaps also their centrosomes, fuse together during conjugation.

Fig. 67. — PlEutogamy in the Rhiiopod, ArceUa vulgarU, After EHpatiewaky. A. PlaBtogftmic union of about five individuals, apparently prepBratory to the formation of zoospores ("pseudopodiospores"). B, Reproduction (formation of " macroamcebiB ") following plastogamy. c, ohromidia; n, nuclei; d, degenerating nuclei.

Most of the more familiar fertilization processes of the Protozoa are ^sentially karyogamic, but, as a rule (as in the Metazoa), not all of the nuclear substance of the cell is involved in the process. For usually, as a preliminary to conjugation, the vegetative nucleus gives off, into the cytoplasm, portions of its substance (chromidia). These may be formed as a result of general nuclear disintegration, or the nucleus may remain quite intact and extrude chiomidia, either directly through its membrane, or by a process of nuclear budding. Some of these chromidia are concerned in reproduction; such are termed idiockromidia. Karyogamy, consequently, involves only a portion of the nuclear aubstance ordinarily, and the remaining chromidia and vegetative nuclear structures may even break down and disappear during the process of fertilization. Altogether these processes of chromidia formation are diverse and often very complicated and the det^B cannot be given here. (Many of these details, and references to the literature of the subject, are given by CaUdns, "Protozoology," New York, 1909).

As a preliminary to the description of typical karyi^amic union we may Tefer to the very special form known as autogamy, which occutsin many of the simplest Protista {e.g., Entammba, Amoeba, some Myxoaporidia) . In autogamy there is refdly no fu»on of cella at all; the characteristic event is the separation of the nuclear chromatic substance of a single cell into a number of separate bodies (Figs. 9S, 99),. which become scattered through the cytoplasm as chromidia, or rather as idiochromidia, for they are concerned in reproduction. After their formation is completed these idiochromidia fuse, by twos, or sometimes in larger groups, forming in effect "new!' nuclei, or at any rate new ' combinations of chromatic substance. These fused chromatin masses then commonly move to the surface of the cell and are budded ofi with small bits of cytoplasm, as small cells or spores (Fig. 99) (Schau-, dinn, Oalkins). Here then the nuclei which fuse tc^ether are the direct, derivatives of a single nucleus, and they remain within the same cytoplasmic mass throughout their formation and fusion. This might readily be regarded as an extreme form of endogamy; it suggests the; roughly analogous process of the reentrance of the nucleus of one of the polar bodies which occurs in afew of the parthenogenetic Arthropods. Fertiliiation by autogamy is considered by some as a primitive method; of fertilization preceding all processes of gamete formation or cell fusions; others regard it as a derived condition in which the nuclei act precociously. However tbie may be it seems more instructive to clsssify the process as karyogamic.

Fig. 98. — Aatogamy in the Flagellate, Trickomattix tacerla. After Prow»iek. A. First miclear diviaion in the CDcysted form. B. The two nuclei completely separated. C. Firat "reducing" division. D. Second "reducing" division. E. Approach of the " reduced " nuclei. F. Fiuion of the nuclei to f orm a siagle Dueleua <8ynkaryon).

Coming now to the con^deration of typical karyogamic fusion we find that all of the fertiliEatioa processes conunon to the Metazoa, as well as those of most of the Protozoa, belong here. And here agfUQ fertilization may be either endogamous or exogamous.

Two general forms of karyogamy proper are usually distinguished, although they are so clearly connected by tTaositional conditions that they must be regarded as merely convenient groupings. These are isogamy, where the pairing cells are similar in size, form and behavior; and anigogamy, where the pairing cells are markedly dissimilar in size, form, and behavior. That is, this distinction is not based upon differences in nuclear structure, or behavior during union, indeed, these are essentially the same throughout karyogamy, but upon external characters of the conjugating cells.

Fio. 99. — AutoKAmy in the Rhiiopod, Enlamtiba hUlolj/liea. From Calking, "Protoioology," after Craig. A. OrgBiusm showing rods and granuleB of chromatin in the nucleus, vacuole irith some atained substance, and dense ectoplaaiD. B. ChromatiD of the nucleus pasunE into the cytoplasm, aa ohroniidia, shown in C D. AggregBtion of chromidia to form secondary nuclei. E. "Spore formation" by budding. F. Spores formed from buds.

Considering first isogamy, as the simpler and less modified process, we find it restricted to the unicellular forms. Isogamic imion frequently occurs between two individuals of the usual vegetative type which do not show, externally at least, any structural modifications usually associated with gametic behavior (Fig. 100). This is most common among the Flagellates, such as the familiar CopromoTias, and Noctiluea, but it occurs also in ActtTiopkrys and in somespecies of Amaba. In other cases the conjugating individuals show some modification in form as compared with vegetative individuals (they are modified similarly of course), but there ia no reduction in size. Thus in Cereomonat and Telramitus, the flagellate bodies of the conjugauts become more or less amceboid and distinctly plastic and viscous. Other genera exhibit various stages of reduction in size (Fig. 101), although in form they still resemble vegetative cells. Tliis is the ca^ in moat of the Foranunifera and many other Rhizopoda ; it is rare among Flagellates I^Slephanoaptugra, Chlamydomonaa) and Ciliates. Finally, we find this reduction in aze accompanied by a modification in form, as in other Rhizopoda whose gametes become flagellated, or where they are amceboid in forms usually flageUate or motbniess.

