A textbook of general embryology (1913) 3

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I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

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

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

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Chapter III. The Germ Cells And Their Formation

The reproductive elements of the Metazoa are single cells, often greatly specialized in form, and always highly differentiated in internal structure and in function. They differ from the reproductive cells of several groups of the Metaphyta, in that they cannot function, i.e., develop, until two single cells, usually derived from two different individuals, shall have met and fused, or conjugated. In many plants single reproductive cells (spores) are formed which develop directly without any such conjugation, and which are therefore to be distinguished from the true germ cells, or gametes, which develop only after conjugation. We shall describe the process of conjugation, or fertilization, in a following chapter, but in order to appreciate the significance of many of the details of germ-cell form and structure, we must remember that they are adapted toward ensuring the conjugation of two unlike cells, egg and spermatozoon.

Reproductive cells are set apart from vegetative cells in many of the colonial Protozoa. In some cases they are distinguishable only at certain times, when cells usually vegetative may give up such characteristics and become reproductive; in others the reproductive and vegetative cells remain permanently distinct though only slightly differentiated structurally. Finally in a few colonial Protozoa the reproductive cells are considerably modified from the vegetative condition, and in form and composition, as well as in function and behavior, are readily distinguished as germ cells. Many of the details regarding these cells have already been mentioned, others can be more conveniently and more significantly considered later, in connection with the process of fertilization (Chapter V) . In this chapter we shall first describe the form and structure of the typical germ cells of the Metazoa, as they appear when fully formed and ready to function, that is, just prior to fertilization. We shall then mention some of the more important modifications of structure shown in different groups, and finally give a brief account of the formation and history of the germ cells up to the time when they are ready actually to enter upon the real process of development. The details of certain phases in the history of the germ-cell nuclei, namely, the maturation processes, are of siich importance that we shall refer to them only briefly in this chapter and devote that following to a more extended account of these events.

Among the Metazoa the fully formed germ cells are always of two very unlike types, the ova or eggs and the spermatozoa or sperm cells. These cells are alike only with respect to their nuclear structure and composition. Their form differences are associated with fundamental differences in function. The egg cell contains by far the greater share of the substance which is to form the material basis of the new individual. The sperm, on the other hand, contributes little substance, and that chiefly nuclear, to the new individual. One of its important functions seems to be that of ensuring to the comparatively passive egg, a stimulus which leads it to react, i.e., to commence development; and the sperm's nuclear configuration, with that of the egg, together appear to determine the course of development to a large extent, if not wholly.

The substance of which the ovum is composed is not a homogeneous protoplasm. The cytoplasm is differentiated and organized into a definite structural and chemical (energetic) configuration. The details of this configuration are uniform in the eggs of each animal kind, i.e., it is specific. This cytoplasmic structure of the ovum, although itself apparently determined primarily by nuclear activity, is of great importance in maintaining the continuity and uniformity of organismal characteristics through successive generations (heredity).

The ova are less modified in external form than the spermatozoa, and often approach the form of a typical cell, except that they are nearly always larger than ordinary cells (Fig. 42). The size of the ovum is not related to the size of the organism producing it, but is in general related to the amount of food substance stored in it, the actually hving protoplasm showing much less variation in amount than the deutoplasm. The smallest eggs are those of the Mammals (Figs. 43, 14, VII), in man only 0.25 mm. (250 micra or 0.01 inch) in diameter, others being still smaller — 0.07-0.10 mm. (70-100 micra) in the deer and only 0.065 mm. (65 micro) in the mouse. The largest eggs are, in volume, the largest known cells; such are the "yolks" of birds' eggs, the largest of which are several inches in diameter and equalled in other groups only by the eggs of one of the sharks (Heterodontus) which are nearly 2 inches (4.0 to 5.0 cm.) in diameter. In a very few cases (some Ccelenterates and Porifera, and a few worms) the ova may be capable of locomotion, performing amoeboid movements (Fig. 44), but in nearly all cases they are quiescent, passive structures although containing a great deal of potential energy which becomes kinetic after fertilization, during the early stages of development.



Fig. 42. — A. Section tbrough the egg of the lamprey, POromyton fiwialilU. After Eerfort. B. SpermatoioGn, drawn to scale. d.en., dense endoplaam; i.ni.. inner membrane (7 vitelline); o.m., outer membrane (T chorion) ; p, granular "polar plaami" t.en., vacuolated endoplasmi v.ex,, vacuolated eioplaBm; /, first polar body; II, seoond polar epindle.


Fig. 43. — Fully erown human oftoyte freshly removed from the ovary. Outside the oocyte are the clear zona pellucida and the follicular epithelium (corona radiata). The ceDtral part of the odcyte containa deutoplasmic bodies and the eccentric nucleus (genniaal vesicle) ; superficially is a well marked exoplasmic or cortical layer. From Waldeyer-Hectwig.


Fig. 44. — Fully grown egg of Hydra viridis, containing nuclei of ingested ceUs. gv, nucleus of egg. The egg is amoeboid. From Waldeyer-Hectwig, after Kleinenberg.


Upon examining the internal structure of the egg we find that the nucleus is unusually large in most cases, spherical or ovoid, and with or without a nuclear membrane (Figs. 43, 45). It is located centrally or eccentrically, usually the latter if the egg contains an appreciable amount of food material. The chromatic network of the nucleus may be either dense, or so open as to give, even after staining, the appearance of a hghter


Fig. 45. — Axial section through the odcyte of the Annulate, ffereia. Aft«t Ijilie, slightly modified, c, cortical protoplasmic layer (eioplastn); n, nucleus; no, nucIeoluH; o, oil vacuoles; i, vitelline mambrane; u. yolk bodiesarea in the cytoplasm (Figs. 43, 45), often known as the germinal wsich. In some eggs the nucleus is in the process of division at the time of egg-deposition (Figs. 42, 46); this condition will be explained later. The actual morphological composition of the nucleus is one of the chief characteristics of the e^, and this too is reserved for consideration in Chapter IV. There is commonly a large plasmosome or nucleolus of varying form and size (Fi^. 43, 45). Centrosome and centrosphere are typically present at this time, though often minute and difficult to observe; later these structures disappear entirely. Frequently the 'cytoplasm contains unusual bodies termed "yolknuelei." The term yolk-^udeus includes organs of several different types, in some way related apparently to the formation


and deposition of the yolk and certain other substances within the egg cytoplasm. Sometimes the cytoplasm appears homogeneous, in other cases it shows considerable differentiation. Often the peripheral layw of the cytoplasm is more vacuolated and less granular than the central portion; the former is then spoken of as exoplasm or as the cortical layer, the latter as endophsm (Figs. 42, 43, 45, 46). There may also be various materials in the cytoplasm which have been laid down during the formation of the egg, under the influence of the nucleus, or yolk-nucleus, and deposited in different regions (Figs. 42-48).


Fig, 46. — Section through the ovariaa egg (o6cyte) of Amphioxus. After Sobotta. X 535. c, vacuolated cortical layer (eioplaam); e. endoplBsm oontaioing deutoplastnic bodies; v, vitelline membiane; J, first polar body; IJ, ■ecoud polar spindle.

