Book - Vertebrate Embryology (1913) 4

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Jenkinson JW. Vertebrate Embryology. (1913) Oxford University Press, London.

Vertebrate Embryology 1913: 1 Introduction | 2 Growth | 3 The Germ-Cells, their Origin and Structure | 4 The Germ- Cells, their Maturation and Fertilization | 5 Segmentation | 6 The Germinal Layers | 7 The Early Stages in the Development of the Embryo | 8 The Foetal Membranes of the Mammalia | 9 The Placenta | Figures
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Chapter IV The Germ-Cells (continued)

III. The Maturation of the Germ-cells

A. In the male

The Urodelous Amphibia have always been a favourite object for the study of these changes, and may conveniently be taken by us as a type.

It will be recalled that duriag the spermogonial divisions the full somatic number of chromosomes is seen. The mitosis is of the ordmary character (Fig. 32). The granules of chromatm increase, run together m the form of beaded rows, which become the V-shaped chromosomes. The nuclear membrane has m the meantime broken down, the centrosome has divided, and around each daughter centrosome an aster is appearmg. The chromosomes then undergo longitudmal fission and, so spht, are placed on the equator of the spmdle now developed between the two centrosomes. The daughter chromosones are then puUed apart by the spmdle-fibres attached to them to the opposite spmdle poles, and there passmg through the same series of changes in the reverse order become the daughter nuclei. Meanwhile a celldivision has occurred m the equatorial plane of the spmdle, m which process the mtermediate bodies-thickenings of the spmdle fibres- play an important part.

All the features of an ordmary mitosis are here : the chromatm is the only part of the nucleus to be divided ; for that purpose it is thrown mto the form of chromosomes, which ^ht lengthways mdependently of any external agency , a division a^tatuLasters and spindle-is ^onst^f ?? e centrosomes and probably by them, the function of pull apart the halves of the aheady divided chromosomes and to LL?the division of the cell. When the W ceased dividing they enter upon a time the nucleus passes through complex changes, whlch are reallty the prophases of the fost of the two maturation divisions. This first division is of a very different character to an ordinary mitosis. There ensues the second division. This, with one important exception, resembles the mitoses of the spermogonia.

Fig. 32. - Stages in the karyokinetic division of the spermogonia of the newt.

The first maturation division (Figs. 33, 34). In the nucleus of a spermogonium the chromatin is in the form of fairly coarse lumps uniformly distributed over a wide achromatic reticulum. As the growth of the cell and its nucleus begin the chromatin becomes subdivided mto finer granules, which soon arrange themselves m rows or filaments ; in each row the granules are connected by threads of the achromatic reticulum, while similar threads pass from one filament to another. This is the narrow thread or leptotene'^ stage. As the nucleus enlarges still more

Fig. 33.- Prophases of the heterotype division in the male Axolotl. 1, Nucleus of spermogonium or young spermocyte ; 2, Early leptotene ; 3, Transition to synaptene; 4, Synaptene with the double filaments converging towards the centrosome ; 5, Contraction figure ; 6, 7, Pachytene ; 8, Early, 9, Later diplotene ; 10, The heterotypic double chromosomes ; the nuclear membrane is disappearing.

it is seen that on one side some of the filaments are arranged in pairs, and converge towards one point, the point where the centrosome m its centrosphere is placed. On the other side of

1 These and the followmg terms were first proposed by von Winiwarter in his classical work on the oogenesis of the rabbit.

the nucleus the filaments pass into the general network. This is the paired thread or synaptene stage.

By coalescence of the component granules the filaments become shorter and thicker : at the same time in each pair the filaments approach one another so closely that only a narrow slit is left between them. On one side the pairs of filaments still converge towards the centrosome, but on the other are inextricably coiled and tangled together into a bunch which is withdrawn some little way from the nuclear membrane. The pairing of the filaments can, however, be seen in the tangle. The several pairs are still imited by achromatic threads, the filaments being toothed at each point of insertion of such a thread. A few threads stretch across the empty space between the tangle and the membrane. This is the contraction figure.

Fig. 34. - First maturation division in the male. 2, Salamander, the remainder Axolotl. 1, 2, The heterotypic chromosomes on the spindle (metaphase) ; 3, Anaphase ; 4, 5, Telophase ; 6, Resting nuclei ; 4-6, Celldivision into two secondary spermocytes.

The members of each pair of filaments now unite throughout their length, so that the longitudinal sHt disappears. The thick filaments still converge towards the centrosome side, where apparently they end against the nuclear membrane. There is, therefore, not one continuous filament or spireme, but several.

The other ends of the filaments pass into the tangle, which is still retracted from the nuclear membrane, but becoming looser as the nucleus enlarges. The coil is soon still more unravelled and occupies the whole of its side of the nucleus. This is the pachytene stage.

The several filaments now separate from one another, so that the polar convergence is lost, and coil in various directions through the nucleus. At the same time the longitudinal sUt reappears in each, and the filaments are once more paired, so reaching the diplotene condition. Their surfaces are still toothed where the connecting achromatic threads are inserted. Soon, however, these cross threads disappear and the filaments become smooth. At the same time the members of the several pairs begin to separate a little from one another, in places if not throughout their length.

The nuclear membrane now breaks down and disappears, the pairs of filaments shorten and thicken, and assume the most various shapes and sizes. A pair may be in the form of two straight parallel rods, or two curved parallel rods, either V-shaped, or C -shaped, or two rods parallel at one, divergent at the other extremity, and so -||--shaped ; or the sht between them may be expanded in two or more places, and then the two may be tAvisted over one another mto a figure of g or or by expansion of the whole sht, while the rods are united at the ends, may be ring-shaped, while finally the ring may be pushed in in four places and assume the form of a cross, =[}:. These bizarre double bodies are the chromosomes of the first maturation division. It seems clear that they are derived from the separate paired filaments of the diplotene stage, these from the thick filaments of the pachytene stage, and these again from the paired filaments of the synaptene nucleus. The origm of these we shall have to discuss later on.

The number of the double chromosomes, and therefore of the several double filaments in the earUer diplotene, pachytene, and synaptene stages, is one-haK that seen in the spermogoma. The reduction from the somatic number {2n) to the germnumber (n) has already taken place. It seems that this half number must be estabhshed in the synaptene nucleus.

The actual division now occurs (Fig. 34). A spindle is formed in the ordinary way, and the double chromosomes are thrown upon its equator in such a way that the two ends of each member of a pair he in the equatorial plane. This is easily seen where the pair retains the original form of two closely-parallel rods separated by a longitudinal slit, and can often be made out in the ring- and cross-shaped and other chromosomes.