Fig. 100. — ConjUBation in the Flagellatp, Capromonat aubtilit. From Calkins, "PratoEoology." after Dobell. A. Vegetative form. B. Beginning of conjugation of two organiama; one fiagellum withdrawn. C. Continued fusion. First stage in nuclear "reduction." D. Second "reducing" division (beteropolar). B. ConjugBtion completed; "reduced" nuclei fusing. F. Zygote within cyst.

We have then among isc^amous organisms a series of forms, at one extreme of which the gametes are morphologically unmodified, at the other they are diminutive and structurally modified, usually in connection with the motor apparatus, in such a way as to render more likely the accident of their meeting, hkelihood of which is largely increased through the fact that reduction in uie ie usually the result of multiple fission or brood formation, which increases the number as well as the activity of the gametea. In all of these forms of isogamy the union of the gametes is permanent, the conjugants fusing completely and thereby losing their identity as individuals. It need hardly be added that in these cell conjugations the essential step seems to be the fusion of the gametic nuclei into a single zygote nucleus.

Fig. 101. — Gamete formation and fusion (iaognmy) in the F1agellBt«, Chlamv domotuui iteinii. Alter Goroschanfcin. A. Vegetative form. n. nucleuB, B, Gioup of gametes formed by multiple fission. Stages in the fusion of gametes (isogametes). O. Zygote. single gamete. D, E, F.

A special form of Isogamy needs particular notice on account of its frequency among the most familiar ProtoEoa — the Ciliata. This is a temporary form of isogamy which involves, not the fusion of two gametes to form a zygote, but the mulual fertilization of the two gametes through the exchange of nuclear substance and perhaps also a small amount of cytoplasm. The details of nuclear behavior in the conjugation of ParamceciuTn, for example, are probably familiar but will bear brief restatement here. This outline refers particularly to that form of Paramecium having only a single micronucleus ; one should recall that in these forma which have both micronucleus and macronucleus, the former ie, or represents, the idiochromidia, the latter the vegetative nuclear structures. Two Paiamcecia of normal vegetative size and external form meet side by side, oral surfaces in contact, in a sort of plastogamic union, The further course of events is exactly similar in each individual of the pair (Fig. l(S). In each cell the micrcnucleus divides and the daughter micronuclei immediately divide again forming four. Of these, three degenerate (c/. maturation), while the one remaining divides once more, this time unequally, forming a larger and a smaller micronucleus in each organism. Each larger or stationary" micronucleus remains passive, but the smaller or ** migratory" nucleus becomes active and moves through the bridge of fused cytoplasms to the stationary micronucleus of the other individual, with which it fuses forming a single compound or zygotic nucleus in each individual. The two Paramoecia now separate each with a nucleus of modified composition. The macronucleus, which has taken no share in the events of fertilization, now fragments and dissolves leaving the fusion nucleus as the only nuclear structure present. Then by three successive divisions the fusion nucleus gives rise to eight small nuclei; four of these in the posterior end of the cell, remain small, as micronuclei, while the other four, in the anterior end, enlarge, forming macronuclei. During the first fission of this cell each daughter cell receives two nuclei of each kind, and at the next division each of the four granddaughters of the ** zygote '* receives one micronucleus and one macronucleus, and the normal vegetative condition is restored. This form of karyogamy is peculiar for at least three reasons; the nuclei alone fuse, both of the gametes undergo nuclear reconstruction, and the individuality of the gametes is not lost.

Fig. 102. — Nuclear histoi? during conjugation in Paramaetum pulrirtum. After Doflein. The macronu cleat structures are omitted in all except A. A. Formation of spindle for first division of micronucleus. B. Telophase of first division. C. Second division of the micronucleua. D. Degeneration of three of the niioronuclei; the fourth, or permanent micronucleuB, preparing for another division. E. Division of the permanent micronucleus into the stationary and migratory micronuclei. The spindle is greatly elongated and has a characteristio mid-body. F. Grouping of micronuclei and exchange of migratory micronuclei. Q. Fertiliiation (mutual). Fusion of migratory with stationary micronuclei. H. Formation of spindle for first division of the fusion nucleus in each conjugant. I. Late phase of second division of fusion nucleus. J. Third division of fusion nucleus. K. Bzconjugant with eight micronuclei. derived from the fusion nucleus. (The degenerating micronuclei are omitted from Q~K.) d, degenerating micronuclei; m, migratory micronucleus; ma, inacronuclena ; mi, micronucleus; p. permanent micronucleus; s, stationary micronucleua; sp, spindle.

The sessile Ciliates show an adaptive modification of this process which is anisogamic in character. In the common Vorticella, for example, while one of the conjugants or gametes retains its normal vegetative form, the other is small and one of a brood of four, which become freeswimming. A small individual upon meeting a large one, is actually absorbed by it. The early nuclear history of each organism is much the same as in Paramcceium, save that the final micronucleus of the megagamete is one of four, that of the microgamete one of eight, the remainder in each gamete having degenerated. But after the equivalents of the stationary and migratory micronuclei* (idiochromidia) are formed in each gamete, the process changes somewhat, for now one of the two micronuclei (the equivalents of one stationary and one migratory body) degenerates in each organism, while those remaining fuse together forming thus only a single fusion nucleus in the single but duplex zygote. Thus there is no mutual fertilization, and while strictly this is anisogamous, it is mentioned here because it is clearly derived from the more typical Ciliate condition as an adaptation to the sessile life of the vegetative form.

