Book - Vertebrate Embryology (1913) 3
Embryology - 10 Oct 2026    Expand to Translate
|
|---|
| Google Translate - select your language from the list shown below (this will open a new external page) |
|
العربية | català | 中文 | 中國傳統的 | français | Deutsche | עִברִית | हिंदी | bahasa Indonesia | italiano | 日本語 | 한국어 | မြန်မာ | Pilipino | Polskie | português | ਪੰਜਾਬੀ ਦੇ | Română | русский | Español | Swahili | Svensk | ไทย | Türkçe | اردو | ייִדיש | Tiếng Việt These external translations are automated and may not be accurate. (More? About Translations) |
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
| Historic Disclaimer - information about historic embryology pages |
|---|
 |
Chapter III The Germ-Cells
The male and female germ-cells - ^the spermatozoon and ovum - are higUy specialized structures and as different from one anoth'er - except in their nuclei - as any two cells could well be, the former being a small, active body, the latter large and inert. But though so unlike in their completed form, they are derived from cells which are apparently identical in the two sexes, the primordial germ-cells.
During the development and growth of the body of the parent which encloses them, these primordial germ-cells pass through a series of changes, the final result of which is the formation of the ova and spermatozoa. The history of these changes is very similar in the male and in the female. In each case it may be divided into three periods, a period of multiplication, a period of rest and growth, and a period of maturation (Fig. 6).
In the male sex the primordial germ-cells divide to form small cells, the spermogonia, which in their turn divide a large number of times. In all these divisions the nucleus divides by karyokinesis, and the number of chromosomes formed at each division is the same as that observed in all the tissue-cells of the body. This somatic number is constant for any given species of animal (or plant), and is (except in certain insects and some other forms) an even number. We shall speak of it as 2 w.
After a time, however, these spermogonia cease dividing and enter upon the second period of rest, during which they grow. The growth is not very great, but quite well marked. During this time the nucleus undergoes intricate changes which are the prophases of the first maturation division. The male germ-cells are now known as primary spermocytes.
At the end of the resting period the primary spermocytes prepare once more to divide. Each is halved to form two secondary spermocytes, the nuclear division being of an altogether peculiar
24
THE GERM-CELLS
III
character, and the number of chromosomes reduced to one-half (n) of the normal number. Each secondary spermocyte then divides again to give rise to two equal cells, the spermatids, the number of chromosomes being again one-half that observed in
Fig. 6. - Diagram to illustrate the history of the germ-cells in the male
(on the left) and the female (on the right).
I. Period of multiplication (many more divisions occur than are here represented).
II. Period of rest and growth,
III. Period of maturation. Sp.g., spermogonia ; sp.c.l, primary, sp.c.2, secondary spermocytes ; sp., spermatids ; sp.', spermatozoa ; o.g., oogonia ; o.c.l, primary, o.c.2, secondary oocyte; o., ovum ; ]).b.l, first polar body; p.b.2, second polar body; p.b.1.2, halves of first polar body. (After Wilson, after Boveri.)
the spermogonia. Each spermatid becomes directly metamorphosed into a spermatozoon, there being no further division.
In the female the primordial germ-cells divide to produce oogonia, and these in their turn divide, the nucleus breaking up into the full number of chromosomes (2»). When the period
Ill
THE^ GERM-CELLS
25
of multiplication has come to an entl each oogonium rests while the nucleus passes through the prophases of the first maturation division. The whole cell then grows into a primary oocyte. This growth is much greater than in the male sex, since it is during this time that the yolk is deposited in the cytoplasm to the accompaniment of other and very complex nuclear changes. In the third or maturation period the ovum, like the spermatozoon, undergoes two divisions, and two only ; but whereas in the male these two divisions are equal, giving four spermatids, eventually four spermatozoa of the same size, in the female they are markedly unequal. The primary oocyte divides unequally into a large cell - the secondary oocyte - and a small cell, the first polar body, the number of chromosomes being reduced to n. A second unequal division results in the production of one large cell - the matme ovum - and a small cell, the second polar body ; meanwhile the first polar body has divided (usually) into two small cells of the same size. The number of chromosomes is again one-half the normal number.
The parallel between these processes in the two sexes is evident, since each primary spermocyte or oocj^e by two divisions produces four cells, each one of which possesses only one-half the number of chromosomes seen in the spermogonial and oogonial mitoses. While, however, in the male the four cells are all of the same size, in the female one, the ovum, is large, while the remainder are small.
The germ-cells are supported, invested, and nourished in the testis and ovary by certam elements known as follicle-cells. These, like the primordial germ-cells, appear at an early stage in the development of the parental body, and our first duty will be to inquire into the origin of both. We shall then be at liberty to discuss the structure and chemical composition of the mature sexual elements, the disposition of the protecting follicles, the nature of the membranes by which the ovum is enclosed, the intracellular and nuclear processes accompanying the deposition of the yolk, the nuclear phenomena involved in the reduction of the number of the chromosomes during the two maturation divisions, the metamorphosis of the spermatid into the spermatozoon, and finally the union of the two germ-cells in the act of fertilization.