The members of the pairs now come apart and travel to opposite spindle poles, where they coalesce and pass into the condition of resting nuclei. The cell, meanwhile, has divided and the two secondary spermocytes have been formed. The nucleus of each of these, it is clear, contains only one -half of the ordinary number of chromosomes.

The division which we have just witnessed is unlike an ordinary mitosis in at least two respects. First, the number of chromosomes is reduced from the somatic to the germ number, and second, the chromosomes are double and frequently of extraordinary shape. For these reasons the division is spoken of as heterotypic, or xmlike the usual type. The term meiotic or reducing, also appHed to it, refers to the numerical lessening of the chromosomes.

We have now to inquire whether this division is or is not like an ordinary mitosis in another respect, the manner ia which the chromosomes are divided. Ordinarily, as we know, the chromosomes are longitudinally divided ; but on this occasion it is held by many observers that the division, albeit in appearance longitudinal, is in reahty transverse.

The uiterpretation of the nuclear changes is a matter of considerable difficulty, and very diverse opinions are entertained (1) as to the origin of the double filaments seen in the synaptene and later stages of the prophase, and (2) as to the mode of formation of the ring-shaped chromosomes seen in the actual mitosis ; different combination of these diverse opinions has led to the formulation of three principal views.

I. It is held that (I) the double filaments of the synaptene stage arise by longitudinal fission of the filament, that the longitudinal split disappears, but reappears (2) to form the cavity of the rings. Hence the actual division is longitudinal (Meves). This is illustrated in the accompanying diagram (Fig. 35, I).

For the sake of simpUcity we will suppose that the full number of chi'omosomes is four, the reduced number two. We will further suppose ttot these four chromosomes are really diff^^' Tne another though apparently identical. Let us caU them 7 A' B, and B: In the prophase of the mitosis two mstead of' four filaments appear. We may suppose that each of th«a consists of two ordinary chromosomes muted end to end say aTo 4' and B to B'. Each filament becomes then spht lengthwa^ll 1). the sHt widens out until each filament assumes a ling shape (I, 2), and the rings are then so placed on the equator of the spindle that the ends of the chromosomes lie in the equator (I, 3). Hence, smce each half ring consists of an A and an A', or of a 5 and a jB', when the halves are separated and travel towards the spindle poles, each daughter nucleus of a secondary spermocyte will receive a chromosome of each kind, A, A', B, and B'.

Fig. 35.-Diagram to illustrate three interpretations of the first matoasee text.)

II. On the second view (von Winiwarter, Schreiner, Agar), (1) while the paired filaments of the sjmaptene stage are believed to arise, not by longitudinal fission of the leptotene, but by apposition of distinct chromatin filaments (that is, chromosomes), the formation (2) of the rings from these double ' filaments is in accordance with the first view.

The diagram (Fig. 35, II, 1) shows the four chromosomes united in pairs by their entire length, though presenting every appearance of longitudinally spht rods : A is paired with A', and B with B'. The chromosomes of each pair then separate to form rings, remaining united only by their ends, and then are placed on the spindle ua such a way that these ends lie in the equator. It follows that A and B face towards one. A' and B' towards the opposite pole, and hence that each nucleus of a secondary spermocyte receives not all tour chromosomes, but only two, say A and B, ov A' and B'.

The division, therefore, is not really but only apparently longitudinal : the result is the same as though A and A' (and B and B') had been united end to end, and then separated by a transverse division of the double chromosome so formed.

III. On the third view (Farmer, Montgomery), (1) the double thread of the synaptene and pachytene is formed by the longitudinal sphtting of the chromatm filament ; but (2) the rings do not arise by the opening out of the spUt. The longitudmal division disappears, and the filament is first gathered up into half as many loops as there are chromosomes in the spermogonia, and these loops then separate as the n ring-shaped chi omosomes. The rings are therefore open at one end only, and the cavity of the ring arises, not by the opening out of the longitudinal split (for that has disappeared), but by the bending of the two halves, united end to end, of each double chromosome upon one another (Fig. 36, III). That is, the filament, consisting of Aj A', B, and B', is first gathered up into two loops, A being bent on A', and B on B', and then the loops separate. In the mitosis (III, 3) the rings are so placed on the spindle that A becomes separated from A' and B from B', so that one secondary spermocyte receives A and B, the other A' and B' (or, of course, A and B', A' and B).

Fig. 36.- Second maturation division in the male (Axolotl). 1, Prophase (split spireme); 2, The homoeotypic spht chromosomes on the spindle; 3, Polar view of the same ; 4, Anaphase ; 5, Telophase ; 6, Resting nuclei and completion of cell-division ; in each spermatid the centrosome has divided, and the sphere has become detached.

The result is therefore the same as on the second hypothesis.

Considering the diversity of opinion, it would be rash to dogmatize, but it may be pointed out that the evidence on the whole is agamst the mode of formation of the rings adopted by the third view. It does seem as though the rings were made by the opening out of the double filaments. We are left, therefore, with the choice between the first and second hypotheses. We can only say that the way in which the members of the pairs of filaments diverge into the general network in the fourth stage (Fig. 33, 4) suggests apposition rather than fission, and this involves ultimately a transverse division of double chromosomes, and that the phenomena of maturation observed in a number of Invertebrate forms corroborate this view.

Before discussing the theoretical significance of this mode of division, we shall describe the second maturation division (Fig. 36).

The nucleus of the secondary spermocyte soon emerges from the resting condition, and a chromatic filament appears. This filament becomes longitudinally split and then divided into a number of V-shaped chromosomes, themselves therefore split lengthways. The number of chromosomes is the half somatic, n. A spindle is developed, the spht chromosomes are placed on its equator, and division takes its ordinary course, resulting in two spermatids, the nucleus of each of which therefore possesses n chromosomes. La the V-shape of the chromosomes, as well as in their longitudinal division, this second mitosis is of the ordinary type. Hence it is called homoeotypic. Each spermatid becomes metamorphosed into a spermatozoon in the fashion already described.

The phenomena of maturation in the male are, as far as is known, similar in other forms {Myxine, Elasmobranchs, Mammaha). Each ripe male cell, therefore, is provided with only haK the number of chromosomes seen in the spermogonia and in the tissue cells of the body. Whether the n chromosomes in all the spermatozoa are or are not alike depends upon the interpretation placed on the first maturation division, as well as upon our views of the nature of the chromosomes.