Coming now to anisogamous karyogamy (Fig. 103), we should note that transitional conditions between isogamy and anisogamy are not infrequent. Thus in the Flagellate, BodOy the conjugants may be either of equal or unequal size, apparently in an accidental fashion. The colonial Paridorina forms gametes of three sizes, small, medium, and large, and conjugation may occur between any smaller and any larger individuals anisogamically, or the small or the medium organisms may conjugate together isogamically. Here then anisogamy is not obligatory, but facultative or accidental. In this case exogamy is the rule since a single colony forms gametes of one aize only, though in isogamy endogamy may occur.

Fig. 103. — FonuatioD of gameteB &nd syngamy in the Sporozoan, Klouia oetopiana. From CalkinB, "Protoioa," after Siedlecki. The cbromatin of the nucleus is distributed throughout the cell, A. B, fioally forming nuclei of the future gametes, C, D, E. The mature gametes, t, swim about, and join a macrogamete. F. The nuclei mingle, Q, and then the cleavage nucleus divides repeatedly by mitoais, to form the spores, if. ^, microgametic nucleus.

In true anisogamy conjugation is practically always exogamous for as a rule a single organism forms gametes of only one size at a time. The essential difference between the gametes is probably that of behavior, i.e., degree of activity, associated with which are constant differences in size. The larger gametes or megagametes, are less uumerpus and leas active than the smaller microgametes, which may be formed in very considerable numbers. Occasionally the gametes differ in form only, as in DaUingeria, where one of the gametes has three flagella, like the vegetative cells, while the other gamete has but one flagellum. In a few forms such as Monas and many of the Gregarines, the only morphological difference between the gametes is that of size, the microgamete being smaller and somewhat the more active, but not otherwise unlike the megagamete. In other forms the microgametes are considerably modified structurally, usually in connection with an increase in locomotor activity. At the same time the megagamete may increase considerably beyond the ordinary vegetative size and may then lose motility more or less completely. So it finally comes about that gametes of two wholly different types are formed, both quite unlike vegetative cells, and the typical Metazoan condition is reached.

Fig. 104. — Micro- and macrogametes of various Protozoa, illustrating various degrees of differentiation. After Doflein, from various authors. /, F, x 662, others X 1126. o, A. Urospora lagidia (Brasil). 6, B, CoUozoum inerme (Brandt), c, C, CMamydomonaa hraunii (Goroschankin). d, D, Vohox aureus (Klein), e, E, Cycloaporia caryolytica (with two polar bodies in cytoplasm of E) (Schaudinn). /, F. Orcheohius herpohdeUa (Kunze).

Several groups of Protozoa, e.g., Gregarines, colonial Flagellates, afford interesting series showing stages in this differentiation of the gametes (Fig. 104). We may outline one such series selecting examples frpm the Volvocine group of Flagellates.

In Stephanosphcera all the individuals of the colony are, or may be, reproductive, and conjugation is isogamous and endogamous. There is no differentiation of gametes. In Pandorina (Fig. 8) all of the individuals may be reproductive, but some of the gametes may be differentiated in size, and conjugation may be either isogamous or anisogamous as described above. Here dissimilarity of the gametes is facultative. In Eudorina two kinds of colonies are found. In one, all the cells may become reproductive, the individuals forming megagametes only slightly larger than vegetative cells; in another only four cells of the colony are reproductive and each of these forms sixty-four very small and active microgametes. Fertilization is here strictly anisogamous and exogamous. The last step is represented by Volvox (Fig. 10), where the number of gamete forming cells is always limited. Here too differentiation of the gametes reaches its climax among the Protozoa, and the Metazoan condition is reached. The reproductive cells lose their motor organs and begin to enlarge. A few of them grow to a relatively enormous size and become the passive megagametes. The others grow to lesser extent and then divide rapidly, each forming, probably 128 microgametes. These are very small flagellated, extremely active cells, with an elongated rostrum or penetrating organ at one end (Fig. 104, (Q. The microgametes are liberated in large numbers and swim about until one reaches a megagamete which it then enters and their nuclei fuse forming a typical zygote which then reproduces a new colony. The resemblance to the gametes of the Metazoa is so complete that they are here termed the oosphere, or ovum, or oogamete (megagamete) and the spermatozodn or spermagamete (microgamete).

It should perhaps be noted here that the process of conjugation or fertilization is not always associated with the reproduction of the Protozoa mentioned above, not even in the colonial forms. For the usual reproductive processes are carried out by the simple fission of ordinary vegetative cells. In the simpler colonial forms, such as Pandorina and Eudorina, as many new colonies may be formed as there are individuals forming the original colony, in Volvox, however, the number of cells which may reproduce in this way is limited, and there seems to be a real distinction between soma and germ, much like that of the Metazoan organism, the mother cell which divides to form the multiple spermatozooids, has even been compared with the testis of a Metazoan.

The principal forms of fertilization and gamete formation are summarized in the accompanying table.

Several facts of prime importance are to be drawn from this account, (a) Among the Protozoa as well as the Metazoa, the process of fertilization is widespread. (6) Out of a variety of forms comes that form of fertilization characteristic of the Metazoa, namely, karyogamy. (c) Accompan3ring this karyogamy is a gradual and finally complete differentiation of gametes, which differ morphologically and physiologically, both from vegetative cells and from each other, {d) The Metazoa show much less diversity than the Protozoa respecting the process of fertilization and the form of the gametes.