26
THE GERM-CELLS
III
I. The Origin of thk Germ-Cells and of the Follicle-Cells
The gonads - testes and ovaries - first appear, at an early date in the development of the embryo, in the form of what are known as the genital ridges. The genital ridges are a pair of longitudinal bands of tissue in the abdominal region, each placed between the root of the mesentery of the gut on the inside and the Wolffian body or mesonephros on the outside, and each projecting downwards into the peritoneal cavity.
Each genital ridge is covered by the peritoneum or coelomic epithelium, which is here usually columnar. Under this epithelium are a number of small cells which are probably derived by proliferation from the epitheUum itseK, together with others which come from the retro -peritoneal tissue behind. In addition there are conspicuous certain large cells - ^usually with large nuclei unlike those of the surrounding cells and in many cases with yolk-granules in the cytoplasm, derived from the yolk of the egg from which the embryo itself has arisen. These large cells are placed some in, some below the epithelium. They are the primordial germ-cells, and their position in the columnar epithelium covering the genital ridge has very nationally given rise to the belief that they are formed by modification in situ of the cells of that epithelium, which has hence been termed the germinal epithelium, The researches of recent years have, however, brought forward very strong evidence to show that the first germ-cells are not formed in or from the germinal epitheUum, but elsewhere in the body, and that they only reach what is to be their final, resting-place by migrating there, the source from which they spring being in general the endoderm or splanchnopleure (mesoderm) of the gut or yolk-sac. Later on, however, it is generally admitted that germ-cells may arise from the germinal epithelium. This is also probably the source of the future follicle-cells, since these are derived from the small cells which are proliferated from that epithelium. The other small cells, retro-peritoneal, give rise to the thecae of the follicles and to the vascular connective tissue (stroma) olf the ovary or testis. Let us consider a few cases.
Fig. 7. - Primordial germ-cells in the dogfish {Sci/llium).
A, Germ-cells in the mesoderm surrounding the gut.
B, Germ-cells (g.c.) creeping up the mesentery (m.).
c. Germ-cells in the germinal epithelium (g.ep.) of the genital ridge. The yolk-granules are beginning to disappear.
Fio. 8.- Primordial germ-cells (r/.c.) in the tadpole of the common frog
{Rana kmpomria). A, In the mesentery (hi.) ; b, In the genital ridge.
g.ep., germinal epithelium ; f.c, follicle-cell.
Ill THE GERM-CELLS 27
In the Elasmobraiich fishes the gerni-celLs are first found in the extra-embryonic blastoderm, either between the yolk and the mesoderm, or under the ectoderm. Thence they migrate into the body of the embryo by way of the yolk-stalk (Fig. 7) ; passing up by the splanchnopleure surrounding the gut, sometimes in the gut epitheKum itself, they reach the mesentery, and thence to right and left into the two genital ridges, where they make their way into the epithehum. The cells are large, the cjrtoplasm crowded with yolk -granules, which, however, are presently digested and disappear, the nucleus large, provided with a large nucleolus, its chromatin in the form of small granules. The accompanying table (from Woods) gives the number found in the unsegmented mesoderm or ventral to the mesentery, in the mesentery, and in the genital ridges, in successively older
Number in genital rid"cs.
34 69 193 710
the germinal
In the lamprey {Petromyzon) the primordial germ-cells, similar m character to those of the Elasmobranchs, first appear in the lateral plate mesoderm, whence they migrate to their definitive position.
So also in the trout and salmon : in these there is also a later formation of germ-cells from the epitheUum of the ridges.
In another bony fish {Cynmtogaster) the sex-cells can be distmguished even in segmentation stages; later they are found at the posterior end of the body (where all the germ-layers are fused together), whence they move forwards into the mesoderm of the genital ridges.
Amongst the Amphibia the germ-cells of the frog appear at
these cells.
Length
of embryo
Number in
Total number
of germ-cells.
unsegmented
mesoderm or
Number in
in mm.
ventral to
mesentery.
2-75
98
mesentery.
98
3-5
230
230
6-0
256
222
8-0
296
154
73
11-5
408
78
137
34-0
710
More germ-cells are
later on formed from
epithelium.
28
THE GERM-CELLS
III
an early stage, in the ncwly-hatchcd tadjiolc ; they are derived from the large yolk-cells of the gut. Being separated off on the dorsal side they move up the mesentery (Fig. 8), and so, passing to the right and left, reach the genital ridges. The cells are at first crowded with yolk, but this soon disappears ; the nuclei are not peculiar.
Fig. 9. - Section of a 12-clay rabbit embryo, showing the migration of
the primordial germ-cells (indicated by black dots). Most are in the yolkstalk mesoderm, some in the mesentery (m), some in the genital ridge {g.r.)
internal to the mesonephros (M). y.sl., opening of the yolk-stalk into
the yolk-sac.