B. In the female

While m the male the first or heterotypic division follows immediately upon the prophases, in the female the two episodes - prophase and division- are separated by an interval, sometimes of great length, a year or more- during which the yolk is deposited in the cytoplasm to the accompaniment of complex nuclear changes.

Prophases of the heterotypic division. The oogonial divisions come to an end at a fairly early period, and growth of the oocyte begins almost at once. The prophases of the heterotype are therefore usually found only in very young animals- in the tadpole of the frog, or the new-born or embryonic Mammal.

These two afford good examples. The nuclear changes which are readily seen in the tadpole's ovary (Fig. 37) are obviously closely j)arallel to what we have observed in the other sex.

A stage in which the chromatin is in the form of scattered granules is followed by one in which the granules run together to form the leptotene filament. Then comes the synaptene, with parallel filaments, followed by the contraction figure. The paired filaments emerge from the tangle to converge to one pole, the tangle itself beiiag withdra-WTi from the other side of the nucleus. The pachjiiene and diplotene follow in due course. A remarkable change now occurs in the straining capacity of the chromatm filaments. Up to the diplotene stage they behave in the usual way, showing great affinity for chromatin stains (carmine, haematoxylin, and basic aniline dyes) ; but from now onwards they lose this faculty and stain only with the acid plasma dyes. Meanwhile, the number of nucleoh (these also stain in acid dyes) is increasing, and presently it is seen that granules of chromatin (that is, granules which are coloured by the ordinary chromatin dyes) begin to settle upon (? be precipitated round) the nucleoh. By what appears to be a continuation of this process the nucleoh become converted into highly chromatic bodies.

The filaments (chromosomes) persist for a while, but will eventually disappear.

Precisely similar phenomena are seen in the young Mammahan ovary (Fig. 38), and only one or two points require to be mentioned. There is a very obvious centrosphere with included centrosome on one side of the nucleus (this usually goes by the name of the yolk-body of Balbiani), towards which the filaments of the synaptene and pachytene converge. In the early stage of contraction the paired filaments are seen to emerge from the rather open tangle on this side, while on the other a few filaments, also paked, stretch out to the nuclear membrane. In the later contraction figure the latter are retracted and the tangle, much closer, hes wholly on the side of the centrosphere.

After the diplotene stage the ruig-shaped figures of eight and other forms of double chromosomes are seen, but then the chromosomes break up into their constituent granules and range themselves along the achromatic threads which make a network through the nucleus. This is the diciyale condition, and in this the nucleus remains through the growth period until the moment of maturation arrives.

Fig. 37. - Prophases of the heterotypic division in the female (ovary of tadpole). 1, Nucleus of oogonium ; 2, Leptotene ; 3, Synaptene ; 4, 5, Contraction figures ; 6, Pachytene ; 7, Later pachytene, multiplication of nucleoli ; 8, 9, Diplotene : the chromatin filaments are becoming achromatic ; granules of chi'omatin are being deposited on the nucleoli.

Fig. 38. - Prophases of the heterotypic division in the female (Mammals). 1-6, Kitten three days old ; 7, Mouse embryo shortly before birth ; 8, Mouse eight days old.

1, Nucleus of oogonium or young oocyte ; 2, Leptotene ; 3, Synaptene ; 4, Contraction figure ; 5, Pachytene ; 6, Diplotene ; 7, Heterotypic clnomosomes ; 8, Dictyate.

In 2-5 the centrosphere and centrosome (volk-body of Balbiani) are shown with the chromatic filaments of the nucleus converging towards them.

Fig. 38*.- Small ovarian egg of the frog surrounded by its foUicle (/.) and theca (th.), which is continued into the pedicle {p.). b.v., a bloodvessel between follicle and theca ; v.rn., vitelline membrane ; ch., clu'omatin filaments, now achi'omatic ; n., chromatic nucleoli, ejected from the nucleus (n'.) and becoming achromatic (w".).

The period of growth and deposition of yolk. The nuclear changes accompanying the deposition of the yolk in the oocyte have only been studied in the Amphibia, to which we must now accordingly return (Fig. 38*).

We have seen that after the prophases the chromatin filaments become achi-omatic, while the nucleoH increase in number and become chi-omatic. The filaments gradually break up into a number of small granules, which disappear, or at least become indistinguishable from the general ground substance - or magma - of the nucleus.

The nucleoli become more numerous, larger, and more chromatic. They pass into the cytoplasm in one of two ways : either they are bodily ejected from the nucleus, lose their staining capacity and break up into small fragments, or else they disintegrate inside the nucleus, the products of their disintegration then passing out - either in the form of small particles or in solution - through the nuclear membrane into the cytoplasm. The nucleoli consist of nucleo -protein, and the result of their transference to the cytoplasm is that the latter first acquires an affinity for the chromatin stains, and then begins to secrete yolk-granules. There is thus a direct connexion between the nucleo-protein of the nucleoli and that which, as we have seen, is demonstrable in the yolk.

It appears that this cycle of changes is repeated many times during the growth of the oocyte, fresh nucleoli being formed, moving to the centre of the nucleus, and there disintegrating.

This passage of material from the nucleus to the cytoplasm of the egg-cell during the time of growth and yolk-formation is of constant occurrence in animals. The material may be solid and bodily ejected or liquid and diffusible, it may be chromatic or achromatic, but it is always given off and is always concerned m yolk-secretion. The chemical changes are, unfortunately, not fully understood.

The material is known generally as ' yolk-nucleus '. It has sometimes been confounded with another quite distinct structure, the sphere and centrosome. In Mammalian ova the sphere has indeed long been known as the yolk-body of Balbiani (Pig. 39). In the Mammals the sphere usually divides into two or more bodies, which persist for some time, but disappear (in the bat) when there are two or three cell-layers in the follicle.

In some Mammals {Cavia, Vespertilio) chromatoid bodies are found in the cytoplasm. These may be of nuclear origin and correspond to the yolk-nucleus of Amphibia.

The yolk-nucleus is a very important contribution made by the nucleus to the structure of the cytoplasm : a second contribution has still to be made.

With the growth of the oocyte the nucleus has been enlarging pari passu, and by the time growth is completed is of considerable size. It lies in the axis, but excentrically, near the surface in the animal half of the egg. The oocyte is now ready for the first maturation division.

Maturation. The nuclear membrane breaks down and disappears. From a very small part of the achromatic substance of the nucleus a spindle - the first polar spindle - is formed (Fig. 41), and on this are placed the heterotypic chromosomes, of which we shall speak in a moment. The whole of the rest of the contents of the nucleus - chromatic nucleoli and achromatic granular ' magma ' - are cast into the cytoplasm. This is the second contribution made by the nucleus to the cytoplasmic structure, and it is of considerable importance, since on it in part depends the diflEerence between animal and vegetative hemispheres.