Such a series of stages as that outlined above, of the gradual differentiation and specialization of gametes, cannot fail to suggest the general subject of sex. It does indeed indicate the nature of the original distinction between the sexes. Among the Metazoa the primary and familiar facts upon which the definition of sex is based, are, that spermatozoa-producing individuals are males, ova-producers are females. In all cases of isogamic conjugation no distinction between the gametes, and therefore between sexes, obtains. In those instances transitional between isogamy and anisogamy, we may see the beginnings of sex distinction, often facultative. True anisogamy involves true sex distinction; at first relatively slight (Pandorina) , in such forms as Volvox or Cocddium and many other Sporozoa, the fundamental differentiation of sex seems to be completely established, t.e., the gametes are markedly unlike and conjugation occurs only between two dissimilar cells.

The essential processes of fertilization are entirely equivalent in isogamy and anisogamy, so that the fundamental distinction of sex is based only upon the external form and behavior of the gameteSy not upon any differences in the nature of the conjugation processes in sexual and non-sexual forms, for none exist.

Most of the colonial Protozoa are monoecious or hermaphroditic, producing gametes of both kinds, but cross-fertilization (exogamy) is the rule here, as it is among the hermaphroditic Metazoa, and frequently for the same reason in both groups, namely, a difference in the times of ripening of the two forms of gametes of a single colony or individual. Many of the Sporozoa are clearly dioecious or unisexual, and in some of these there are also secondary sexual characters, usually size differences, such that macrogamete-forming or female individuals can be distinguished from microgamete-forming or male individuals, some time before the gametes are actually formed; in a few rare instances this distinction can be made throughout the life cycle, and individuals can be identified at any time as males or females.

We now come to a consideration of the meaning and theoretical significance of these processes of fertilization or syngamy. Probably there is, in the whole field of Biology, no process of such widespread occurrence and obvious importance, where the phenomena axe so well known, which at the same time is so little understood. Why fertilization should occur, what is effected by it, and how syngamy brings about the results which do follow it, are questions to which to-day, after decades of speculation and research, no sure answers can be given.

Although we may be on uncertain ground, it will be profitable to review some of the suggestions and hypotheses that have been proposed in this connection, even if we accomplish little more than to point out the possibilities and difficulties of this fascinatmg subject. And furthermore, while no thoroughly demonstrated solutions of the problems of fertilization have been reached, there are several carefully worked out hypotheses in the field, some of which are certainly to be regarded as close approximations toward the correct explanation of some of the problems of fertilization. We may conveniently arrange these current ideas as to the results and primary "purpose of fertilization in four groups. The results of fertilization may be connected with (a) reproduction, (b) rejuvenation, (c) the process of variation, (d) the process of heredity. In considering each of these we shall state as briefly as possible the essentials of the evidence for and against the central idea.

That fertilization is primarily a reproductive process was the original view held by Harvey and his successors. There are now two forms of the hypothesis. In one form we find the idea that the ovum quite obviously seems to contain only a part (i.e., the cytoplasm and one-half a nucleus) of the mechanism necessary for development, and that the spermatozoon brings into the ovum those parts (i.e., one-half a nucleus and the centrosome, or the stimulus to its formation) which complete this mechanism and enable development to proceed. In its other form this idea is that the ovum is a quiescent, passive body which needs to be stimulated to its normal activity (development) by the entrance of the spermatozoon, the kinoplasm (centrosome) of which is the part chiefly acting as the stimulus. In short, fertilization is to be regarded normally as the necessary antecedent, as the cause of development.

There are many facts opposed to this view. Of these we shall discuss two chief classes, first, those of parthenogenesis, both normal and "artificial," and second, those drawn from the relation between fertilization and reproduction among the Protozoa.

For present purposes we may extend the definition of parthenogenesis to include all those cases where single cells, specialized for the purpose, develop without undergoing sjmgamy. In the plant kingdom parthenogenesis, in this broad sense, is very widespread. Development from spores is very conmion, even among the higher (vascular) plants, and in some instances (some of the Fungi) reproduction by single unfertilized cells is the exclusive method. And the development of ova, typical in every respect save that of needing to be fertilized, is not uncommon. . Among the single-celled animals phenomena equivalent to development from spores are frequent, and among the multicellular animals normal parthenogenesis is known in the Rotifera, some of the Crustacea, and in several orders of Insfecta. In most of these forms fertilization does occur at some period in the life cycle, after a widely variable number of parthenogenetically produced generations, but there are a few Metazoa, e.g., the wasp, Rhoditis, and the Crustacea, Cypris, Limnddia, and sometimes Apits, in which males never develop and fertilization is therefore entirely unknown (Weismann). In some of the parthenogenetic Crustacea the nucleus of one of the polar bodies seems to act as a fertilizing nucleus (see Chapter IV), so that a sort of autogamic process occurs, recalling that of some of the Protozoa and perhaps analogous with it. In all of these Metazoa the form and history of the: parthenogenetic ova, the occasional presence of vestigial! spermathecse, and other similar conditions, clearly indicate that this is a secondary or derived condition, a special adaptation, which therefore throws but little light upon the fundamental significance of the fertilization process; this proves only that fertilization is not in all cases a necessary antecedent to development, and that the ova may, in certain cases, contain in themselves complete developmental mechanisms, and need neither the addition of sperm structures nor the special form of stimulation afforded by the entrance of the sperm cell.