The extent of the cytoplasmic differentiation varies greatly in eggs of different species. In many forms it can hardly be demonstrated in the egg at the time it is fully formed; in such eggs this differentiation appears later, during or after the processes of fertilization, or even still later, during cleavage. We shall have to return to this subject in connection with the subject of cleavage, after we have described the fertilization process. But there are two or three fundamental aspects of this fact that should be mentioned here. There are quite commonly three, sometimes more, distinct forms of cytoplasmic material arranged as definite regions of the ovum, occasionally as zones, or layers, or as localized masses (Figs. 42, 45). These may be distinguished by the more or le§s vacuolated character of the protoplasm, or by the collection of varioua pigments and differently colored granules, or by forms of deutoplasmic materials otha* than yolk, or in various other ways. The disposition of these substances usually expresses, incompletely, however, an underlying organization or morphology of the egg substance as a whole, which is considered a fundamental structure of the egg as a specific organism. This organization is practically always polar, i.e., disposed symmetrically with reference to one chief axis (Von Baer), and in the eggs of most bilateral ' animals examined, it is bilateral also (Roux, Van Beneden). In some way this morphology of the egg is . related to the morphology of the embryo developed from the egg, and hence is called its promorphology. ,,

This promorphology is better termed organization, for it is not only grossly material, but also dynamic, i.e., energetic, depending upon chemical and physical arrangements not

Fig. 47.-SBmidiapammatio often visible directly. The extent drawing of a mediui section

, . t J.-L- • i- through the fertiliied egg ot the

and nature of this organization are ay, Mu>ca. From Korecheit

often obscure, but this, and the and Heider, after HenkiDB and

Blochmann. en, chonon; d,

nuclear structure of the ovum, are flattened doraal side of the egg;

probably its most important chax- f^S"«""K.'":£jS

aCteristicS, for together these deter- truded through the micropyle;

■. J 1 i *' cortical layer; m, micropyle; mme the course OI its development this also marks the anterior find

as a specific creature. °' the egg; p, egg and «petm

«« a ojjvv I, I. VOIU.V. ^ pronuclei in prooeBS of fusion; r,

Polarity is one expression of this polar bodies; n, ventral side of

organization. The polarity of the '^'

fully formed ovum is related to the polarity of the egg cell

as it was placed in the epithelium of the gonad, the chief egg axis corresponding with the axis passing through the attached and free surfaces of the epithelial cell. This polarity apparently determines the primary position of the egg nucleus and centrosomes, and thus secondarily determines also the arrangement of the cytoplasmic substances which develop through the interactions between the nucleus and cytoplasm, processes which may frequently be observed during the growth period of the ovum. The two poles of the egg are conunonly unlike (Figs. 42, 48), so that we distinguish an animal and a vegetal pole, corresponding in most cases with the originally free and attached surfaces respectively, although this relation may occasionally be reversed (Echinoderms). In general the animal pole is that toward which the nucleus is eccentrically displaced, and nearer which the centrosome, or similar body, is located; it is the more protoplasmic, and therefore the more active region of the egg. The vegetal pole is frequently occupied largely by the relatively inert food substance, the materials in general related with the vegetative organs of the developing embryo.

As regards the nature of the organization, or promorphological relations, of the egg, two views have been taken and will be discussed in Chapter VII. The first is that the differentiated substances visible within the cytoplasm are genuinely "organ-forming substances," or at any rate tissue forming, in their potentiality. Thus they represent the organs of the embryo in an intracellular form. The second view is that these substances are only secondarily related to the real morphology of the embryo, and that both embryonic structure and the differentiated substances of the egg, are the result of an underljdng, invisible, and as yet little-known organization of the ground substance of the egg cytoplasm. According to this view the correspondence between the "organforming substances" of the ovum, and the organs and tissues of the embryo, is not in itself direct, but results from their common relation to the primary underl3dng arrangement or organization of the substance of the egg. In some cases the arrangement and position of these substances may be considerably altered experimentally without disturbing the normal course of development. It should be added, however, that in some other cases such a disarrangement does effect a corresponding disarrangement of the organs or tissues of the embryo.


In addition to the formative substances mentioned above eggs may contain varying amounts of nutritive substance of many different kinds, collectively termed yolk or deutoplasm. The yolk may be in the form of granules, small spherical bodies, large plates, fluid drops of various sizes, or in compact masses (Figs. 45, 48). These substances may be of different chemical compositions and staining reactions in a single egg. They may be formed within the egg by its own activity, or they may be contributed indirectly by cells associated with the egg during its formation. The arrangement of the food substances in the egg has an important bearing upon its later development, especially upon the form of its cleavage (Balfour). Eggs in which the yolk is distributed quite uniformly through the cytoplasm, and in which the protoplasm is therefore more or less completely intermingled with the yolk granules, or plates, are termed homolecithal or isoledthal eggs. Some eggs have been described as dLedthol, i.e., without yolk, but many of these have been found really to contain a small amount of quite uniformly distributed deutoplasm, and a truly alecithal egg is rarely if ever found. Eggs of some species among nearly all the large groups are of this homolecithal type, for example, the star-fish, sea-urchin, and also the Mammals, which were formerly- thought to be alecithal (Fig. 43). More frequently the yolk and cytoplasm are not uniformly mingled but are chiefly accumulated in different parts of the cell. Ordinarily these materials occupy opposite poles of the egg so that this retains a radial or rotatorial symmetry; the yolk is accumulated towaxd the vegetative pole, the protoplasm toward the animal pole (Fig. 48). Such eggs are termed telolecithal. They show great variation in the relative amount of yolk contained. On the one hand it is often difficult to distinguish the telolecithal egg from the homolecithal type, for the tendency toward polar accumulation of the yolk may be very slight. The egg of Amphioxus illustrates such a transitional condition. At the^ opposite extreme we find eggs such as those of the Reptiles and Birds, which are relatively immense cells, in which it is difficult; to distinguish, before development begins, any definite region which ia entirely free from yolk. Between theae extremes all intennediate conditions are found. This telolecithal type of egg is very common among the Invertebrates, and is characteristic of all the Craniata except the true Mammals. Among the Chordata successive stages in the accumulation of yolk are represented by Amphioxus, Lampreys, Ganoids, Dipnoans, Amphibians, Reptiles, and Birds. In the last two groups the protoplasm is extremely limited in amount, and is found only as a small disc or layer on the surface of the spherical yolkmass, at the animal pole. A third and less common arrangement of yolk is that seen in the cenirolecithal eggs of many Arthropods, chiefly Insects. Here the yolk occupies a greater or lesser portion of the center of the egg while the protoplasm forms a superficial layer all around it (Figs. 47, 117, 118).


Fig. 48. — Egg of the Teleost, Fundvliu htterotiilut. Toted view, about an hour after fertilization, e. chorion; d, protopIaBmic germ diac or blaatodiao; o. oil vacuoles; p, perivitelliDe apace; v, viteUiae membraDe; n, yolk.


The various constituent materials of the egg differ, often very considerably, in density, and since they are definitely disposed with reference to the chief egg axis, the eggs tend to assume a definite position with respect to gravity when they are free to move. Usually, in such cases, the yolk is heavier than the protoplasm, and the animal pole is therefore directed upward; this position is reversed occasionally, particularly when the deutoplasm is in the form of oil drops, e.g., Nereis and most Teleosts.

In some forms the egg cells are naked, without cell coverings or membranes, as in many Ccelenterates and some Molluscs. Or the egg may be naked at first, but soon after becoming free from the parental body it may acquire a thin membrane over its surface, as inEchinoderms. Ordinarily the egg is surrounded by definite membranes of varying nature and origin. The primary egg membrane is the vitelline membrane, or true egg membrane. Typically this is a thin membrane secreted by the superficial protoplasm of the egg and closely applied to its surface (Figs. 42, 45, 46, 47). In most cases it is quite structiu-eless; sometimes it is thicker and may be perforated radially by minute canals or pores, when it is termed

Fig. 49,— SecUoa

ill J-- through the eagmero the zona radiata. Occasionally the vitellme branes of the Eiasmembrane may appear double, showing an fo^^^J.^'^^ inner zona radiata and an outer structureless ziegier, after Balfour.

1 /in: An ci-e, Folliculftc epithe layer (Figs. 49, 50). In such cases it is iium;rf,outBrpt>rtion possible that the membrane is not wholly "* ^^ vitelline mem . „. . , Tirana (zona pellu vitellme, or it may be the case more fre- dda); i/t, surface of quently that the pores originaJiy passed pottioTSt tl"' ^td' completely through the membrane and dis- "nf. membrane (zona api>eared from the outer portion of it upon contact with an external medium.

The vitelline membrane may envelop the egg completely, or there may be left a minute, funnel-shaped perforation through it at the point where the egg was attached or otha-wise especially related to the epithelium of the ovary. This aperture is the microfyle (Fig. 50), and, ae we shall see, this sometimes affords an aperture for the entrance of the sperm cell.