As we have already had occasion to observe, there is a definite relation between the polar structure and symmetry of the egg and the structure of the embryo which is to come out of it, inasmuch as the anterior end is always developed near the animal, the posterior end near the vegetative pole. The structm-e of the embryo is at this moment bemg predetermmed in the egg, by the dispersal of the contents of the nucleus.

This is a fact of universal occurrence. When the germinal vesicle breaks down, only a smaU part of it is utihzed m the formation of the chromosomes which take part in the maturation mitosis. The remainder is given to the cytoplasm, of which it forms henceforward a definite and integral part. Experiment has sho^vn that that part is causally related to the development of certain organs, is therefore a vehicle of inheritance. It will be noticed that this process is without parallel in the male sex.

We return to the first polar spindle and its chromosomes.

The chromosomes appear first, as beaded filaments of heterotypic form - wrings, crosses, figures of eight, curved rods, and so on (Fig. 40). Their number is the half -somatic or germ-number n. It has been disputed whether these chromosomes are identical with those which were formed at the end of the prophases, in the young oocyte.

It must be remembered, in discussing this question, that the hypothesis of the individuahty of the chromatin A does not necessarily involve 'p^-'\X^ that of the individuality of the chromosomes. We have so™1-from^?SSi? th?o»y?; seen elsewhere that there is of the Axolotl (Siredon) just before reason for beheving that the membrane breaks down.

chromatia of the nucleus comprises a number of quahtatively unlike bodies- not merely that the chromosomes are different, but that they are composed of individually different granules. It is also probable, to say the least, that chromosome formation is a nratter of precipitation from solution, for there is certainly much more chromatin in a dividing than in a resting nucleus, and the chromatin often disappears from view in the latter condition. But a body endowed with certain properties wiH retain those properties in solution and emerge from solution with the same, and in a mixture of unlike bodies each wiU retain its own properties in solution and exhibit them afresh when reprecipitated. The chromatin granules are such bodies, and wo may weU suppose that they do retain their properties in spite of their disappearance. It does not foUow, however, that the granules are associated always in the same order to form chromosomes, though that may be so. Hence the chromatin granules may well retam their individuaUty while the chromosomes do not.

The chromatin, therefore, of those heterotypic cliroinosomes that now apj)ear may, on this view, bo regarded as identical with the chromatin of the prophases.

Fig. 42.- The maturation divisions in the female (Axolotl). 1, First polar spindle with heterotypic chromosomes ; 2, Extrusion of first polar body ; 3, Appearance of second polar spindle ; longitudinal division of chromosomes in egg and in first polar body ; 4, Second polar spindle radial ; homoeotypic chromosomes on equator (metaphase) ; 5, Polar view of the same ; 6, Anapliase ; 7, Extrusion of second polar body ; 8, Second polar body with resting nucleus ; 9, Female pronucleus in resting condition, closely surrounded by yolk-granules.

When the spindle is formed the chromosomes are placed on it and shorten and thicken (Fig. 41). The spindle then moves to the surface at the animal pole, Avhere it takes up a radial position, closely surrounded by yolk-granules. The actual maturation divisions now occur.

Fig. 41. - Germinal vesicle of the oocyte of the frog just before maturation (after Carney). The nuclear membrane has disappeared. The first polar spindle, bearing the heterotypic cliromosomes, is seen in the middle of the nucleus {p.s.). n., nucleoli ; v.m., vitelline membrane ; /., follicle ; th., theca.

Fig. 42. - Oocyte of mouse with heterotypic spindle from the Fallopian tuDe. ihe oocyte is still surrounded by the cumulus of follicle-cells.

The first maturaiion division (Fig. 42, 1, 2). The heterotypio chromosomes are placed upon the spmdio in the same way as in the male - that is, Avitli the extremities of the half -rings in the equators. The half-rmgs break away from one another and pass to the spindle poles. Cell-division now occurs. This is extremely unequal. The outer group of chromosomes, with a small quantity of cytoplasm, is cut off as the first polar body from the egg ; it lies in a depression at the surface. The inner group of chromosomes remain in a clear area in the egg, now the secondary oocyte.

The first polar spindle is found (in Siredon, and generally in Amphibia, also in Bnds) in the egg as it passes mto the oviduct. Li Mammalia - ^where it is also known to be heterotypic (Fig. 43) - ^it may be formed while the egg is in the ovary, or after it has passed into the Fallopian tube.

The first maturation division in the female evidently involves similar changes to those seen in the male : the prophases, the number and form of the chromosomes are all exactly the same. The interpretation of the manner of division of the chromosomes - ^whether longitudinal or transverse - which is adopted for the one, may therefore be applied to the other.

The second maturation division (Fig. 42, 3-9). Without passing into a resting condition the V-shaped chromosomes in the egg undergo longitudinal fission, as also do those in the first polar body. A number of parallel fibres, tangentially placed, now appear - the second polar spindle. The spindle is soon rotated into a radial position and the V-shaped chromosomes, already split, are thrown upon its equator with their apices towards the spindle axis, as in the male. Their number is, of course, n. The halves of the chromosomes then separate and pass to the spindle poles. Another miequal cell-division now occurs. The outer group of chromosomes, together with a httle cytoplasm and one or two yolk-granules, is extruded as the second polar body, while the inner group remain in the now mature ovum as the female nucleus, or rather pronucleus, to employ the more usual term.

In both the second polar body and the ovum the chromosomes break up, a membrane is formed round them, and the nucleus passes into the resting condition.

Since the chromosomes are V-shaped, are longitudinally divided, and are present in half the normal number, this division is evidently homoeotypic, as in the male.

The second polar spindle is formed as the egg passes down the glandular region of the oviduct (in Siredon and most other Amphibia). In the uterus the polar spindles are in metaphase (with the chromosomes in the equator). The division is not completed until after the egg has been fertilized (which is just after the egg is laid).

Where fertihzation is internal (Elasmobranchs, Birds, Reptiles, Mammals) the second polar body is extended while the egg is in the oviduct.

Although the chromosomes of the first polar body have divided, cell-division (in Siredon) does not usually follow. In other cases the first polar body does divide.

A centrosphere - if not an actual centrosome - is present at the poles of both the first and second spindles. In the mature ovum there is, however, no trace of it. The female pronucleus is immediately surrounded by yolk-granules (Fig. 42, 9).