It is remarkable that, in the female at any rate, many of these young germ-cells are expelled from their follicles and disintegrate in- the peritoneal cavity. There is an extensive formation of fresh ones by modification of cells of the germinal epithelium, which is also the source from which the folUcle-cells are derived. The theca comes from retro-peritoneal tissue. (An account of the follicle and the theca will be found in a subsequent section.)
When we pass to the Reptiles we find the same migration taking place. In the tortoises {Glmjsemys) the sex-cells arise in the endoderm of the yolk-sac posteriorly. Becoming amoeboid
B.
Fig. 10.- Primordial gorm-cells {g.c.) in the rabbit.
A Early stac'e in the formation of the genital ndge, covered by the germinal epithelium {g.cp.). Below this are some germ-cells and connective tissue and blood-vessels {b.v.). Germ-cells arc also seen in the mesentery {m.).
B, A germ-cell in the epithelium of the gut.
c, One from the yolk-stalk.
D, An epithelial cell from the yolk-sac.
P. 29
Ill THE GERM-CELLS 29
they migrate towards the middle line into the embryonic region. Passing out of the endoderm into the splanchnopleure (the mesoderm covering the gut), they travel up the mesentery into the ridges. Many, however, fail to reach their destination, and remain for some time in the epithelium of the gut. It is stated that in this animal no germ-cells are ever formed from any other source.
In the Birds, again, the germ-cells appear early, in a chick on the third day of incubation. They seem- to originate from the splanchnopleure of the yolk-sac, and pass, in the way already described, to the genital ridges, and there into the germinal epithelium. By the fifth day the migration is complete and the cells begin once more to multiply.
In all these cases the identification of the primordial germcells is considerably facilitated by their retention of yolk-granules at a time when these bodies have disappeared from the surrounding cells.
In the (placental) Mammals, however, where there is practically no yolk, the distinction of these cells from the surrounding elements is a matter of some difficulty, and it has been, and is still contended, that the germinal epithelium is their only place of origin. Nevertheless, there is good reason for believing that the Mammalia are no exception to the general rule.
In a rabbit embryo of eleven or twelve days (Figs. 9, 10) there are to be found in the splanchjiopleure of the yolk-stalk large numbers of rounded cells, distinguishable from the surrounding cells by their cytoplasm- which includes large oxyphile granules - and their nuclei, which are round and large, with a fine achromatic reticulum bearing small granules of chromatin. The nuclei consequently look pale. There is one, sometimes more, large nucleolus. In all these respects the cells bear a close resemblance to the large cells in the endodermal yolk-sac epithelium (Fig. 10 d). Precisely similar cells may be found in the body of the embryoround the sides of the gut, and sometimes in the gut epithelium in the mesentery, and finally below and in the columnar epithelmm (germinal epithelium) internal to the mesoncphros which IS the beginning of the genital ridge. In earlier stages the same ceils are found in increasingly smaller numbers in the genital
30
THE GERM-CELLS
III
ridges and mesentery, in increasingly larger numbers in the yolkstalk and endoderm. It can hardly be doubted, therefore, that the same migration of these cells from the yolk-sac to the genital ridges is occurring here as we have already observed in other forms. There is equally little doubt that these cells, arrived at the genital ridges, become germ-cells.
In the rabbit embryo of twelve days the genital ridge (Fig. 10 a) is very slight, consisting of a band of columnar epithelium, below which are a few cells derived probably by immigration
Fig. 11. - Ovary of rabbit fiom embryos of a, 18 days, and B, 21 days,
showing formation of cortex (G) and medulla, b.v., blood-vessels in
stroma {str.) of medulla ; md.c, medullary cords.
from that epithelium. These cells differ from the germ-cells amongst which they lie in their nuclei - which are oval in shape, have more than one nucleolus, and a more open reticulum with coarser granules of chromatin, and are of smaller size - and in their cytoplasm - which includes no granules. Those which lie below the surface are destined to give rise to the follicle-cells, which will eventually be disposed in layers round the germ-cells. Deeper still are connective tissue-cells and blood-capillaries derived from the retro-peritoneal tissue ; from these will come the thecae and the vascular stroma. Let us follow the development of this genital ridge into the sexual organ, and first into the ovary.
The whole genital ridge is enlarged and made to project into
P.31
Ill
THE GERM-CELLS
31
the body-cavity by the increase of the connective tissue elements and blood-vessels, or stroma, which thus forms a central core or medulla to the whole organ. At the same time, by continued proliferation of the germinal epithelium at the surface, an external layer or cortex is formed. The germ-cells lie mainly in this cortex, but a few - those presumably which in migrating to the genital ridge have never reached the surface - lie in the stroma, where they are grouped in rows known as medullary cords (Figs. 11 A, 12 a). They seem to degenerate.