A secondary membrane called the chorion often suirounds the egg outside the vitelline (Figs. 42-50). This is a secretion formed while the egg is still contained within the ovary, from the cells there surrounding the egg, and its presence depends upon the arrangement of these ovarian cells in the form of a definite layer or epithelium surrounding the egg, termed the follicle. The chorion may be a thin flexible membrane, or a tough resistant shell, as in Insects and Teleosts. Penetrating the chorion there is nearly always a continuation of the micropyle. The formation of this results from the fa«t that one of the cells of the follicle usually acquires a very intimate relation with the ovum, through a fine pseudopodial process, so that at this point no membranes are laid down (Fig. 50). When the egg is fully formed and leaves the follicle, this process is withdrawn, leaving a funnel-shaped canal. In a few instances (some Insects) there are several micropylar perforations through the egg membranes. Such openings are to be regarded as specializations of the minute canals, mentioned above, which give the appearance of the zona radiata to the membranes.




Fig. 5O. — A. Animal pole of the egg of the Cephftlopod, Aroonauta. From Wilson, "Cell," after Ubhow. SuiroiindinB the egg is the chorionic membrane perforated by the funnel-flhaped micropyle, m. Beneath the mieropyle lies tho egg nucleus in the cortdcol protoplBsmic layer, p.b. polar bodies. B. C. Sections through the egg membranes and micropyle of the egg of the Teleosta Esox (S) {ovarian egg) and Pj/goeteue (C) (ovarian egg, 0.4 mm. in diameter). After Eigenmann. X 375. a, zona radiata externa; /, egg follicle; u, lona ladiats interna; tn, micropylar cell; p, egg protoplaiim ; pp, protoplasmic piocesBea; y, yolk in egg; z, loua radiata.


Finally there is a great variety of tertiary membranes formed by the walls of the oviducts, or by special glands in connection with the reproductive system. These are applied outside the chorion, or if this is absent, directly upon the vitelline membrane. These envelopes may be of slime or jelly of an albuminous character, fibrous, or shelly coverings of chitin, lime, or other substance. In forms depositing the eggs in the water this is sometimes a thick jelly, holding the eggs together in strings or masses, or serving to attach them, either singly or in masses to plants, sticks, or other solid objects {e.g., Amphibia). These tertiary membranes serve also, in special instances, as protection against drying, temperature changes, pressure, or mechanical injury, and against the attacks of food-seeking organisms, or infection by bacteria or other parasitic organisms. Frequently they are nutritive in character, as the albumin or " white " of the birds' eggs or the dense oily substance surrounding the eggs of the snails.

Eggs may possess none or all three of these classes of membranes; sometimes only primary and secondary, or primary and tertiary membranes are present. This usually depends upon the nature of the egg-laying habits, method and duration of development, and various other conditions.


Fig. 51. — Flagellate spermatozoon. A^B. Two views of human sperm cell. After Retzius. X 2000. C. Diagram of the structure of a generalized type of flagellate spermatozodn. After Meves. a» annulus; oc, anterior centrosome; ci/, axial filament; c, centrosomes (end knobs) ; e, protoplasmic envelope; ^, head; m, middle piece; mi, mitochondria; n, nucleus; n«, neck; j), perforatorium (acrosome) ; pc, posterior centrosome; «, spiral filament; /, tail piece; </", terminal filament.


The spermatozoa, when fully formed, bear little resemblance to ordinary cells, yet their individual history clearly shows them to be such. In a few forms, chiefly among the Crustacea, the sperms do resemble ordinary cells, and are often provided with long radiating processes, sometimes, though rarely, pseudopodiaJ. But by far the most conmion form is that known as the flagellate spermatozoon, found in all groups of animals from Protozoa to man. These are minute thread-like cells in which three general regions can usually be made out (Fig. 51). One end is enlarged forming the so-called head. This is in reality chiefly made up of the nucleus of the cell, and it stains densely with all nuclear dyes. The chromatin of the nucleus is solidly packed, and though the head of the sperm is much smaller than the egg nucleus, the two contain practically, perhaps precisely, equal amounts of chromatin. Just behind the head is a smaller middle piece which is the chief cytoplasmic portion of the cell; the cytoplasm is really continued as a very thin envelope over the head, at the anterior, end of which it is usually produced as a sharpened perforating or attaching organ called the acrosome or perforatorium. In some spermatozoa (e,g., some Mammalia) the head is connected with the middle piece by an intermediate section called the neck (Fig. 51). The middle piece is quite highly differentiated. It contains the centrosomal structures of the spermatozoon, and its center is occupied by the proximal portion of a kinoplasmic structure, the axial filament. Surrounding this are frequently one or more differentiated layers, and often a spirally wound thread; the whole is covered with a dense outer sheath. In some instances (toad) the centrosome is said to be included in the region of the head piece. Extending posteriorly from the middle piece is a long flxigellum or tail, in some species flattened and provided with a fin-like undulatory membrane (Fig. 52, L).


Fig. 62. — Vuious types of apermatoioa. A, B. The Teleost, Leuci$eii* (Ballowita). C, D. The bicds, PhyOojmeiite and TadorTia (B&lloniti). E, F. Two tonna of the Bpenn of the anail, Paludina (von Bruno). O. The Nematode, A.tcan$ (Van Beneden). H. The Annulate, Mj/tostoma (Wheeler). I. Tha but, Vttperuoo (Bftllowiti). J. The oposaum, Diddphya (Wilwin). K. Tho l«t (Wilson). L. The Utodele, Amphiuma (McGregor). M. The Cnutaoeui,


Through the middle of the tail is an axial filament connecting with the centrosome of the middle piece, a relation which is very common in flagellate or ciliate structures. Proximally the axial filament passes through a ring-like structure, the annvliLS, in the end of the middle piece. Distally the filament may continue beyond the cytoplasmic envelope of the tail as a terminal filament or end piece. There is the greatest variety in details of form of the sperm, affecting chiefly the form of the head and acrosome, length of tail, and total size; a few of the more conmion or more striking forms are illustrated in Fig. 52. (In cases where a neck region is distinguished, the middle piece is often regarded as the proximal part of the tail.)

The smallest spermatozoa are found in Amphioxus and are only 0.016-0.020 mm. (1&-20 micra) in length; the largest are found in some of the Amphibia where in SaUmiandra they are about 0.7 mm. (700 micra) in length, and in Discoglossus 2.0 mm. (2000 micra), the maximum length known. The sperm cells of the Crustacean Cypris, are also of this gigantic size (2 mm.). The spermatozoa of most of the Vertebrates are 25-75 micra in length. The human spermatozoon (Fig. 51) represents about the average size; the dimensions of this are ' (Krause): total length, 52-62 micra (1/400-1/500 inch); length of head, about 4.5 micra (between 1/5000 and 1/6000 inch) ; length of middle piece, about 6 micra (about 1 /4200 inch) ; length of tail piece, 41-52 micra (1/500 to 1/600 inch); width of head 2-3 micra (1/12000 to 1/8000 inch); thickness of head, (this is one of the few instances where the head is flattened), about 1 micron (1/25000 inch).

The number of sperm formed by a single individual is very large in most organisms and can be only roughly estimated. It has been computed (Lode) that the total number formed in man may average about three hundred and forty billion, or Ethusa (Grobben). N. The Crustacean, Intichu8 (Grobben)« O. The Crustacean iSida (Weismann) . P. The Crustacean, Bj/fAo^rep^es (Weismann). A;, end knob; m, middle piece; n, nucleus; p, perforatorium; u, undulatory membrane* Not drawn to same scale. A-F, I-K, from Wilson.


approximately eight hundred and fifty million sperm for each one of the four hundred ova matured during the reproductive period of the female. The volume of the human sperm is roughly only about 1/195000 that of the egg (egg =0.25 mm. in diameter). The sperm of the sea-urchin contains only about 1/400000 to 1/500000 the material in the egg (WUson).

In several forms, both Vertebrate and Invertebrate, atypical giant" spermatozoa are occasionally found (Fig. 52, E). In most instances these are abnormalities resulting from some •deviation from the usual course of events during sperm formation. But in a few instances {Euschistus) a dimorphism of the sperm seems entirely normal (Montgomery). In such cases the larger sperm heads contain the normal amount of chromatin, but an excessive amount of linin and karyolymph.