Nature of the reducing division. We have already assumed for the purposes of illustration that the several chromosomes of a nucleus are genuinely different from one another. We may now add that there is experimental evidence (which we cannot discuss here) in support of this ; it is further probable that the granules of which each chromosome is composed are again of different values. Secondly, there are cases where the chromosomes are of different sizes (certain Insects), and in these cases they are found in pairs (in tissue- and in yomig germ-cells), the two members of a pair being of the same size. In the heterotypic division of maturation the members of the paks get separated from one another, so that each secondary spermocyte (and consequently each spermatid after the second homoeotypic division) receives a similar set of different-sized chromosomes.

Attention has akeady been called to the difference in size of the ring-shaped chromosomes in Siredon.

Now when a row of granules (or chromosome) is divided lengthways each half contains its due portion of each granule, and hence each daughter nucleus receiving half of each chromosome receives ipso facto a specimen of each different gi'anule. The two daughter nuclei are therefore alike and a longitudinal division of the chromosomes is merely quantitative.

If, on the other hand, the row of granules (or chromosome) is transversely divided, or, what is the same thing, if two different chromosomes are separated from one another, each daughter nucleus will not receive a specimen of each different granule or chromosome, but only one-half, the remainder passing to the other nucleus, and the division is qualitative.

The first condition may be represented by some such formula as this (where a-h are the quahtatively different granules in a chromosome, A,A',B, B', &c., whole chromosomes) : abcdefgh A A' B B' G C D D' abcdefgji' A A' B B' G C D D the line being the division, while the second condition will be represented by

abed A B G D or efgh A' B' G' D' Ordinary somatic mitoses are therefore quantitative, and so is the second homoeotypic maturation division. If, however, we adopt the view that in the heterotypic mitosis a transverse division of the chromosomes is involved, then we must further beheve that the division is quahtative, and consequently that the secondary spermocytes, and eventuaUy the spermatozoa, receive chromosomes of different kinds. Of every four spermatozoa produced from a single primary spermocyte, therefore, two wiU be aUke of one kind (containing, say, A, B, Sec), while two will be alike of another kind (containing A', B', &c.).

But it is evident from the foregoing that identical nuclear changes occur during maturation in the two sexes. The prophases of the first division- with the leptotene, synaptene, pachytene, and diplotene stages- are the same, and whatever view is taken of these phenomena must hold good for both sexes. In the female the growth period intervenes between the prophases and the actual division, but when this division occurs it is of the same form as in the male, heterotypic. The second division is homoeotypic in both sexes.

While, however, the cell-divisions are equal in the male- resulting in four spermatozoa - in the female they are unequal - giving one large ovum which receives practically the whole of the cytoplasm and the yolk, and three small polar bodies. The similarity of the nuclei shows that in spite of their small size these polar bodies are in reality potential ova, and there are cases where they are large - as large as the ovum - and can be fertilized and develope.

Like the spermatozoa, the ovum (and polar bodies) receives only one-half the somatic number of chromosomes. As we shall see more fully in the next section, these chromosomes form a complete set, as do those of the male. If- as is probably the case- there are varietal differences between hidividual spermatozoa in respect of these chromosomes, the same will be true of the ova.^

But what the further significance of these difEerences is, if they exist, we do not know. The chromosomes of the spermatozoon and ovum are certainly vehicles of inheritance - ^that is, concerned in the transmission of at least some of the inheritable characters of the species from one generation to the next. But since every spermatozoon or ovum can perform this function as well as every other, we are driven to conclude that each one possesses a complete set of the necessary specific chromosomes ; but that in different spermatozoa or ova the chromosomes may be of different varieties- that is, be concerned in the transmission of different varieties of the same inheritable character. This may be expressed by the followmg scheme. A B, G, D, Sec, are the n different specific chromosomes. In the tissue-cells and young germ-cells there are 2n, each kind bemg represented by two slightly different varieties, namely, A and A', B and B', G and G', &c. In the prophases of the heterotype division A and A' unite,^ and so B and B', G and G'.

In the actual heterotype division A and A', B and B', G and G' are separated from one another, so that each secondary spermocyte or oocyte has A or A', B or B', and so on.

1 Provided of course that priraary oocytes differ inter se in the arrangement and distribution of the heterotypic chromosomes.

2 If the union is by parallel apposition it is further possible to suppose that the individual granules of"^ which 4 and^' are composed pair ofi each with each, namely a with a', b with b', and so on.

The homoeotypic division is quantitative, hence each spermatozoon or ovum obtains A or A', B or B' , and so on ; that is, a complete set of the various kinds of chromosomes.

In only one respect are there chromosomal differences between the two sexes. In certain forms (Insecta), and possibly in others also, there is an accessory chromosome or heterochromosome (often paired), which not only differs in size and behaviour from the ordinary chromosomes, but is not the same in spermatozoon and ovum. The variations in the behaviour of this body or bodies are too complex to be discussed here, but those who have investigated it beheve it to be concerned in the determination of sex. Apart from the heterochromosomes and the varietal differences of the ordinary chromosomes, the germ-nuclei are exactly alike.

We have now to see how the two nuclei - each containing one-half the somatic number of chromosomes - are brought together when the germ-cella unite in the act of fertilization.

IV. Fertilization

The Axo\otl- 8iredon- wi\\ serve as a type (Fig, 44). The spermatozoon- which is of the same form as that of the newt and salamander - after passing through the mucin jelly surrounding the egg, reaches the surface of the latter. It approaches the egg with its anterior end- acrosome - and always in the pigmented animal hemisphere, sometimes near the equator, but more usually near the animal pole.

The acrosome pierces the surface-layer of the egg-cytoplasm, and immediately the egg reacts in a remarkable manner. From all sides there begins to flow towards the acrosome what appears to be a watery albuminous fluid : it is hyaline, but coagulable. This becomes concentrated round the acrosome in the form of a conical plug, the base of which projects at the surface, the apex towards the interior of the ovum (Fig. 44, a). This plug is the entrance-funnel, its base bemg known as the entrancecone (' cone of attraction ' is an erroneous expression, as it is not formed prior to the contact of the sperm with the egg). The entrance-funnel enlarges and extends more and more . into the interior of the ovum, being directed usually towards the axis : it carries in with it a number of the superficial pigment granules and the spermatozoon. The latter, therefore, after moving actively up to the surface of the ovum and penetrating it with its acrosome, is passively carried in by the inflow of the entrance-funnel ; this movement is apparently due to a difference in surface tension between the entrance-funnel and the surrounding cytoplasm. The acrosome presently gets caught in the side of the entrance-funnel, but the substance of the latter, still moving on, carries the head and tail of the sperm with it. The result is that the anterior end of the head now faces outwards, while the posterior end Kes at the bottom of the funnel, where the head is bent on the tail, and the whole sperm-head has been rotated through 180°. Between the head and the tail - and therefore now at the inner end of the funnel - is the large anterior centrosome (Fig. 44, c).