The cortex increases in thickness and becomes divided up into columns or blocks by the ingrowth of vascular connective tissue from the stroma. These columns - ^which were at one time believed to be produced by hollow invaginations of the germinal epithelium and known as the epithelial tubes of Pfliiger - are the sex-cords (Figs. 11 b, 12 b). They consist of folHcle-cells derived in all probability from the germinal epithelium, and of germ-cells which have migrated into then- present position from their source, the yolk-sac." The germ-cells have been increasing in numbers : in the resting condition their nuclei present the same characters as before. The cytoplasm, however, loses the oxyphile granules. At about the twenty-second day the germ-cells cease to divide and enter on the period of rest : they are in fact primary oocytes, and their nuclei begin to undergo the changes characteristic of the prophases of the first maturation division.
This mode of origin of the germ-cells does not of course preclude the formation of others from the cells of the sex-cords, that is, from the germinal epithelium, and it is Indeed quite possible that this occurs.
In the mouse and other Mammals (guinea-pig, mole, cat) the germ-cells appear to come from a similar source. In the mouse they are large cells with rounded bodies, dense, rather deeplystaining cytoplasm, and large nuclei, with a close reticulum, small, scattered chromatin granules and one or two large nucleoli (Fig. 13 A). They divide by mitosis for a time, but pass into the resting condition at a comparatively early date, about the fifteenth day (Fig. 1 3 B, c). They He intermingled with a number of cells, the future follicle-cells, which may be regarded as of epithehal origin. The meduUa of the ovary is formed late
32
THE GERM-CELLS
in
(Fig. 13 e), when the nuclei of the germ-cells have already reached the pachytene ^ condition (see below), by ingrowth of connective tissue from the base ; prior to this the whole thickness of the organ is composed of follicle-cells and germ-cells, with but a few capillary blood-vessels. The sex-cords (' tubes ') of the cortex arise, therefore, not so much by downgrowths of epithehum as by rearrangement of the cortical, that is, foUicle and germinal, and medullary, that is, stroma cells (Fig. 13*).
B
C
â– p-rp 13* _A B, Formation of medulla in ovary of mouse. sir stroma of medulla; md.c, medullary eords; c, cortex of gerra-cells and
^Sestisof embryo mouse, ep., eoelomie epithelium ; L, seminiferous tubules.
A few germ-cells are found in the medulla, but as in the rabbit, these probably never become mature. When the gerincell nuclei have reached the diplotenic or dictyate stage (J^ig. 13 F) (in the new-born animal) the formation of folUcles begnis, by the grouping of the follicle-cells round the oocj^es, to form a single flat layer. At the same time the oocytes enlarge. These two processes always occur first at the deep end of the sex-cords. Later (Fig. 14) the cells of the follicle become cubical, and then increase in number till several layers are formed. Hence the
X A full explanation of this tern, will be found in the section of the next chapter deahng with maturation.
Ill
THE GERM-CELLS
33
cortex of the young ovary comes to comprise several layers of small oocjrtes, each surrounded by a single layer of flat cells under the surface, and larger oocytes, surrounded by cubical foUicle-cells, disposed in the more advanced deeper down in
Fig. 14.- Part of cortex of ovary of young mouse (8 davs) In the
deeper parts the foUieles (/. 3) consist of two- or three-cell layeS" and Se
oocytes are arge. In the middle layers the follicles (/. 2) areTntlayered
but the cells are cubical, the oocytes smaUer. Under the s^SfaTe the
oocytes are smaUer still and the follicle-cell flat (/. 1). ep surf^e enithe
lium (germinal epithelium) ; h.v., blood-vessel ; strl\tfo^T,Z tW
many layers. It is not, therefore, that the folHcles and oocytes enlarge as they pass in from their (supposed) origin at the surface but that those which are inmost are the first to enlarge The oocytes of the outermost layer often lie practically in the epithehum at this stage. Their nuclei, however, are not in the condition seen in newly-formed germ-cells (oogonia), but in the dictyate stage characteristic of oocytes
1355 _
34 THE GERM-CELLS m
In the male (Figs. 13 Bo^, 13* c) the sex-cords become early shut ofi from the suiiace epithehum (peritoneum) by the formation of a sheet of comxective tissue [iunim albuginm), and from one another in the same way. The sex-cords are the rudiments of the seminiferous tubules of the testis. Each consists of an outer layer of foUicle-cells, and an inner mass of germ-cells, presenting the characters already described. These are spermogonia and divide mitoticaUy many times. Intermmgled wi h Lm are a few of the folUcle-cells. In this sex there seems to be no doubt that many of the first-formed g-m-cells d^^^^^^^^ and that in the adult fresh spermogoma are difierentiated from
"Lfy"on of the germ from the body or somatic " ee^thet first appearance in a part of the body remote from, ^iCr gradualmigrationto their ^f^^f^^^ eesses which find a parallel in many, if not m aU, groups ot tne aSmal ldngdom. Thus in the Hydroid Coelenterates, the germ on xo become medusae or gono
tLrir^s Sgr^LX often pass foxward. and backphores. In tins migi<^t j round-worm
waxds from one 87;7„V ;t:eUs is distinguishable Ascaris the parent cell ol tne g ^ t^e
"y --'-^ ^S^t^- n C,!o,s. In
segmentmg oTum. Similaily ™ ^^Me Oephalopod Moltaca and in S<=°:^P ^/^^^ i^cts during the formation of the ^e™-' ^appearance they may be -P--"f ^j'^^Cre^Ue difierLiation of the blastoderm-or rather later, ^^e
the germ-layers or ater ^J^.^y ^o^Hged to migrate mesoderm, and m aU these oases J j ^^i^^.