In marked contrast to the egg, the sperm cells are in very active movement on account of the rapid vibration of the tail. They are always contained within a fluid medium, the seminal fluid, which is either the fluid of the cavities of the body, or a special secretion of certain glands in connection with the reproductive system. In the latter case this fluid is equivalent to a tertiary egg envelope, and like this is sometimes of nutritive value to the germ cells.

The form diflferences between ova and sperm give a nice illustration of the modification of structure accompanying a physiological division of labor, which is so well marked here. Both cells contain equal amounts of nuclear substance, but the ovum possesses in addition a large amount of cytoplasm, and often a much larger amount of food substance, while the sperm contains an amount of cytoplasm which represents practically an irreducible minimum, aiid no deutoplasmic material whatever. The egg, therefore, provides practically the whole of the extranuclear substance of the developing organism. At the same time the ovum is a passive, non-motile structure. The spermatozoa, on the other hand, are not only extremely motile, but they are produced in very large numbers, conditions correlated with their function of finding the inactive ova and of ensuring the initial stimulus to activity (development) of the passive egg material. This differentiation affords the material basis for development while at the same time it ensm*es the fertilization of practically every egg produced, though the eggs and sperm may be shed freely into the water some distance apart, a distance often very great as compared with the size of the cells. The relatively very large amount of cjrtoplasm in the egg, and small amount in the sperm, constitute the most marked exceptions to the nucleo-cytoplasmic {kern-plasma) relation, mentioned in the preceding chapter; these conditions are entirely special and are to be regarded as adaptations to the very unusual functions of these cells.

The details concerning the form and number of the sperm and egg cells, the amount of yolk in the eggs, the character of their membranes, etc., are significant only from the viewpoint of adaptedness to the conditions under which they must function. This adaptedness of the reproductive phenomena toward ensuring the final bringing to maturity of a number of organisms sufficient to maintain the specific group in undiminished numbers is a general biological topic of especial interest. In strictness this lies outside our province, but to omit entirely any reference to this subject, leaves without significance many of the details of structure and behavior mentioned in the preceding paragraphs. We may therefore suggest briefly a few of these relations, not only as regards the germ cells, but also the general processes of spawning, etc., which are all concerned in finally bringing together an ovum and a spermatozoon.

All these varied, and often complex, phenomena of habit and morphological specialization of the reproductive cells are correlated with the special conditions of life which affect the chances that a single egg shall finally become a mature organism. They are conveniently grouped under three chief heads: (a) the ensurance of mating, (b) the ensurance of the actual meeting and fusion of the germ cells, (c) the chances of death before maturity, involving such factors, as abundance of food, enemies, adverse conditions in the inorganic surroundings, necessity for reaching special conditions of development, food, etc., duration of the period of development, and the like.

A few forms, especially in the warmer climates, appear to breed quite continuously throughout the year (many Coelenterates, MoUusca, etc.), but commonly the germ cells are produced at regular periods, which Inay have a duration of only a few days or hours, or they may extend over several months. Breeding or spawning periods are nearly always seasonal and usually annual, but a few forms, particularly the Mammals, breed or spawn at shorter intervals. In many Mammals it is true that there is only a single annual breeding season or period of cestrus; this condition, known as moncestrous, is characteristic of most Carnivora and is found also in the Chiroptera and Marsupials. Others, however, are polycestrous, and exhibit two or three annual breeding seasons (Insectivors), and in still others the period of oestrus may occur at intervals of a few weeks (man), or it may be quite continuous, as in most Rodents and some Carnivors. (See Marshall, Physiology of Reproduction, 1910.)

Among the higher animals the breeding season is often preceded by a "nuptial season" during which, especially among the males, there may develop various special morphological and physiological peculiarities. The Fishes, Birds, and Mammals exhibit the frequent development of special external markings or colorings, special secretions, and unusual modes of behavior. Both these and the breeding habits proper, are to be regarded as responses to stimuli, frequently climatic in origin^ resulting from changes in temperature, light, moisture, food characters, etc. These phenomena are commonly regarded as indications of an increased metabolism that affects not only the organs of reproduction, but secondarily the whole body.

In a few rare instances, chiefly among the segmented worms, the annual spawning season is very definitely fixed and varies within limits of only a few calendar days. More usually the time of spawning is subject to wide variation and is dependent upon temperature and other seasonal conditions. The species of the Palolo worm afford one of the best marked instances of a fixed spawning season. It is not quite aa regular and limited as tradition would have it, but in the Tortugas, most of the individuals of the Atlantic Palolo {Eunice fucata) swarm and spawn during one or two mornings which fall within three days of the moon's last quarter between the latter part of June and the end of July (Mayer) . The Pacific species {E, mridis) spawns similarly on and near the last quarter in October and November. Somewhat similar relations have been determined for other Annulata, such as Amphitrite (Scott) and Cerdtocephals, The determining factor in these and similar cases seems to be the character of the tides, combined with factors of temperature and light. Other organisms spawn at a definite time of day, individuals coming to maturity at any time during a longer breeding season. Thus Amphioxus and some Hydroids spawn only about sun down or shortly thereafter.

During the intervals between the breeding periods the formation of the germ cells may almost or quite cease, to recommence shortly prior to the next period. In some creatures, however, the eggs are formed continuously and are stored in secondary reproductive cavities pending the time of their production. This is more likely to be the case in sperm production. But in all such cases the rate of formation of the germ cells is rhythmic, increasing just before the breeding period.

The sperm cells are always passed outside the body of the organism forming them (save in self-fertilizing hermaphrodites); the eggs may or may not be thus extruded. Animals used to be described (even classified) as "oviparous" or "viviparous," according to whether the female extruded undeveloped eggs or living "young," but these terms have now lost all precise meaning, for m any case eggs are formed, and in different species the developing organisms may leave the body or reproductive cavity of the parent at almost any stage.

The unfertilized eggs may be simply thrown outside the body of the female, as in most aquatic animals, the sperm being thrown out at the same time and in approximately the same place ; in such cases fertilization is ensured chiefly by the production of immense numbers of spermatozoa. Such a process is very common among the Sponges,* Coelenterates, Echinoderms, Annulata, Mollusca, Fishes, and many Amphibia. Eggs thus thrown off into the water may float at or near the surface, aa pelagic eggs, or they may sink to the bottom among the debris {demersal) . Or the extruded eggs may be deposited with reference to definite and often very special conditions affording, to the. new organisms, protection, food, etc. Among land animals which deposit the eggs outside the body, these are usually very definitely placed with reference to such conditions ; the Insects afford a great variety of excellent illustrations of relations of this kind. In some cases, among both aquatic and terrestrial forms, definite "nests" are constructed in which the eggs are deposited, and where the newly hatched organisms may remain for some time. The eggs and young then may or may not be guarded or fed, by either or both of the parents. The nests may vary from simple depressions or pockets in the mud or sand, like those of many fresh water Fishes, to the structures of very complex architecture of many Birds.

Among the forms which do not liberate their eggs at an early stage in their development, there is a great variety of habit. In some Crustacea and Amphibia, for example, the eggs are first extruded, but are immediately placed upon the surface of the body, of either the male or female parent, and develop there. Or they may become embedded in the skin (many Amphibia) or may be deposited in some cavity not primarily a reproductive cavity, such as the pharyngeal cavity, in some of the Siluroid and Cichlid Fishes. In one of the Cyprinoid Fishes {Rhodeus) the eggs are placed in the mantle cavity of a clam, where they are fertilized and develop on the gills. In most cases where the eggs are retained in "brood cavities," these are modified portions of some part of the reproductive system proper; here the eggs may remain until a comparatively late period in their development. In such cases fertilization must be internal, and the sperm are then definitely introduced into some reproductive cavity of the female.