Instances of merogony, or "male parthenogenesis," where egg fragments containing no nuclear substance develop after penetration by a spermatozoon (Echinoderms), also show that the addition of egg and sperm structures, each incomplete in itself, is not a necessary feature of fertilization.

It may truly be said that the obviously secondary character of normal parthenogenesis renders the phenomenon of little value as evidence regarding the real meaning of fertilization in the vast majority of instances. But such an objection cannot be brought against the evidence from experimental parthenogenesis, and probably the clearest evidence upon this phase of the subject is that of "artificial" or induced parthenogenesis. In view of this fact, and of the great general importance of the subject, we may consider this matter rather fully.

The eggs of many animals, belonging to many different classes and phyla, normally requiring to be fertilized, may be stimulated to begin their development by chemical or physical treatment. Thus the ova of many Coelenterates, Echinoderms, Annulates, Molluscs, and even some Chordata (Teleosts, Amphibia), may be induced to commence their development parthenogenetically by being subjected to the action of a great variety of organic and inorganic substances in solution, to unusually high or low temperatures, to physical shock, or to various other conditions.

In this process of artificial parthenogenesis two phases must be kept separate, first, the phenomenon of maturation, in those cases where this is not completed at the time fertilization normally occxuts; and second, the phenomena of cleavage and differentiation, occurring subsequently to maturation. Certain acids and some other substances seem to have, according to Morse, a specific effect in bringing about maturation, either not producing cleavage or actually inhibiting it, in which latter case it may then be induced by other treatment. The precise actions upon the egg of those chemicals inaugurating cleavage are varied, but for the most part they appear to effect certain changes in the egg which are similar in nearly every instance. For example, according to Loeb, who is the pioneer in this important work, in the sear-urchin, the best result, that is, the closest imitation of natural fertilization, is secured by treating the eggs, first with a solution of a monobasic fatty acid, such as butyric acid, for one or two minutes. In many cases the butyric acid can be replaced by an alkaline solution of equivalent strength or by a solution of almost any fat solvent. This treatment results in the formation of an apparently typical fertilization membrane. Then second, the eggs are treated for some minutes or hours (sea-urchin eggs, thirty to fifty minutes at 15° C.) with a hypertonic sea water, that is, sea water whose osmotic pressure has been raised about 60 per cent, above normal by the addition of salts, such as sodium chloride. Finally the eggs are retm^ned to normal sea water and cleavage then follows in quite the usual fashion.

What is actually accomplished within the egg by such treatment as this is largely conjectural. Loeb suggests that the process may be as follows. The mature oviun is surrounded by a relatively impermeable surface film which prevents the oxidations necessary to development. The butyric acid or similarly acting substance, by dissolving certain fatty constituents near the egg surface, frees from association with these, certain other osmotically active materials which then form the permeable fertilization membrane, and thus rapid oxidations are permitted. Loeb believes that the nuclear substance possesses a catalyzer which, in the .presence of oxygen, brings about a synthesis of nuclein, one of the chief constituents of chromatin, and that this sjmthesis of nuclein is the chief chemical action of the segmenting ovum. This process of cleavage once started in the right direction, leads then in a perfectly natural and normal way to the later processes of development. Loeb suggests farther that the spermatozoon may contain certain substances, enzymes, which form within the egg, materials capable of producing efifects similar to these, and that herein lies the natural stimulating effect of the spermatozoon.

Many facts regarding this hypothesis, both j)ro and con, have been forthcoming in recent years, but it is still too early to say how closely this approximates the truth. We might add, however, that Masing and others have not been able to detect any marked synthesis of nuclein such a Loeb describes during cleavage. And quite recently Conklin has determined that the synthesis of nuclear substance is, in Creptdvla, at least no greater than the S3aithesis of cytoplasm.

The eggs of other forms are more successfully treated by other methods, and each may have a particular treatment which is most effective. But in very many of the instances of artificial parthenogenesis the essential result of the treatment seems to be a process of membrane formation accompanied by the withdrawal of fluids from the egg. This has led to the suggestion (Loeb) that the spermatozoon acts in this fashion, for it is relatively very deficient in fluids, and upon entering the egg reduces to some extent the relative fluidity of its cytoplasm, thus acting as a stimulus.

That the action of the spermatozoon is not specific, and that fusion, of the two germ nuclei is really not necessary to inaugurate development, is clearly shown by the fact that almost any spermatozoon, of whatever species, that can gain entrance to an ovum, is capable of initiating development, and of effecting the apparently normal cleavage of the ovum; to what extent the internal processes of fertilization and cleavage are entirely normal in such a case, we shall see later; suffice it to say here that frequently a foreign sperm nucleus remains quiescent and takes no part in the formation of the mitotic figure.

It is true that the development of artificially fertilized ova seldom proceeds farther than the cleavage stages. As a rule the lower the organism in the evolutionary series, the farther its development may proceed. And while some artificially fertilized Echinoderm eggs have been carried past the larval stage (Delage), the Chordate ovum (Teleost, Cyclostome, Urodele) will cleave only a few times. This indicates clearly that the parallelism between natural and artificial fertilization is not complete, although it is not unlikely that ultimately a form of treatment may be found which will produce just the same result as normal fertilization, save in so far as this is concerned in the inheritance of individual characteristics. In all cases of artificial parthenogenesis the cleavage figures are essentially normal except that the reduced or number of chromosomes is present (exceptions to this have been reported by Tennent and Hogue, and others) ; the poles of the spindle are occupied by typical centrosomes, formed anew by the substance of the egg after the disappearance of the oocyte centrosomes of the maturation spindle. This is also true regarding the centrosomes of normally parthenogenetic eggs. And there are several instances known where the specific effect of certain reagents or external conditions is the formation, out of the cytoplasm of the ovum, of numerous centrosomes, apparently of normal structure and each with a small aster (Yatsu) .