forwards into their ^f^Z::::Z ^ n Z from the germinal
saos ; lastly, they may be drfferent ^^^^
epitheUum of the ^j'-^^ ^ere we y,,,,^,,^, either
the double origm which we have .^^ subsequent
at an early date, independently °' ^ aate, from
migration into the f f no hard-and-fast the germinal epithehum itself. It is clear
Ill
THE GERM-CELLS
35
rule can be laid down. All the germ-cells may be precociously separated from the somatic cells and elsewhere than in the generative organ, or some may have such an origin, while others arise in the generative organ itself, or lastly, all may be developed by the second method, as appears to be the case in most Annelid worms and in Ascidians.
Nor need the conversion of what look like tissue-cells into cells endued with the capacity of reproducing all the characters of the species cause any particular astonishment when the widespread capacity for regenerating lost parts possessed by the adult tissues, and the remarkable facts of bud-reproduction, are borne in mind. In these cases germ-plasm or reproductive substance must be present in the regenerating or budding tissues, and yet there is no obvious continuity between this germ-plasm and that of the germ-cell from which the regenerating or budding individual sprang ; as little should we expect to find a demonstrable continuity in the case of sexual reproduction.
II. The Structure of the Mature Germ-cells A. The Ovum
The egg-cell is large and inert : it is quite incapable of locomotion ; only occasionally does it exhibit peristaltic contractile movements, as in the formation of the polar rings at the time of fertilization in Annelids, or slow changes of shape as in the protrusion of the animal ends of the blastomeres in Petromyzon, or the flattening at the animal pole of the frog's egg prior to segmentation.
In shape it is nearly always spherical : exceptionally, as in Myxinoids and Amia, ellipsoid or ovoid.
Size of the ovum. The ovum is always a large cell compared to other cells of the body, even where, as in Placental Mammals, it is actually very small, and it may be very large indeed, as in the large-yolked ova of most fishes, and of birds and reptiles. The size of the ovum is due to the contained reserve food material or yolk, the amount of which varies very greatly in the different groups.
O 2
36
THE GERM-CELLS
III
A small-yolked (microlecithal) egg is found in the lamprey (Peiromyzon) and in the Anurous and Urodelous Amphibia. In the frog the diameter of the ovum is about 1-6 mm. In the Gymnophiona, and most 'Ganoid' fishes {Acipenser, Amia, Lepidosteus), there is more yolk in the egg, while in the Myxinoid Cyclostomes, Elasmobranch and Teleostean Fishes, Reptiles, Birds, and Monotrematous Mammals, the egg is large-yolked (megalecithal). Finally, in the Placental Mammals- which are descended fiom large-yolked forms- the yolk has been reduced to a very small amount.
The following table brings out the contrast between the size of the eggs in the large-yolked Monotremes and the smallyolked other forms. It will be seen that amongst the latter the Marsupials have the largest ovum. In this respect, as m others, they are intermediate between the Monotremata and the PlacentaUa.
Monotremata
MarsupiaUa :
PlacentaUa :
Echidna Ornithorhynchus
Dasyurus Didelphys
Canis
Homo
Lepus
Ovis
Talpa
Cavia
Erinacells
Mus
3-4 mm. 2-5 mm.
0-28 mm. 0-13 mm.
0-18 mm. 017 mm. 015 mm. 0-15 mm. 0 09 mm. 0-08 mm. 0 06 mm. 0 06 mm.
The yolk. The yolk is frequently termed deutoplasm m distinction from the living substance or protoplasm m which it hes. ir s Losited in the cytopl-m of the ovum during the period o ^oXh t the form of smaU bodies spoken of a. granules, g ofls or platelets (Fig. 15). The size, shape, and structure
Ill
THE GERM-CELLS
37
of these vary. In the lamprey, frogs and toads, newts and salamanders, the granules are oval or ellipsoid bodies, sometimes vacuolated. In the Elasmobranch fishes they are oval plates, sometimes spherical and vacuolated. In the Teleostean fishes the separate yolk-globules run together at an early stage to form one continuous yolk-mass. In the Birds there is white yolk and yellow yolk, the former consisting of small globules enclosing still smaller ones of varying size, while the latter is made up of
Fig. 15.- Yolk-gvanules. a, Dogfish, b, Axolotl, the smaller from
the anmial, the larger from the vegetative hemisphere, c, White volk.