An interesting series of relations may be traced illustrating the gradual increaae in the certiunty with which fertilization shall be accom


Fig. 63. — Forms of aperm&tophor^B. A, B, C. The Insects, Larieera, Loeugla, &nd an Ichueumoaid. From Koi^chelt and Heider, after GiUoD. D, E. The Urodeles, AmUysloma and Diimyctylus. From Bertram Smith. X 3. F. Feripatui edaardrii (incompletety developed). After von Kennel (Korachelt and Heidai).

plished, whether it be external or strictly internal. First, among many of the Crustacea, Annulata, Gasteropoda, Cephalopoda, and Amphibia, there is a process of pseudo-copulation or ampisxus, where, sometimes after a complicated "courtship" (Urodeles, Jordan), the sperm are received by the female in or near the reproductive cavities or openings. Usually in such instances the sperm are not scattered freely, but are contained witliin definite packets or cases called spermalophores (Fig. 53). In the TJrodeles these are simple masses of spermatozoa enclosed in a thin envelope; they are discharged by the male and then picked up by the cloacal margins of the female and stored until the eggs are ready to be fertilized. In the amplexus of the earthworm and many Gasteropods, there is a mutual exchange of spermatozoa between two hermaphroditic individuals, the sperm being received into storage cavities and retiuned until the eggs are deposited. Such a receipt of sperm or sperm packets into storage cavities is quite common, and the sperm may in these cases remain alive for long periods, even for three or four years (honey bee, some snails). The spermatophores are sometimes very elaborate affairs conttuning a complex mechanism arrai^ed so as to discharge the sperm just at the time the female is depositii^ the eggs (Fig. 53, C). Among the more complex are those formed by the male squid {Loligo) and transferred to the buccal membrane of the female, where they remain attached pending the time of spawning (Drew).

Among forms where internal fertilization is the rule, this is more frequently the result of a definite act of copulation, by which the sperm are transferred directly to a reproductive cavity of the female through a male intromittent organ. This occurs in most Insects, many Turbellaria, Crustacea, Molluscs, and in most of the higher Vertebrates. This general relation, carried to an extreme may result in symbiosis, or even in the parasitic character of the male upon or within the body of the female. Thus in some of the Cirripedia, degenerate " complemental " males are found living semiparasitically within the body of the female. Several of the Trematoda live in pturs within a single cyst. In Bilkarzia (Trematoda) the female lives permanently in a groove on the body of the male (Fritsch) (Fig. 54). In Diplozodn (Trematoda) two hermaphrodite individuals, at first entirely separate, become permanently fused so that the openings of the reproductive ducts are in apposition. In Syngamus (Nemathelminthes) also, the male lives permanently attached to the female. The climax of this relation is afforded by Trichosomum (Nemathelminthes) where several (2-5) dwarfed males live within the uterus of the female (Leuckart). Or it might be said that the climax is to be seen in the cases of self-fertilizing hermaphrodites; these are usually internal parasites, where the chances of the meeting of males and females would be practically nil (e.g., many of the Trematodes and Cestodes).

The retention of the eggs during their development, within a brood ca^^ty is primarily a protective arrangement, but it often leads to the establishment of an organic nutritive relation between the embryos and the wall of the cavity. This is the case in some Tunicates, Elasmobranchs, etc.y and of course it reaches its climax in the intimate and extensive organic relation between embryo and oviducal (uterine) wall among the Eutherian Mammalia.

The number of eggs produced during each reproductive or spawning period varies enormously, and is related to a variety of conditions of development. In general the number of eggs is larger when there are few or no other means of ensuring the complete development of a number of organisms sufficient to maintain the species numerically. Any structure of the egg or habit of deposition, adapted to ensure development, is Ukely to be associated with a reduction in the number of eggs formed. The number is largest among forms which discharge the eggs at random or where they are subject to unfavorable external conditions, to liability to the attacks of parasites, or use as food by other organisms. For example, the marine fishes produce very large numbers of pelagic ova, the codfish is said to form 8 to 10 millions in one season ; and in a species of sea-urchin {Echinus) a single female is said to have discharged, in a single season, as many as 20 million ova. Where very special conditions of development are essential, as in the complicated life histories of many internally parasitic worms, the number of eggs is very large and fertilization is ensured by hermaphroditism.

The number of eggs is smallest where they develop within a brood cavity, or where some degree of parental care is exercised. In a few Mammals and Birds only a single egg is formed at each breeding period, and in these groups the number rarely exceeds eight or ten. Further the number of eggs produced, in general varies inversely with the amount of food substance contained, or with the chances of the young Ending food for themselves by the time they become free living.

The number of sperm cells formed is always larger than the number of eggs, and often reaches many millions. The number is likely to be smaller among forms in which the sperm are directly or indirectly introduced into the reproductive cavities of the female. Some of the Crustacea afford interesting illustrations of this. In some Ostracods only a few hundred very active spermatozoa are formed; these are inserted directly into the seminal receptacles of the female. They are most remarkable for their size — 2 mm. in length in a few, or more than twice as long as the body of the male. In Daphnia the number of sperm may be only twenty, or even less, six to eight in some species. These too are liberated directly into the brood cavity of the female, which forms only two eggs at a time; these sperm are very adherent, and are said to be somewhat amoeboid. Indeed the spermatozoa of many of the Crustacea are unusually interesting on account of their atypical form and behavior (Fig. 52). Some {Bythotrephes) are quite like ova, large (O.I mm.), rounded, and quiescent, depending upon a pecuhar viscid or adherent quality for their likelihood of attachment to the egg. Others (some Decapods) have a number of stiff radiating processes which seem to function by catching in the hair-like bristles surrounding the openings of the oviducts, where they are placed in amplexus.

The amount of food yolk contained in the egg is related also to the duration of the embryonic period of development, or to the rate of development, a prolonged embryonic life requiring an abundant supply of food materials, and an unusually rapid rate of development depending upon a supply of easily assimilable nutritive substance. Such a relation is illustrated by the difference between summer and winter eggs, formed by Rotifers, and many Insects and Crustacea; the winter eggs, subject to unfavorable conditions and passing a longer period in development, contain considerably more yolk, and are covered with much tougher and more resistant membranes, than the summer eggs which develop rapidly and under favorable surroundings, indeed often within the brood cavity of the female. Thus in Daphnia the small summer eggs are formed by only three nurse cells (see below), while the large winter eggs are supplied with food by forty or more nurse cells . When the developing embryo acquires special nutritive relations with the parental tissues, the eggs are of course practically yolkless.

The provision of egg membranes is associated not only with a reduction in the number of eggs formed, but also with the duration of the embryonic period, liability to unfavorable external conditions, prevalence of food-seeking enemies, etc. The membranes which are functional under such conditions are of the secondary and tertiary classes described above. The nature of these varies from the common thin fibrous coverings, to tough and impervious membranes capable of resisting extreme dryness, or the leathery or calcareous " shells " of the Sauropsids. Among the most complex and perfectly adapted membranes or shells, are the egg cases of many Elasmobranchs, and particularly those of the Holocephali (Dean) (Fig. 55), which often remain intact and functional, in their passive way, for more than a year. In some cases the egg membranes may have a nutritive value and may augment or replace the food supply in the form of yolk; the eggs of birds and snails are good illustrations of this relation.

We must now trace briefly the steps leading to the formation of the ova and spermatozoa as the highly specialized cells we have described. In the lowest Metazoa, Sponges and some Hydroids, the germ cells are scattered through the tissues of the organism as separate, free cells, which may migrate from place to place, feeding and growing, often at the expense of the other cells (Fig. 56). But in other Hydroids, and in all forms above these, the germ cells are localized in a definite reproductive tissue and organ, or series of organs, the gonads — ovaries and testes. The simplest gonads are merely masses of rapidly proliferating cells (Fig. 57), usually bordering a cavity which is the ccelom, and which is supposed to be primarily this reproductive cavity simply. In most of the higher forms the ccelom comes to have many secondary relations, and forma in addition to the reproductive and other smaller cavities, the very extensive body cavity. In the embryos of the Craniates there is a pair of longitudinal ridges, either side of the attachment of the ^mfeof thTnoft^ dorsal mesentery, through a considerable ex- cephaian, Chimara


tent of the body cavity; these are the genital ^^^""rwo-tbirds




ridges, and the peritoneum covering these becomes thickened by the enlargement and proliferation of the cells (Fig. 58). These are the rudiments of the gonads. The cells composing these rudiments are often of two kinds. Some of them, indifferent cells composing in gen



Fig. 56. — Origin of tha germ cells in the Hydro-inediisa. Cladimema, From Wilson, "Cell," after Weiamann, A. Youqb stage: section throunb the wall of the manubrium. Ova developing in the ectodenn, ec; en, endoderm. B. Older stage, showing Ova, o, and nutritive cells, n. The ova oontain small nuclei prob. ably derived ft«m ingested nutritive cells.