Fig. 44. - Fertilization in the Axolotl.

A and B. Meridional sections of the whole egg. a, Formation of entrancefunnel (first part of sperm-path), b, Formation of sperm-sphere and aster ; o3 male pronucleus ; ? female pronucleus ; p.b the t^^^ polar bodies.

c Formation of the sperm-sphere round the middle piece (anterioi centrosome) ; narts only of the head (black) and tail are sho^vn.

X. Formation of the sperm-aster. The centrosome has disappeared ; the head besinning to be vacuolated, is separated from the tail.

T'FuSr shortening and vacuolation of the sperm-nucleus. There is still no centrosome.

F, Appearance of the definitive centrosome. g, h. Division of thn centrosome.

(In c-H the arrow marks the direction of entrance of tlie spermatozoon.) I, Approach of the two pronuclei. Formation of spindle-fibres J, i?ormation of asters, elongation of spindle, further enlargement of pronuclei, and appearance of clu'omosomes.

K, Further elongation of spindle, and formation of a ccntrosnhere

irfhe'Toindr^r""" ^he pronudear men.branes are breakSg 5ow,x ana trie spindle-hbres passing in.

L, The fully.formed fertilization spindle. In the equator are the chromoomes, now longitudinally split, and attached to large spiiidle fi'bre Tu each centrosome the centriole has divided.

The entrance-funnel soon disappears, but the pigment carried in by it remains for some time as a streak, usually known as the first part of the sperm-path (Fig. 44, b).

A clear, yolk-free area now appears round the centrosome ; this is the sperm-sphere (Fig. 44, c). Very soon radial fibres or processes of some kind begin to pass out from the sphere amongst the yolk-granules ; this is the sperm-aster (Fig. 44, d). Meanwhile the head or sperm-nucleus has become detached from the tail, and the centrosome which was between them has totally disappeared. It seems that the formation of the sperm-sphere and aster- like that of the entrance-funnel- is due to the extraction of water from the cytoplasm, in the case of the entrancefimnel by the acrosome, in the present case by the centrosome ; and that the centrosome is completely used up, in fact dissolved, in the process.

The tail of the spermatozoon will not concern us : it degenerates and vanishes. The head of course remains to become the sperm-nucleus or male pronucleus. It shortens and thickens : as it does so it becomes vacuolated. By further shortening and vacuolation it becomes transformed into an ordinary nucleus (Fig. 44, E). It Hes on the outside of the sperm-aster.

It is at this moment that the definitive centrosome makes its appearance (Fig. 44, v). On the side towards the sperm-aster the nuclear membrane breaks down, and through the aperture something comes out of the nucleus which appears, when outside, as a rounded granular body. This is the definitive centrosome. It is not preformed in the sperm-nucleus and then ejected, but, probably, is due to a precipitation of the albumins of the cytoplasm by the nucleic acid of the sperm-nucleus. But, whatever interpretation be put upon the process, the centrosome is of male origm.

The male pronucleus, preceded by its centrosome and aster, now advances to meet the female pronucleus which has aheady left its position at the animal pole and is retm-ning towards the centre of the egg. The line in which the male pronucleus is now moving is knomi as the second part of the sperm-path. This does not necessarily lie in the same straight line, nor even in the same meridional plane as the first or entrance part of the path. This depends in part on the position of the female pronucleus (Fig. 46).

The first or entrance part of the path is usually directed towards some point in the egg axis, that is, it Hes in a meridional plane of the egg. If, as also is usual, the female pronucleus hes in the axis, it is evident that the second part of the sperm-path or line of union of the two pronuclei will he in the same plane. In that case it may be in the same straight line with the first part, or, more usually, make an angle with it, smce the pomt in the axis at which the pronuclei meet is at a fairly constant distance from the animal pole, while the point of entrance o the spermatozoon in the animal hemisphere is variable. If, however while the first part of the path is in a meridional plane the female pronucleus is not in the axis, then the sperm -nucleus must turn out of its meridional plane to meet the female pronucleus at some point which is not in the axis. The converse of this is seen when the entrance-path is not m a meridional plane while the female pronucleus is in the axis ; m this case also the sperm must turn aside. Thirdly, both sperm-entrance path and female pronucleus may be out of their normal direction

Afterwords, the meridional plane which includes or is parallel to the entrance-path does not necessarily coincide with the meridional plane which includes or is parallel to the line of union of the pronuclei.

During the advance of the sperm-nucleus the centrosome divides (Fig. 44, g, h) at right angles to the direction in which the sperm-nucleus is travelKng, that is, to the second part of the sperm-path, and also to the meridional plane in which the path lies. The daughter centrosomes therefore lie in a plane parallel to the equator of the egg. Hence, when the pronuclei have met, they lie together between the daughter centrosomes, which lie in a plane parallel to the equator of the egg.

The two pronuclei are now closely apposed, but not fused, inside the sperm-sphere and aster. Next, the centrosomes send out fine fibres in all directions (Fig. 44, i, j). On the one hand these impinge upon the pronuclear membranes - ^these are the begimiing of the fertilization spindle ; on the other hand they radiate out until they pass into the radiations of the original aster inside which they He.

The pronuclei enlarge, and presently in each granules of chromatin appear and run together in rows to form chromosomes (Fig. 44, j). The number of these in each pronocleus is the same as that which entered into it at the close of maturation, namely n, the germ-number. Meanwhile the asters round each centrosome have been growing larger, the spindle-fibres longer, and the latter now break through the pronuclear membranes to meet their fellows from the opposite pole (Fig. 44, k). The membranes, achromatic network, and nuclei are now all dispersed, and the two sets of chromosomes, paternal and maternal, are placed side by side on the equator of the fertilization spindle, where they undergo longitudinal fission as in ordinary mitosis (Fig. 44, l). Hence, when the daughter chromosomes pass to the spindle poles, each daughter nucleus will receive a complete set of paternal, and a complete set of maternal chromosomes. The full somatic number, 2 n, is now restored, and with each repetition of nuclear and cell-division each cell in the body comes to possess 2 w chromosomes, one-half of which are derived from the father, one-half from the mother. With the apposition of the two sets of chromosomes in the equator of the division apparatus - asters and spindle - the act of fertilization may be said to be complete.