D, yellow yolk, from the Hen's egg.
larger spheres, each including a multitude of minute droplets. In both kinds of yolk the smaller bodies are often set free by the rupture of the larger enclosing envelopes. In the Placental Mammals the yolk-granules are usually globular (Fig. 18, b). All the granules which have been mentioned are protein in nature, but in addition to these fat globules are not uncommon. Fat is present in the hen's egg, in some Mammalia (guinea-pig) (Fig. 18, c), while in the Teleostei a single large oil-drop is characteristically present (Fig. 72).
The chemical composition of the yolk of a hen's egg is as follows. The yolk- that is, the ovum- weighs from 12 to 18
38
grammes include
THE GERM-CELLS IH 47-2 % of this is water ; the remaining solids
Protein . Salts . Fats Lecithin Cholesterin
15-63 %
0- 964 % 22-84 % 10-7 %
1- 75 %
51-884 %
The proteins include ovo-vitellin (for the greater part) and some albumin.
The former is not a globulin but a nucleo-proteid ; on digestion with pepsin it yields an iron-containing body, a pseudo-nuclein known as haematogen, since it is supposed that it is the source of the haemoglobin of the embryonic blood corpuscles. With the ovo-viteUin the lecithin of the egg is closely associated. The fats are oleates, palmitates. and stearates. With them must be included certain phosphatides.
The salts are chlorides of sodium, potassium, magnesium, and
calcium. , . • i a
The reaction of the yolk is alkaline. The colouring is due to
lutein, a lipochrome. , . ,
Other ova have not been so fully investigated, but it is known that the ichthulin of certain fish eggs (carp, cod) is a nucleoproteid, and lecithin (6 %) and nucleo-proteid (94 %) can be demonstrated in the yolk of the frog's egg. The significance of the presence of nucleo-proteids will be more evident when we consider later on the part played by the nucleus during the
deposition of the yolk.
The yolk of the Monotreme egg is of a yellow colour, in the lamprey it is a faint yellow, in the dogfish greenish, in the Ganoid fish Amia brown. In Placental Mammals the yolk is
colourless. ,
The yolk is not scattered Irregularly through the cytoplasm, but arranged in a very definite fashion, known as the telolecithal ; that is to say, while the cytoplasm (or protoplasm) is concentrated on one side of the egg, the yolk (or deutoplasm) is conceiitrated on the opposite side. This does not imply, of course, that all
in
THE GERM-CELLS
39
the yolk is on one side, all the protoplasm on the other side, but that most of the cytoplasm is on the one, with fewer and smaller yolk-granules, while on the other the yolk-granules are more abimdant and larger, with less cytoplasm in between them. The transition from one extreme to the other in a small-yolked egg such as that of an Amphibian is quite gradual (Fig. 16) : there is a graded diminution in the concentration of cytoplasm,
A
I
Fig. 16. - Diagram of a meridional section through a full-grown oocyte of the frog. The yolk-granules are represented by stippUng, the pigment by the thin black line. The arrow marks the egg-axis, its head the animal pole.
an increase in the concentration of the yolk in passing from one side to the other.
As the yolk increases the distinction between protoplasmic and deutoplasmic portions becomes more and more marked, until the limit is reached in the megalecithal type. Here the amount of yolk is so enormous that the cytoplasm is reduced to a small cap or disc- the blastodisc- at one side, the bulk of the ovum being occupied by the yolk (Fig. 17). Yet eVen here small yolkgranules are found in the blastodisc, and the transition from blastodisc to yolk is not absolutely abrupt.
In the Placental Mammals the telolecithal arrangement of the yolk can stiU be seen, in spite of the small amount, at least when
40
THE GERM-CELLS
III
the nucleus, with some cytoplasm, goes to the surface just before maturation (Fig. 18).
In the ovum of the Marsupial Dasyurus (Fig. 18, a) the yolkglobules run together at this time to form a single rounded mass - the yolk-body - placed on the opposite side to the nucleus.
In the ova of Birds (Fig. 17) the white yolk is disposed in the form of a central plug - the latebra - under the blastodisc. This is surrounded by successive layers of yellow and white yolk, alternately. The same feature is observable in the ova of Reptiles, Gymnophiona, Amphibia, and Elasmobranch fishes, where sheets of coarse and fine granules alternate.
The telolecithal disposition of the yolk confers upon the Vertebrate ovum a very definite structure and symmetry. In most cases the ovum is a sphere, and it is evident that a line may be drawn passing through the centre of the protoplasmic portion, at the surface, the centre of the egg, and the centre of the deutoplasmic portion at the opposite surface. This line is the egg-axis, and it is clear that its two ends, or poles, are unlike. The former, the protoplasmic, is known as the animal pole, the latter as the vegetative pole. These terms took their origin in the observation that in such an egg as that of the hen the chick or animal is developed from the blastodisc, at the side opposite to the inert
or vegetative yolk.