To summarize the evidence from parthenogenesis, both normal and artificial, we may say that, among the Metazoa, the ovum contains within itself a mechanism sufficiently complete to function for a time at least, although the spermatozoon, when it enters, does add to this mechanism and supplies some parts not present in the egg; these parts either are not absolutely necessary or they may, under certain conditions, be supplied from the structure of the ovum in the absence of the sperm. And further, while the egg may be stimulated to develop by means other than the entrance of the sperm, this is normally the form of stimulus which inaugurates the series of reactions we call development. Taking this view of fertilization the formation of the spermatozoon is a means of insuring the properly effective form of stimulus, which might otherwise be lacking in the environment of the egg.

Turning now to the evidence which the Protozoa offer regarding the relation of fertilization and reproduction, we may approach more closely the problem of the fundamental significance of syngamy. Nearly all the known life histories of Protozoa are cyclic in character. The process of reproduction by simple fission may proceed uninterruptedly for a longer or shorter period, but finally this is interrupted by some form of syngamy. Formerly this seemed obviously to mean that the ordinary reproductive processes depended ultimately upon a process of conjugation or fertilization, and that the life cycle in the Protozoa was essentially similar to that of the Metazoa, the divisions of the somatic cells of the latter being equivalent to the simple fissions of the former, and the fertilization processes of both being essentially similar (homologous) . It was overlooked at first that the process of fertilization might just as well be considered the result of vegetative divisions as the cause of them; to this phase of the relation we shall return shortly.

It is true that in some Protozoa, e.g., Noctiluca, TrichospfuBr^ turn, some Gregarines, fertilization is really followed by a marked increase in reproductive activity. And it is often true that multiple fission tends to follow conjugation. But in other cases reproductive activity seems not to be affected by fertilization. And in many, probably most Protozoa, fertilization tends to inhibit reproduction. In many Rhizopods, Flagellates, and Ciliates, a pause in the succession of fissions may be quite marked after conjugation. In many of the Sporozoa a period of encystment follows, and the same is true of many Algse. In such cases, therefore, fertilization seems opposed to reproduction, or at least to any immediately ensuing processes of multiplication; it may still be true that the ultimate effect of fertilization may be increased rate or duration of fission. Conjugation may occur without reproduction; reproduction may occur without conjugation. And that conjugation is frequently to be regarded as determined by external rather than internal conditions is indicated by the occurrence of so-called "epidemics" of conjugation which may often be observed in Protozoan cultures. In such cases conjugation may often be artificially induced by regulating the character and amount of the food supply (Jennings).

It may be concluded, therefore, that among the Protozoa the processes of reproduction and fertilization are not fundamentally related, and the primary significance of fertilization must be sought in some other relation. This view is widely accepted to-day and it consequently becomes necessary to explain the practically universal association of the two processes among the Metazoa, the only exceptions being, as we have seen above, the secondary and obviously derived instances of normal parthenogenesis. The commonly suggested explanation is the following.

Whatever the real significance of fertilization may be, it seems, for reasons which will appear later, a condition for continued existence of specific forms of protoplasm that occasionally some disturbance of its inner structure should occur, such as would result from the mingling of the substances of two distinct individuals. Among the single-celled organisms this may occur at any time, whenever that action would form a natural response to internal conditions of the organisms. Among the Metazoa, on the other hand, such a complete fusion of cells, and particularly of nuclei, can occur only when the organisms are in the form of single cells, i.e., gametes, and differentiation of the organism is at a minimum. Thus whereas the two processes are originally distinct and unrelated in their origin in the Metazoa, they have come to be related, and now fertilization appears as the first step in reproduction.

Another general hypothesis regarding the function and significance of fertilization is the rejuvenation hypothesis, associated chiefly with the names of Biitschli, Maupas, and Richard Hertwig. This is based to a large extent upon the phenomena of the Protozoan life cycle. It involves as a starting point the assumption, partly based upon observation, that protoplasmic activity tends gradually to diminish in intensity, and that associated with this diminution are certain morphological alterations in the structure and composition of the cell. Altogether these modifications are known as senescence. A frequent characteristic of a senescent cell, in both Protozoa and Metazoa, is the relatively large proportion of cytoplasm as compared with nuclear substance. It is further assumed that syngamy and the consequent admixture of nuclear and cytoplasmic materials of two individuals, perhaps representing different races, causes the restoration of the senescent protoplasm to a condition of vigor, in a word, brings about rejuvenation. It would follow from this, that protoplasmic activity is cyclic, and that periods of senescence would be followed by death unless fertilization, or an equivalent process, should occur.

Precisely what is involved in the process of rejuvenation cannot be stated definitely. Richard Hertwig suggests that senescence is due chiefly to changed nuclear-cytoplasmic rela^ tions resulting from repeated cell-fissions, and that in rejuvenation there is essentially a restoration of the normal nuclearcytoplasmic ratio, as well as a certain chemical and physical reorganization of the protoplasm through the combination of materials from two more or less unlike individuals. Loeb and others, as we have seen, also regard the rapid synthesis of nuclein as the most important consequence of fertilization. Still others (Minot, Bernstein) believe that rejuvenation is not only a nuclear-cytoplasmic phenomenon involving or resulting from an increase in the relative amount of nuclear substance, but that it further includes an increase in the property of growth, i,e., the formation of new protoplasm, both nuclear and cytoplasmic, out of non-living substance.