Fig. 67. — Diagram ot the structure of the developing ovary of the Annulate, AmpkitrUe rubra. From Korschelt and Haider, after E. Meyer, g.dr., rudiment of ovary; g.e.. germinal epithelium; 0.Z., fully fonnod ova, scattering; pm., peritoneum; V.t,, vaa ventrale.



era] the frame-work of the gonad, have been formed in situ from the proliferating peritoneal cells. Others, the primordial germ ceUs, are often first distinguishable In some other region of the developing embryo (Fig. 59, A); they then make their way into this germinal epithelium as development proceeds.

The gonad may consist almost entirely of the reproductive cells proper, and may be then a more or less periodic structure, almost if not quite disappearing between the periods of reproductive activity. Or it may be a permanent organ of considerable though varying size, consisting of a complex stroma of a variety of cells — nutritive, vascular, nervous, connective tissue, and other cells, in addition to the true germ cells. When highly developed the gonad may also contain various cavities, of ccelomic character, into which the germ cells are passed when ripe. Thence they may pass directly into the body cavity from which exit is made to the outside through simple perforations in the body w^l or through special tubes or ducts. In the testes these ducts may lead directly to the outside from the cavities of the gonads. The structure of the gonad can nearly always be reduced to that of a complexly folded epithelium the essential elements in which are the germ cells; the ccelomic surface is the free surface of the germinal epitheUum. This relation becomes important in describing ttie fundamental morphology of the germ cell.



Fig. 58. — X. Part of a section through the body of a youDE lizard. Lacerta off^it, ahowing the genital ridges and aaaoclated stnicturea. B. Genital ridge, enlarged, showing young follicles containing oveu From Korschelt and Heider, after Braun. ao, dorsal aorta; i, cardinal vein; me, mesentery.


It is a question whether the germ cells are to be considered as originally undifferentiated cells, which become modified during the life of the organism for the reproductive function, or whether they are set apart from the beginning of the organism's multicellular existence as reproductive cells, and become visibly modified only in later stages. In those few forms where the germ cells are diffused it seems that any tissue cell which is not completely specialized in some other direction may assume reproductive characters. In forms which develop special gonads, however, there are many reasons for believing that the genn cells are always to be distii^uished as such very early in the history of the individual organism. In Ascaris the history of the primordial germ cells has been traced back to one of the two cells resulting from the very first division of the fertilized ovum; not all of the descendants of this cell are germinal how ever (Fig. 32). In some of the bony fishes germinal cells are recognizable in the fifth cell generation, i,e.j in the thirty-two cell stage. And in many other forms, including some of the Mammalia, the germinal cells can be distinguished from the somatic cells very early, even in the blastula (Fig. 59). This may indicate that, although not visibly distinct, the germ tissue is, after all, in reality distinct from the somatic, in most if not in all forms. It would seem more consistent with the present conception of development, however, to say that this distinction exists only potentially and comes about as a real differentiation in the developing organism, for however early this differentiation may occur, a stage is always found where germinal and somatic substances are contained undifferentiated within a single cell and are then indistinguishable.



Fig. 6fl. — A. BectioD through ftn early embryo of the Teleoat, Aficromelruj aiTpreiHrfua, showiog the distinct germ cells. After Eigenmann. ee, ectoderm; TO, eododerm; g, germ ceils; so, somatic layer of mesoderm; sp, Bplanehnic layer of mesoderm. B. Section through forty-cell stage of the Cruatacean. Cj/clopg brema/mig, ahowing, g. the ceil that ^vcb rise to the germ cells, en. the primitive endodetm cell in the process of its first division. Alter Hftcker.



The visible distinction between the gonads of different sexes may occur very eaxly. In some forms this distinction between sexes can be made out in the fertilized ovum. And in many forms the two kinds of gonads can be distinguished soon after they are first marked out, though there is reason even here for supposing that the distinction is really, though not visibly, present in the fertilized egg.

The processes involved in the later differentiation or histogenesis of the eggs and spermatozoa are collectively termed oogenesis and spermatogenesis respectively. They are conveniently divided for description into three periods or phases. These are (1) the period of cell multiplication, during which the simple epithelial cells, or primordial germ cells, divide more or less continuously, increasing the bulk of the gonad; (2) the period of growth, when cell division is less rapid or altogether inhibited, and the cells enlarge rapidly, the egg-forming cells much more considerably than those forming the spermatozoa; (3) the period of maturation, when the germ cell nuclei undergo profound modifications during their last two divisions as germ cells. Sometimes the terms oogenesis and spermatogenesis are used to indicate only the events of this third period, which are of such importance that we shall make them the subject of the next entire chapter. In the history of the spermatozoa a fourth period is to be distinguished, namely, the period of transforman Hon or metamorphosis; for the highly differentiated structure of the spermatozoon is rapidly assumed after the process of maturation is completed. This period is not marked in the history of the ovum, for this, with the exception of its imusuaJ size, is not so markedly differentiated in structure.

During the first of these periods, that of multiplication^ the cells of the reproductive tissue are termed odgonia and sper Primordial Germ

CeU CTrimitive

Ovum").


Period of Multiplication . « c h r o m osomes. (The number of cell genera- , . , ^ l^-v . tions ismuch _/ \ / \ ^Odgonia. greater than indicated here.)


Period of Growth. a chromosomea


Period of Maturation, i^ chromo- ( aomes.



Fig. 6U. — Diagram of the chief events of oogenesis.

Compare with Fig. 61.


Primary Odcyte.


Secondary Odcyte and First Polar Body.


Mature Ovum and Three Polar Bodies. Adapted from Boveii.


matogonia, and of these there may be a great many generations during this period, before growth commences. As the oogonia and spermatogonia become older, division becomes slower and ceases as the cells enter upon their growth period. At the close of the growth period, while still contained within the ovary or testis, the cells are known respectively as the primary oocytes, or ovarian eggs, and the primary spermatocytes. From this point onward the histories of the eggs and sperm are not quite identical, although entirely equivalent (Platner, 0. Hertwig).


As said above, the chief events concern the nuclear structure and we can only point out here that during the period of maturation two more cell divisions occur.

In the testis each primary spermatocyte divides once^ forming two cells called the secondary spermatocytes, and then each of these divides again, forming altogether four cells called the


Period of Multiplication. « chromo.somes. (The number of cell generations • much greater than is indicated here.) _ _ _

Spermatogonia.


Period of Growth. 8 chromosomes.


Period of Maturation. — chromo-. somes.


Period of Metamorphosis, -^chromo-' somes.



Primary Spermato< cyte.


Secondary Spermatocytes.


Spermatids.


Spermatozoa.


Fig. 61. — Diagram of the chief events of spermatogenesis. Adapted from Boveri.

spermatids. These are all alike and each becomes metamorphosed into a spermatozoon without further division. Thus are formed, from each primary spermatocyte, four similar spermatozoa. In the ovary, or sometimes after the primary oocyte has left the ovary, this too divides, but here the division is very unequal, resulting in the formation of one large cell, the secondary oocyte, and one very small cell, called the first polar body. Typically each of these then divides again. The secondary oocyte divides unequally as before, forming a large cell the mature ovum, and another small cell, the second polar body. Meanwhile the first polar body divides equally forming two similar polar bodies. In some cases the division of the first polar body is suppressed. Thus each primary oocyte typically gives rise, like the primary spermatocyte, to four cells, but these are not all alike in form and size, although they are fundamentally equivalent, i.e., homologous, to each other, and to the four spermatids. Of these four cells, however, only one, the ovum, is functional; the polar bodies degenerate without functioning. The parallel events of spermatogenesis and oogenesis are shown diagrammatically in Figs. 60-61. (See also Figs. 76, 90, 94.)