From what has akeady been said it further foHows that the yolk and protoplasm are distributed about this axis in such a way that the egg would be divided into precisely similar halves by any section which included the axis, but by none other. Hence the egg is said to possess a polarity and a radial symmetry about the axis. In any one plane at right angles to the egg-axis all radii are alike. The plane at right angles to the axis and including the centre of the egg is equatorial.
In cases where the egg is ovoid or elUpsoid (Myxinoids, Aima) the egg-axis is the major axis.
Yolk is heavier than protoplasm. Hence the Amphibian egg which, after fertilization, is free to rotate inside its jeUy membranes, always turns over till its axis is vertical with the white, vegetative pole below. The fuU-gromi ovarian egg-whether alive or dead-behaves in the same way when floated m a fluid
I
kl.
I.
VJ/l
I^ct 17 - Hen's longitudinally biRected. (After Balfour, modified.)
The section includes" the axis of the ovum, the animal pole bemg to the
upper side of the figure, sli., shell, underneath it the external shellmembrane ; i.m., internal shell-membrane ; a.di., au' chamber ; c7i., chaiaza ;
hi blastodisc ; I., latebra of white yolk ; v.m., vitelline membrane.
Between iO and i 1
B. 3 C.
Fig. 18. - Mammalian ova.
A, Dasijurus (a Marsupial). 1, The ovarian egg (oocyte) ; the nucleus is near the surface at the animal pole ; the cytoplasm contains spherules of jo\k. 2, The second maturation division. The first polar body has been extruded, and the second polar spindle is seen. The yolk-spherules have run together to form the yolk- body [y.h.) placed at the vegetative pole. (After Hill.)
B, A ha,t [Vesferiilio). Both polar bodies have been extruded and fertilization is taking place. The two pronuclei are seen. In the cytoplasm are numerous globules of yolk (protein). (After Van Beneden.)
c, The guinea-pig (Cavia). Full-grown oocyte. In the cytoplasm are mitochondria (cln'oraatic bodies) and fat globules (the former are black, the latter clear in the figure). (After Lams and Doorne. ) z. , zona pellucida.
Ill
THE GERM-CELLS
41
of the same specific gravity as itself. Similarly the ovum (yolk) of the hen's egg always turns over inside the shell till the blastodise is uppermost. So in Elasmobranchs.
Pigment. The polarity and radial symmetry thus conferred upon the egg by distribution of the yolk may be further emphasized by the disposition of the pigment where that is present apart from the colouring matter of the yolk itself. In many Amphibia (Anura and Urodela), in Ceratodus and Acipenser, pigment is present in the egg. The dark brown, almost black pigment of the frog's egg will be familiar. Chemically it is a melanin. In other cases {Siredon, for example, and the edible frog) it is of a much lighter colour.
The pigment lies (Fig. 16), in the form of minute droplets, in a dense superficial layer in the animal hemisphere of the egg, extending a greater or less distance into the vegetative hemisphere. There is left round the vegetative pole as a centre a circular unpigmented area. The symmetry of the egg, as determined by the position of the yolk, coincides with that due to the distribution of the pigment. There is also a less dense mass internally in the animal hemisphere.
The Nucleus. The nucleus - germinal vesicle - of the fullgrown oocj^e is characterized by the presence of one (Placental Mammals) or more nucleoli, usually chromatic. The history of these nucleoli and of other parts of the nucleus wiU be dealt with later. What interests us at the moment is the position of the nucleus. This is always in the axis of the egg, but excentric (Fig. 16), and always nearer the animal than the vegetative pole. In a microlecithal egg the nucleus lies in the protoplasmic portion, in. a megalecithal egg in the blastodise (Fig. 17). It is placed, therefore, in what is termed, in Oskar Hertwig's first rule, the centre of its field of activity. The importance of this will be appreciated when we come to the study of the phenomena of segmentation.
Structure and symmetry of the ovum. It will be obvious from the foregoing that the egg is no homogeneous body, but heterogeneous with a definite polar structure - radially symmetrical about an axis determined conjointly by the disposition of the yolk, the distribution of the pigment, when that is present, and
42
THE GERM-CELLS
HI
the position of the nucleus. The first two characters are purely cytoplasmic. The significance of this initial structure of the
/
2
Fig. 19.-0vary of the tadpole, showing development of the ovarian
cavity (o.c.) and numerous germ-cells in diSerent stag^ 1 ^Wntene
with the nuclei in different conditions. 1, Earliest stage ; 2, bynaptene
Tpachvtene 4 Diplotene ; 5, Formation of nucleoli. The largest oocytes
L sSnded by f ofiicles. ep.', coelomic epithelium (germmal epithehum).
egg cytoplasm in development cannot be over-estimated, for it is related in a perfectly definite way to the structure of the embryo which will come from it. Thus, to take one example.
um.
Fig. 20. - Small ovarian egg of the frog surrounded by Its follicle (/.) and theca {ill.), which is continued into the pedicle (^.). h.v., a blood-vessel between follicle and theca ; v.m., vitelline membrane ; dr., chromatin filaments, now aclu-omatic ; n., chromatic nucleoli, ejected from the nucleus in' .\ and becoming achromatic (54".).
n
^ -Liu
A
r
'J
-O.