The real evidence for the cyclic character of the life processes of the Protozoa is chiefly that of Maupas and Calkins, who showed that in Paramcedum and some other Ciliates, when conjugation is prevented, there occur, under laboratory conditions, periods of depression in vital activity, accompanied by changes in structure, i.e., periods of senescence. This depression leads finally to death unless conjugation is permitted, or unless the organisms are subjected to some form of stimulus. If stimulated by chemical or physical means, or naturally through conjugation, the organisms may in some cases recover their original vigor and begin a new cycle with youth renewed.

But rejuvenation is by no means always the result of conjugation, for frequently the senescent organisms perish in spite of conjugation; and it may even be the case that the descendants of cells which have conjugated before the signs of senescence have appeared, perish sooner than their immediate relatives of approximately the same age, which have been prevented from conjugating. Moreover, Jennings has shown that in certain races of Paramcedum aureliorcaudatum conjugation may occur at intervals of only one or two weeks, while in other races of the same species conjugation occurs only at intervals of a year or longer, and in still a third race no conjugation was observed during a period of three yeaxs, although during this time observation was not so continuous as to preclude the possibility of conjugation having occurred.

Valuable evidence upon the question of the cyclic character of the Protozoan life history is afforded by the work of Woodruff, who has shown that if more natural conditions are substituted for the artificial and more uniform conditions of the laboratory, no cyclic relation appears, in some strains of Paramoedum at least. By continually altering the character of the food, and by imitating in other ways the naturally variable conditions of pond life, he has been able to continue a single race of ParamcBcium for over five years. During this period more than 3000 generations were formed by simple fission, and in all this time conjugation did not occur, and no periods of depression or signs of structural modification could be observed.* Finally, Woodruff has been able to carry a culture of Paramcedum on a uniform diet of beef extract, which is supposed to contain all of the materials necessary for their life, for ten months (about 450 generations) without any indication of senescence.

Such facts as the foregoing show, first, that protoplasmic activity among the Ciliates may not be cyclic in character under

On Sept. 27th, 1912, Professor Woodruff writes that this culture is in its 3265th generation, and still normal certain conditions, and second, that when cyclic periods of protoplasmic depression do occur the protoplasm may be restored to a condition of normal vigor, either by physical or chemical stimuli, or by fertilization. Supposing, and the supposition is highly probable though not completely demonstrated, as a fact, that the living processes do tend, in the absence of continued stimulation, to diminish in intensity or otherwise to deviate from the normal, then we find in the process of fertilization a natural means of insuring the receipt of stimuli which might otherwise be lacking. The onset of those structural and physiological modifications called senescence, leads to a modification in the behavior of the organisms, i.e., they form gametes and conjugate.

This becomes clearer when we recall that life itself is response — ^reaction to the stimuli resulting from changed relations. Such a changed relation may result (o) from changes in both the environment and the cell or organism, or (6) from changes in the environment alone while conditions within the organism remain comparatively uniform, or (c) from changes within the organism while the external conditions remain comparatively uniform. Of these three possibilities the first two are certainly the most frequent in the lives of most free-living Protozoa. But we may interpret fertilization as fundamentally a means of ensuring a changed relation through the realization of the third possibility in the absence of the other two. In a way the Ciliates act so as to ensure automatically a changed relation between organism and environment; when external conditions become too uniform to bring forth the normal vegetative activities, the form of reaction actually changes and is modified into gamete formation and fertilization, which immediately leads to an internal disturbance and the condition of uniformity is corrected, whenever it may occur.

Among the Protozoa we find this division of labor between vegetative and conjugative or fertilizing cells occurring whenever internal-external relations demand it. Among the Metazoa, however, such a division of labor must occur at a certain period in the life history, on account of the impossibility of the complete fusion of two whole organisms at any time other than when they are in the form of single cells; consequently we find vegetative and gamete-forming tissues differentiated side by side, and since these are, as components of a single organism, in a fairly constant environment removed from continuously rejuvenating stimuli, it is the function of the gamete-forming tissues to form single cells which can fuse with cells of other individuals and thus, by altering the composition of the organism, alter its relation to external conditions. And the almost universal association of reproduction and fertilization consid^ ered as a rejuvenative process among the Metazoa may have an added significance; the two occur together, not because they are directly related to one another, but because they are both occasioned by the same condition, or rather by the same limitation of opportunity, for the complete fusion of multicellular organisms can occur only when these are in the form of single cells, or gametes.

Reference to the third possible significance of fertilization may be more brief because of its extremely hypothetical character. The idea that the process of fertilization is primarily related to the phenomena of variation is associated chiefly with the names of Weismann and Oscar Hertwig. The scanty and uncertain character of the evidence here is indicated by the fact that there are two exactly opposed views as to the nature of the relation. Hertwig maintains that the effect of fertilization is to limit variation within a species, by tending to bring back to the normal, through the'process of heredity, the progeny of extreme fluctuations and unusual or abnormal variations (mutations), because the likelihood of their mating with the much more common mediocre or average individuals is so much greater than that of their mating with their likes. Weismann, on the other hand, maintains that the effect of syngamy or "amphimixis" is to cause or promote variation, which would result from the new organic combinations in the continued admixture of the gametic nuclei of different individuals. In this way the process of fertilization becomes of great evolutionary significance in that it accounts, in part at any rate, for the presence, indeed the origin, of variations and fluctuations, the raw materials" of evolution.