Fig. 62. — LoDgitudiual section through the ovary of the Copepod, CanihoeamptvM. From Wilson, "Cell," after H»eker. og. the younBest germ-cellB or oBgonia (dividing at og.'); a. upper part of the growth-Eoue; oc, oficyte,,ot gtowiag ovarian egg; m, fully formed egg, with double cbromfttin-Toda.

All these processes may be going on in the ovary or testis at the same time, occiuring progressively from the basement membrane of the germinal epithelium toward its free surface, so that a section through such an epithelium shows practically every step in the history of a single cell (Figs. 62, 68, 69). Before considering in detail the nuclear changes involved in the maturation processes we must consider the more important facts concerning the history of the cytoplasmic parts during this phase of the genesis of the germ cells.


Fig. 63. — Diagrams of the egg-tubea (ovaries) ot Insects. Aftei Korschelt and Heider. A. Orthoptera, without groups of nutritive cells. B. Coleoptera, with many such groups. C. Bemiptera, with terminal nutritive group and nutritive channdB extending to the ova. c nutritive channel; /, egg follicle; m, lone of multit^oatioa; n, nutritive cells; o, ova.


In the growth of the egg the chief aspects are those associated with nutritive relations of the developing ovum to the adjacent cells, especially in those forms whose eggs contain a considerable amount of yolk. In the non-localized ovary such as that of the Sponges and some Hydroids, the ovum grows at the expense of whatever cells happen to be adjacent to it (Fig. 56). In the common Hydra, as the ovum grows to be considerably larger than these tissue cells, it becomes amceboid and actually ingests these neighboring cells, digesting their substance and growing rapidly (Fig, 44). The nuclei of these ingested cells are relatively indigestible and remain for some time scattered through the egg cytoplasm. Among all those forms with definitely localized ovaries growth of the ova ia accomplished ' very difierently. When the eggs are small and contain relatively little food, no special nutritive mechanism is developed, the egg forming the food substances in its own cytoplasm from materials drawn from the circulating fluids in the cavities o the ovary. Such eggs develop independently of the neighboring cells intermingled with the ova. J'-JLT'^SS Wh™ tte fully formed eggs contain large part of the egg-tube amounts of food substance this is usually DytUcut marginaiu. obtained by one of two chief methods. In After Koracheit. n. ^]jg simplest cases Certain of the ovarian

groupB of nutntive y _

cbUb; o, ovum contain- cells adjoining the egg take on the characp^tiy"'»^^dund^*by teristics of nurse cells. These may either nutritive aubMance contribute their own substance directly to

(deutoplaBm). - ^ i

the ovum or they may become intensely active, forming deutoplasm which is then drawn from them by the growing ovum (Figs, 63, 64). There may be a single nurse cell for each ovum (Fig. 65), or the riiu^e cells may be scattered irregularly through the ovary so that several may be related to each ovum. The nurse cells in many or even in most cases, are cells which were potentially germ cells, but which have lost thdr germinal potentiahty and become wholly nutritive, contributing to the formation of the true germ cell, degenerating and disappearing completely after the ovum is grown and has left the ovary. In the other cases, which are conunonest among the Vertebrates, almost universal among them in fact, the ovarian cells adjoining the ovum —themselves potentially germ cells originally, form around the ovum a definite layer termed the follicle (Fig. 66). The follicle cells have the arrangement of an epithelium; they may form either a single layered, simple epithelium or in other cases s many layered stratified epithelium (Fig. 43). They not only provide for the nutrition of the enlarging ovum, with which they are frequently connected by definite intercellular protoplasmic strands, but toward the close of the growth period of the egg they may become secretory and form certain egg envelopes of the secondary type, t.e., chorionic. We have ateady men' tioned how the micropyle is formed through the chorion and vitelline membrane by the Losertion of one of these follicle cells with a long process, preventing the membranes from forming at that point. When the eggs are fully formed and ready to be laid the F,o. 67.-Seotion through the follicle luptures allowing the eggs egg oi the Tunicate, Cynihia xq escape freely. Often there de porttio. After ConkUa. In the periphery of the egg are the nuclei Velops m the folhcle a definite



Fig. 65. — Sections through ovanan ova and nurse cells in the Annulate. Ophryotrocha. From Wilson, "Cell." after Korschelt. A. Young slsKe, th» nurse cell, n, larger than the egg. B. Growth of the ovum, o. C. Late stage, the nurse cell degenerating.


Sion along which it bursts; this later history ace Fig, 91) . X 357. weakened region is called the

C eioplasm or cortical layer; n.

egg nucleus or germinal veaicle, ClCOinX {e.g., COmmOn fOwl). In

fonicle cells are actually taken into the egg and absorbed, much as in the case of those ova which ingest adjacent interstitial cells. This is the case in many of the Tunicata where some of the so-called "test-cells" lose their cell outlines and are directly taken into the cytoplasm of the egg; their nuclei remain distinct for some time (Fig, 67), The remaining test cells then form a distinct follicle outside the whole structure. These nuclei no longer function as nuclei of course; as the growth of the egg is completed they are extruded again, along with a portion of the superficial protoplasm forming then a thick vitelline membrane resembling a chorion and often so called.



Turning now to the formation of the sperm we find, as we should expect, processes on the whole entirely comparable with those of egg formation. The chief differences result from the fact that in the ovary conditions are associated with the formation of a few relatively large cells, while in the testis small cells are formed, but in very large numbers. In Sponges and Hydroids there is the same non-localized formation of the sperm as of the ova, the germ cells being distinguished not so much by position as by size. Apparently any ectoderm cell may enlarge and become reproductive. In all forms above these there are definite testes. Among many Ccelenterates and Echinoderms the testis is composed purely of germ cells, but usually the testis, like the ovary, contains other interstitial or accessory cells, and frequently these are directly nutritive in function. The general structure of the testis differs from that of the ovary in that its epithelium is thrown into folds, forming either simple columns or dciniy each with an efferent duct or pathway which is to be considered coelomic in origin. In the testicular epithelium, which is ordinarily reducible to the stratified type, we find sperm cells in all stages of formation, from spermatogonia to fully formed spermatozoa. As we have already said, the process of sperm formation is usually continuous, though frequently periodic (seasonal) in its rate or intensity. When the testis is composed of acini, each is usually surrounded by a follicle, equivalent in function to the egg follicle. But when arranged in lobules and columns, each of which may be derived from a single primordial cell or prespermatogonium, the nutritive cells do not show this follicular arrangement, but are fewer in number and scattered along the basement membrane of the epithelium, or along the connective tissue partitions bounding the spermatic columns of the lobule, and to some extent among the germ cells proper.

Among the Craniates the typical arrangement is that shown in Figs. 68, 69. Here, along the basement membrane, are several generations of spermatogonia with the scattered nutritive bdsal cells, sometimes called also the Sertoli cells, usually larger than the spermatogonia. As the spermatogonia increase in number, through continued mitoais, they begin to increase in size, though not nearly to the same extent that the oogonia do. There ia not always the same distinctness between the phases of multiplication and growth here, and the two final divisions of the full grown spermatogonial cell, then known as the primary spermatocyte, are the reducing or maturation divisions. These lead, as we have seen, to the formation, from each primary spermatocj'te, first of two secondary spermatocytes, both alike, and then to four spermatids, all alike. The cells of the column then become arranged so that groups of spermatids become related with each of the basal cells, which often leave their original position and move out toward the free surface of the epithelium. Through their attachment to the basal cells the Bpermatids draw a supply of food and energy during their rapid and extensive metamorphosis into spermatozoa. In some cases a very close relation is established by the actual embedding of one end of the spermatid in the substance of the basal cell. It should be noticed that the function of the follicle or basal cells of the testis is to supply nutrition to the germ cells, not so much during their period of growth as after that is completed, during the period of metamorphosis; while in the ovary the corresponding ceils function during the growth period; this is correlated with the smaller size of the spermatocyte and with the need for aa easily available food supply for the large number of sperm cells during their metamorphosis.