The whole falls into two periods. In the first the spermatozoon is carried into the egg by means of the entrance-funnel, which in turn is due to a stimulus of some kind imparted to the egg cytoplasm by the acrosome ; the acrosome is the modified centrosphere. In the second the definitive centrosome is formed from the male pronucleus and the division apparatus made between its two halves while the pronuclei meet. The mechanisms involved in both periods are therefore centrosomal.

The details of fertilization have been studied in many animals, including several Vertebrates. In Vertebrates it is a rule for the sperm to enter during the second maturation division of the ovum, as in the Axolotl {Petromyzon, Salmo, Triton, Mus), but in other cases it may enter at an earlier or later period. The tail may be left outside {Mus), but is more often taken in : it always degenerates.

The pronuclei may fuse to form a segmentation nucleus, from which 2 n chromosomes arise {Pristiurus, Salmo, Petromyzon) ; but the newt and the mouse resemble the Axolotl in the separate formation of the chromosomes in each pronucleus.

It is certain that in all cases the female centrosome disappears. Whether the definitive cleavage centrosome is identical with the centrosome seen in the spermatozoon, that. is, in the spermatid, or is, as in the Axolotl, a new formation from the sperm-nucleus, is not certainly known, but there is little doubt that it is invariably a male centrosome.

As a rule only one spermatozoon enters the egg, and the presence of more than one leads to serious derangements of development (pathological polyspermy).! in what is known as physiological polyspermy, however, two or more, sometimes a great number, normally get in, as in some Amphibia (including the Axolotl), Reptiles, Birds, and Elasmobranch fishes, in which last they are very numerous and known as 'merocytes' (Riickert). In these cases only one of the sperm-nuclei fuses with the egg-nucleus. The remainder lie about in the yolk, each develops its own centrosome and aster, and may divide (with n chromosomes) many times. Ultimately the accessory sperm-nuclei degenerate without contributing to any embryonic structure.

1 As in the sea-urchin, where the several nuclei fuse and their chromosomes become irregularly distributed. Where, however as m the frog Srseveral nuclei remain apart the polyspermy need not cause abnormal ^eveToprenHM. Herlant, Arch, de Biol. xxvi. 1911) although the superfluous sperm-nuclei do take part in the edification of the embryo.

It remains for us to discuss the significance of fertilization.

It has commonly been supposed that its essence is to be found in the union of the pronuclei of the germ-cells, both nuclei being held to be necessary for the development of a normal individual. This view is based partly on the phenomena of conjugation in certain Infusoria, but also very largely on the assumption that the nuclei of the germ-cells are the sole vehicles for the transmission of inheritable characters ; this again rests upon the fact that it is only in their nuclei that the germ-cells are alike, while in every other respect they differ, and upon the supposition that the paternal and maternal contributions to the total inheritance are equal.

Now, whatever view we may take of the parts played by nucleus and cytoplasm respectively in the handing on of the characters of the species, it is most assuredly certain that for the production of a normal individual both pronuclei are not a necessity. In the first place, there is the phenomenon of parthenogenesis, natural and artificial. In the former the ovum develops without fertilization by the sperm and without artificial assistance (as in Aphidae and some other Insects, and in certam Crustacea). In the latter the stimulus usually given by the sperm is replaced experimentally by some physical or chemical agent. Thus the ovum of a sea-urchin or Mollusc may be stimulated by treatment with hypertonic sea-water, or butyric acid or other substance, or by mechanical shock, or a lowering of the temperature ; in the case of the frog it is sufficient to pierce the egg with a fine needle. In all these instances some physical or chemical alteration (or both) IS produced in the egg, as a result of which it begins to segment and develop. The process, if care is taken, may be perfectly normal, and the individual reach the adult condition A sexually mature (male) sea-urchin has been reared in this way In all cases of parthenogenesis only the female pronucleus is

The converse is seen in what is called merogony, where the egg (of a sea-urchin, Worm, or Mollusc) is divided into two halves, only one of wlucli contains the nucleus. Both halves can be fertilized, the nucleate and the enucleate, and will develop into normal larvae. In the latter case only the male pronucleus is present.

On the other hand, a nucleus must of course be present, and actual experiment has shown that what is really necessary for normal development is the presence in the ovum, and ultimately in every cell of the body developed from it, of a complete set of the n unlike chromosomes characteristic of the species.

Hence, both male and female pronuclei are not necessary, and we must look elsewhere for the significance of fertilization.

As we know already, the germ-cells of both sexes pass through two maturation divisions, and two only, after which their capacity for reproducing themselves is lost. The first effect, or almost the first effect, of their union is that their product, the fertilized ovum, begins to segment and continues to do so. In other words, the power of reproduction by cell-division which was previously lost is in fertilization restored. It is mutually restored.

That the ovum regains the power of nuclear and cell-division is obvious : we see the maternal chromosomes undergo longitudinal fission, as they lie on the spindle, and subsequently we see the egg cytoplasm divide. In the case of the male Ave see the male chromosomes divide in ordinary fertilization as they lie alongside the female ; in the fertilization of enucleate eggfragments the stimulus imparted by the female cytoplasm to the male chromosomes is still more evident.

A study of fertilization reveals the mechanism by which this stimulation is effected. For ordinary nuclear and cell-division an apparatus is necessary, the spindle with its asters ; this apparatus is made by the centrosomes in the cytoplasm, the two centrosomes proceeding from the division of one, and its function is first to pull apart the halves of the divided chromosomes, and second, to ensure cell-division by the cell-plate or intermediate bodies developed in the equator.

The mature ovum possesses no centrosome : the mature spermatozoon possesses little cjrtoplasm, and that only in the tail. In fertilization the centrosome is either introduced by the male cell or made by it after entering the egg : the necessary cytoplasm in which this centrosome can divide and make the asters and spindle is provided by the female. The wholly different structures of the two germ-cells are therefore mutually complementary in the stimulation by which the lost power of cell-division is restored, and this is the significance of fertilization.

The experiments on artificial parthenogenesis suggest that a physico-chemical expression may be found for this stimulus.

This is not, however, its only effect. A very common, if not universal, result of the approach of the sperm is the exudation by the ovum of a peri vitelline fluid. In some cases (for instance, the sea-urchin) a membrane which prevents the entry of more spermatozoa is secreted at the same time and pushed out by the peri vitelline fluid. In the frog it remains as a thin fluid layer between the ovum and the jelly ; it is the exudation of this fluid which enables the egg previously adherent to the mucin jelly to turn over till its axis is vertical and the white pole below : this occurs shortly after insemination.