Both of these views are based upon the more fundamental and underlying hypothesis of the representative particle nature of the elements of the chromosomes, or perhaps of other portions of the germ cells, which themselves vary in their structure or their combinations. Here again the occurrence, among the Metazoa, of fertilization only when the organisms are in the form of single cells, grows out of the fact that complete nuclear fusion can occur only when in this state.

There is little direct factual evidence for or against these views, either one of which can be maintained upon theoretical grounds. In a few cases it is known that the amount of variability is not significantly different among sexually (gametically) and asexually (parthenogenetically) produced individuals of the same species. And from the standpoint of more recent studies upon heredity and variation the evidence is chiefly either negative or opposed to the idea that this relation constitutes an important element in the origin or present function of fertilization. The present aspects of this relation between fertilization and variation merge m the larger question of the relation with heredity which we may refer to next.

Whatever the significance of fertilization may prove to have been originally, its relation to the phenomena of heredity is to-day undoubtedly its most important aspect, at any rate among the Metazoa. The general subject of the relation of the structure of the germ cells to the main facts of heredity is reserved for consideration in Chapter VII, but we should pomt out here some of the underiying conditions involved in the fundamental fact of the union of the two germ cells derived in nearly all cases from two different individuals of the same group or species.

As pointed out in the introductory chapter, the germ cells are not to be regarded as the material links between successive generations of specific organisms, for organismat specificity is not discontinuous, but continuous, and the germ cells are no less specific, no. less the organism, than a^ the mature individuals producing the germ cells or produced by them. From the standpoint of the fertilization process and of heredity, the essential fact is not that the zygote develops into an individual of the same species to which belonged the organisms producing the gametes, for in parthenogenesis, for example, specific organisms are produced in the absence of fertilization. The significant fact here is that offspring may possess some of those characteristics which are the individiuil possessions of either of the parents. On the whole, offspring inherit, or may inherit, equally from both parents, and such a possibility must depend upon the fact that the zygote is composed of substances or structures derived from both the parent organisms.

Of course the only substance of the zygote which is derived in equal or approximately equal parts from the two parents is the chromatic portion of its nucleus, and it is frequently said that therefore it must be the nuclear structures of the germ cells which are involved in this fact of equal biparental inheritance. And yet the fact should not be disregarded that the sperm does bring mto the egg a certain though indeed a small amount of cytoplasm. The fact that the individual parental characters are inherited equally does not necessarily mean that all nonindividual characteristics are thus inherited, for all of the more general species characters are common to both parents, and the offspring might conceivably inherit these wholly from either parent. From this point of view the contribution by the ovum of practically the whole of the cytoplasm of the zygote might have at least two meanings. It might mean that the general species characters of the offspring are determined by the structure of the cytoplasm, and only the individual traits by the nuclear structures; it would follow from this that the spermatozoon takes a relatively subsidiary part in heredity. Or it might mean that the two gametic nuclei are from the beginning equally involved in the determination or direction of development, while the cytoplasm of the ovum merely affords the great bulk of the material basis for this development, and is itself in no wise involved in the qualitative determination of either* specific or individual characters of the offspring. It would follow from this that the two germ cells are of more nearly equal importance in the process of heredity.

We may finally conclude from all of the foregoing discussion that little can be very definitely asserted regarding the real function of fertilization now, and still less regarding the original significance of the process. Some of the questions involved here are to-day the most interesting and important of the unsolved problems in the fields of Embryology and Biology.

It is reasonably clear that fertilization is not, at the present time, a simple process, although it may have been so originally. Doubtless there has been an evolution both of the process and of the consequences of fertilization, just as there has been of all organ structure and organ physiology. Furthermore, it seems clear that the various possibilities described above, as to the significance of fertilization are not mutually exclusive: fertilization may be important for several of these reasons, even in a single case, and probably it has no one meaning that is exclusively true. It is quite possible that normally, among the Metazoa to-day, the spermatozoon may bring about in the ovum the formation of centrosomes which do, as a matter of fact, take an important part in the succeeding cleavages of the zygote, it may also chemically and physically stimulate the ovum to develop by bringing about initial changes in its chemical or physical structure or organization, it may at the same time introduce substances, the effect of which is "rejuvenation" of the specific protoplasm apart from the reproductive phenomena, and finally the structure of the spermatozoon which does these things may also affect the course of development so that individual characteristics of the male parent, as well as of the female parent, may appear. And to say that the result of fertilization is, for example, rejuvenation, need not mean that it is not also a stimulus to reproduction, a controlling factor in variation, and a means of heredity.

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WooDRUFP, L. L., The Life Cycle of Paramecium when subjected to a Varied Environment. Amer. Nat. 42. 1908. Two Thousand Generations of ParamcBaww. Arch. Protist. 2L 1911. A Summary of the Results of certain Physiological Studies on a Pedigreed Race of ParamcBcium, Biochem. Bull. 1. 1912.

Yatsu, N., The Formation of Centrosomes in Enucleated Egg-Fragments. Jour. Exp. Zool. 2. 1905.

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

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