Fig. 68. — Diaer&m of a, seclloo through port ofthe testis of the rat, Bhowing some stages in sperm atogenesis. From Korschelt and Heider, after Lenhoss^k. bz, basal cells; tpc, spermatocytes; ape, spermatoKoma; apt, spermatidB; ara, sperm atoioa.



Fig. 69. — Diagrammatio outline of the BpermatogeuesiB of the Tat in thittytwo stages. After v. Ebner. Basement membrane toward the left. 1-8. Period of multipUaation (the number of cell generations is actually very large). 9-18. Period of growth. 19-24. Period of maturation. 25-33. Period of met&morphosiB. b, basal cells or Sertoli cells. 1-16. Spermatogonia. 17, 18. Primary gpermatocytea preparing tor division. 19. First spermatocyte divi■ion. 20. Secondary spermatooyteB. 21. Secoodory Hpermatocjrte dividon. 22-26. Spermatids. 20-31. Transformation of spermatids. 32. Fully fotined spermatosoa.



Probably the most interesting phase of the cytoplasmic aspects of spermatogenesis is this metamorphosis of the spermatids into spermatozoa. After growth and maturation the spermatids have much the same external appearance as any typical cell; they are more or less spherical cells with a pair of centrosomes or centrioles, and a large spherical nucleus with a dense chromatin network. Internally we know that the nuclei are unlike those of the somatic cells on account of the presence of only - chromosomes. Without any further division each is converted into the special form of the spermatozoon typical of the species. While the details of this metamorphosis vary considerably in different groups, the essentials of the process are everywhere the same. The spermatid (Figs. 70, A, jP; 71, A) contains, in addition to the typical cell organs just mentioned, a modified region of the cytoplasm which is sometimes a centrosphere or idiosome, sometimes of rather doubtful character and origin, which for convenience may be termed the spermatosphere. Close by lie the remains of the last division spindle. The spermatids are further characterized by the presence of a collection of chromidial structures termed the mitochondria (Fig. 71).


Fig. 70. — Formation of the HpermatoioOn in Urodelea. From Wilson. "Cell," A-E. SaUtmandra, after Meves; F-K. Amj'hiuma, after McGregor. A. Spermatid with periphcrai pair of centrosomes (c) lying outside the sphere, and aiial filament. B. Centrosomea near the nucleus, outer one ling-ahaped ; a. Bcroaome. C. Inner eentrnsome inside the nucleus, enlarging to form middle piece; n. nucleus. D. Portion of much older spermatid, showing divergence of two halves of the ring, r. E, Portion of mature spermatozo5n, showing upper half of ring at r, and the axial filament proceeding from it. F. Spermatid of Ampkiuma, showing sphere- bridges and ring-shaped mid-bodies. O. Later stage; outer centrosome ring-shaped, inner one double; sphere, «, converted into the acrosome, a. H. Migration of the centrosomes. /. Middle-piece at base of nucleus. J. Inner centrosome forms the end-knoh within the middle-piece, which is now inside the nucleus. K. Enlargement of middle-piece, nt, end-knob within it; elongation of the ring.


The details of the metamorphosis of these structures into the parts of the spermatozoon are subject to wide variation; the following account is based upon the history of the mammalian spermatid (Fig. 71). The first step in the process is the migration of the centrosomes to the surface of the cell, and at the same time the migration of the nucleus to the opposite side of the cell. In most cases it is difficult to determine the relation of the axis thus marked out, but in many instances this is perpendicular to the basement membrane of the germinal epithelium, thus expressing a polarity which coincides with that of the ovum; the nucleus lies toward the membrane, i.e., the attached surface of the cell, the centrosomes toward the free surface. As the centrosomes and nucleus are taking these new positions the spermatosphere moves up to the nucleus and around it to the side opposite the centrosomes, quite to the surface of the spermatid. The two centrosomes now separate, one approaching the nucleus, the other remaining peripherally.

Following these changes in the relative positions of the parts: come the real modifications of structure. The nucleus becomes; elongated or ovoid, and the chromatin condenses, first into a heavy reticulum, and then into a dense mass in which no visible structure remains; finally it acquires the form of the head of the mature spermatozoon. The spermatosphere meanwhile is largely converted into the acrosome or perforatorium, at the tip of the elongated nucleus; a smaller portion is transformed into a very delicate envelope covering a part of the head, in many cases scarcely distinguishable on account of its thinness. At the other end of the cell a fine flagellum begins to grow out in connection with the peripheral centrosome, either from the substance of the centrosome itself, or from the cytoplasm under its influence. Then the two centrosomes move farther in toward the nucleus. In the simpler cases the distal centrosome now divides into aSterior (toward the head) and posterior portions. The posterior part grows out peripherally into a rapidly elongating fiber which becomes the axial filament of the flagellum, while at its base it becomes ring-like; then through this ring or anniilus the axial filament grows in, finally connecting with the anterior portion of the centrosome. The anterior portion itself remains in the middle piece as a posterior centrosome of the spermatozoon. The proximal or anterior centrosome partly disappears, and partly is converted into that part of the middle piece which connects with the head (the neck).


Fig. 71. — Metamorphoaia of the epermatid of the guinea pig. Ctaiia eabam.

After MevBs. L. Side view, sbowing flatteiuiig of head, a, axial filament; e, centrosomes (centrioles) ; e, neck, containing end knoba (proiimal centriole)! k, chromidial " nebenkorper ;" ni, middle piece; n, nucleus; «, oentroaphere (acroHome) ; it, annulua [poaterior portion diatal centrosome) ; t, mitochondria (partly becoming a portion of the tail envelope) ; y, cytaplasinic portion of speiniBtid, being thrown oS in K and L: iuJ.K.L containing mitochondrial n



The cytoplasmic part of the spermatid seems to be largely consumed in providing the energy for this transformation, in addition to that drawn from the nurse or Sertoli cells. But the cytoplasmic membranes of the middle piece. and tail, including the undulatory membrane when this is present, are derived directly from the cytoplasm of the spermatid. The mitochondria of the spermatid seem to be transformed into the spiral layer of the middle piece. In many instances, particularly among the Mammals, the larger part of the cytoplasm remains for a time connected with the middle piece of the developing spermatozoon and then is cast off, and finally degenerates without taking any further part in the structul'al formation of the functional sperm cell. The chief structural correspondences between spermatid and spermatozoon are shown in the accompanying table.


Comparison of the Structures of the Spermatid and Spermatozoon With particular reference to the mammalian condition. (Partly from Gegenbaur-Furbringer^ Lehrbiich der Anatomie des Menschen, Leipzig, 1909)


Spermatid Nucleus.

Spermatosphere (centro sphere). Proximal centrosome (cen triole).


Distal centrosome (centriole). (a) Anterior portion. (6) Posterior portion.


Cell body.


Mitochondria.


Spermatozo5n


Head.


Acrosome (perforatorium) and sheath covering the anterior part of the head.

Forms an undifferentiated part of the middle piece; in Mammals, the neck. In part may disappear.


Centrosome of middle piece. Aimulus and axial filament of middle piece and tail.

Partly used as source of energy during metamorphosis. Partly thrown off. Remainder forms cjrtoplasmic envelopes of middle piece and tail (including imdulatory membrane).

Spiral membrane of middle piece.


Whatever the details of the metamorphosis of the spermatid may be, the facts of essential importance are always identical. These are, that the nucleus of the spermatid is directly transformed into the head of the spermatozoon; the centrosomes of the spermatid become the centrosomes and kinoplasmic structures of the spermatozoon and are contained within the middle piece, or partly in the tail; the cytoplasm of the spermatid in part goes to form a thin cytoplasmic investment of the spermatozoon or is in part cast off.

The fully formed spermatozoa now lose connection with the nurse cells and pass by way of the canals or ducts of the testis into the efferent reproductive ducts, vasa efferentia and vasa deferentia, to the outside, either directly, or after being stored for a time in special cavities such as the seminal vesicles. The sperm may remain alive in these storage cavities for a long time, awaiting the period of extrusion, upon the approach of which they become very active.

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

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


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