Of greater importance than this is the change in the cytoplasmic structure of the egg brought about at this time.

A few hours after insemination there appears in the frog's egg a crescentic grey patch on one side along the border of the pigmented area (Fig. 45). The grey crescent is due to the immigration of pigment from the surface into the interior, and this in turn is caused by the entrance of the spermatozoon. The grey crescent always appears on the side of the egg opposite to that on which the sperm has entered. We know that a watery fluid flows towards the sperm from the cytoplasm (the entrancefunnel flrst, and later the sperm-sphere, are due to this), and we may suppose that this streaming movement drags the pigment granules away from the surface on the opposite side, whence the grey crescent.

The grey crescent is actually opposite to- that is, in the same meridional plane as- the first or entrance part of the sperm-path (Fig. 46). Hence it does not necessarily lie in the same meridional plane as that which includes the line of union of the pronuclei.

We shall see in the next chapter that the meridional plane of the first division always includes the line of union of the pronuclei, and hence does not always coincide with the meridional plane of the grey crescent.

It is clear that, whereas the unfertilized egg was radially symmetrical about its axis, it can now be divided into similar halves by only one plane, that which includes the axis and the middle point of the grey crescent. About this plane it is bi-laterally symmetrical. The greatest interest attaches to this alteration of symmetry, since the side of the grey crescent will become the dorsal side of the embryo, the side on which the sperm entered its ventral side. Since the animal and vegetative poles mark respectively the future anterior and posterior ends (approximately), it follows that the plane of symmetry of the fertiUzed but unsegmented egg coincides with the median longitudmal or sagittal plane of the future embryo. The whole bilateral symmetry of the embryo is now predetermined in the cytoplasmic structure of the egg.

Fig. 45. - Formation of the grey crescent in the frog's egg (R. ternporaria). a, b from the side ; c, d from the vegetative pole. In A, c there is no crescent, in b, d a part of the border of the pigmented area has become grey.

That the blastodisc has a bilateral structure in Birds and Elasmobranch fishes also seems to foUow from the fact that the cells in both these cases are larger at one end of the blastoderm than at the other. Further, this structure is definitely related to that of the embryo since the large-celled end becomes anterior.

Whether the change from the original radial to the definitive bilateral symmetry is in these cases also brought about by the spermatozoon, future researches must show.

In the Teleostei the concentration of the superficial cytoplasm (periblast) to form the blastodisc is an effect of fertihzation.

Fig. Diagrams to show the relation between the first and second parts of the sperm-paths. The paths are projected on a plane perpendicular to the axis. In a the two parts are in the same meridional plane, in B m different meridiona.1 planes. 1, First part of the sperm-path ; 2. Second part; o^, male pronucleus ; ?, female pronucleus ; ^.c, grey crescent : on the opposite side (side of entrance of the sperm) the superficial pigment Z rS; ' «,<^^ has divided in a plane perpendicular to thf a^ at right angles to the second part of the path. '

In conclusion we may attempt to estimate the parts played by the cytoplasm and the nucleus of the germ-cells in inheritance.

That some at least of all the inheritable characters of the species- and not only specific but varietal and individual characters as well- can be inherited from the father as readily as from the mother is obvious. Since the nucleus, beside the centrosome which is merely an organ of cell-division, and the acrosome which merely provides for the entrance, is the only part of the male cell which is always incorporated in the fertilized ovum for the tail may be left outside, we are obliged to regard the nucleus, that is, the chromosomes, as the vehicles by which tliese characters are transmitted.

The chromosomes of the nuclei of the germ-cells - which, as we have already pointed out, are different from one another - are in some sense the determinants of inheritance in the offspring : on their presence depends the ultimate appearance in the offspring of certain characters, and, in respect of their capacity for transmitting these characters, the two germ-cells are similar : each possesses a full set of the necessary chromosomes. In ordmary sexual reproduction the offspring receives two such sets, but one will suffice, as in parthenogenesis and merogony.

It does not, however, follow that the determinants for the whole of the inheritance are located in the nucleus.

As we have just seen, the material for the different parts of the body of the embryo is present in the cytoplasm of the fertilized but unsegmented egg ; to that structure the spermatozoon has . contributed nothing, beyond the rearrangement of material, the substitution of a bilateral for a radial symmetry. Experiment teaches us that the various parts of this structure are so many organ-forming substances, causally related to the development of certain organs, and therefore determinants of a part of the whole inheritance ; and recent researches on heterogeneous hybridization show clearly what this part is. The ovum of a sea-urchin, if the proper precautions are taken, may be fertilized by the sperm of a starfish, a feather-star (both of which of course are, like the urchin, Echinoderms), or even of a Mollusc or Worm. The result is always the same. A typical sea-urchin larva is developed. Even an enucleate egg-fragment will develop a little way when so fertilized, and exhibits the maternal characters alone. The spermatozoon employed does nothing but convey to the egg a stimulus, which sets the process in action ; its chromosomes sometimes persist, sometimes do not.

Hence the characters, the determinants of which reside in the cytoplasm, are the large characters which put the animal in its proper phylum, class and order, which make it an Echinoderm and not a Mollusc, a Sea-urchin and not a Starfish ; and these large characters are transmitted through the cytoplasm and therefore through the female alone. The smaller characters - generic, specific, varietal, individual - are equally transmissible by both germ-cells, and the determinants of these are in the chromosomes of their nuclei.

And yet the cytoplasm of the egg-cell is indebted very largely for its structure to the activity of the nucleus. As we have seen, the nucleus makes two contributions to the cytoplasm, first, the so-called ' yolk-nucleus ', the substances concerned in the deposition of the yolk, and second, the contents of the germinal vesicles dispersed when the latter breaks down at matm'ation. These processes are perhaps independent of the chromosomes. Further, they find no parallel in the male sex.

Even if, therefore, the cytoplasmic determinants are ultimately to be assigned to the nucleus, the share taken by the female in the transmission of the whole heritage is greater than the part played by the male.


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Jenkinson JW. Vertebrate Embryology. (1913) Oxford University Press, London.

Vertebrate Embryology 1913: 1 Introduction | 2 Growth | 3 The Germ-Cells, their Origin and Structure | 4 The Germ- Cells, their Maturation and Fertilization | 5 Segmentation | 6 The Germinal Layers | 7 The Early Stages in the Development of the Embryo | 8 The Foetal Membranes of the Mammalia | 9 The Placenta | Figures

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