-th.
B.
Fig. 21. - a, young, and b. older oocytes from the pigeon's ovary. r'^T cytoplasm of the oocyte; v.m., vitelline membrane;
/., follicle ; Ih., theca.
P. 12
CO
ill
8
Ill • THE GERM-CELLS 43
the anterior end of the embryo is developed always near the animal pole of the frog's egg, the egg-axis making a certain constant angle with the longitudinal axis of the embryo ; or, in other words, the anterior and posterior regions of the embryo are predetermined in the structure of the egg.
That the relation is a necessary and causal one is shown by those experiments- performed on the eggs of various animals - in which, some one part of the cyioplasm being removed, some definite organ of the embryo or larva is lacking. The different portions of the egg cytoplasm are therefore so many organforming substances, and since the organs are part of the sum total of the inheritable characters of the species, the cytoplasmic substances, on which their development depends, are factors determinant of inheritance.
The egg-follicle. In the ovary the egg-cell is invested by one or more layers of follicle-cells, the function of which is not only to protect, but also to nourish, the growing oocyte. These are derived, as we have seen, from the germinal epithehum of the genital ridge. The follicle in its turn is surrounded by a theca of flattened connective tissue-cells.
In the Amphibia there is but one layer of cells in the follicle ; they are flat. The ovary is hollow (Pig. 19), and the theca cells are continued into the stalk by which each ovum is suspended to the wall of the ovarian cavity (Fig. 20). Between theca and foUicle there are blood-vessels.
In the Elasmobranchs and Birds (Fig. 21) there is but one cell-layer in the young follicle, but the number is subsequently increased to two or more. The cells are cubical or polyhedral in shape.
Li the Monotreraata the number of cell-layers is only one or two, but in all other Mammals (Fig. 22) it is greatly increased, and a cavity filled with an albuminous fluid - the liquor foUicuh - is developed in between the cells, thus leading to the development of the characteristic hollow Graafian follicle. The cavity appears first on one side of the ovum as a narrow crescentic sUt ; soon this enlarges and extends round the ovum, which is then attached to the wall of the cavity only by a short stalk- the so-called discus prohgerus. On its free side a few layers of follicle-cells remain adherent to it, the cumulus proligorus. Finally, by the further extension of the cavity, the stalk is ruptured and the ovum, with its corona of cells, floats freely in the folUcular cavity. The ripe foUicle, which has now returned from the deep parts to the surface of the ovary, bursts, and the ovum, with its corona, is expelled and passes into the mouth of the oviduct (Fig. 43) to be fertilized.
The expulsion of the ovum is known as ovulation. In multiparous Mammals several are, of course, expelled at the same time and from both ovaries.
Fto 22 - Part of the cortex of an adult mouse ovary. </./., cavity of
JSaSan follicle ;/.,fomck^
proligerus ; ej)., surface (coelomic) epithelium ; th., theca , b.v., Dlooa vessel.
After ovulation the foUicle collapses, but it does not immediately degenerate. It becomes altered mto a corpus luteum. The foUicle-cells divide for some little time, and then, ceasmg to do so, hypertrophy (Fig. 23). They secrete fat and lutem (to which the corpus luteum owes its yeUow colour). Amongst these enlarged folUcle-cells grow vascular strands from the mnermost layer of the theca. The theca cells, which mcrea.e m numbers by division, are fusiform, and. lymg obliquely, or tangentially, or radially, iii the follicle, divide up the luteal tissue into ii-regular blocks. The larger strands contain blood vessels. There is a central cavity filled with stellate cells and extravasated blood corpuscles.
It has been shown that the corpus luteum secretes a substance which passes into the blood, and by that channel reaches the wall of the uterus, Avhere it appears to be necessary for the proper attachment of the embryo by means of the placenta.
Fig. 23.- Marginal portion of a section through the corpus luteum of a mouse 14 days after parturition (i.e. after ovulation). /., hypertrophied follicle-cells ; s., septa of connective-tissue cells ; th., theca ; b.v., blood vessels.
The membranes of the ovum. These may be of three kinds, primary, secondary, and tertiary. A primary membrane is one secreted by the cytoplasm of the egg itself; a secondary, one secreted by the follicle-cells and often termed ' chorion ' ; while tertiary membranes (albumen, shell) are secreted by the epithehum of the oviduct as the egg passes to the exterior.
The ovum of Vertebrates is always immediately surrounded by a vitelline membrane, frequently termed a zona pellucida (Figs. 16, 17, 18, 20, 21). This membrane may be traversed by fine radial pores, by nieans of which nutrient material passes from the folhcle -cells to the ovum : it is then spoken of as a zona radiata.
It is a matter of great difficulty in most cases to determine whether the vitelline membrane is primary or secondary, but it is stated that there is a membrane secreted by the ovum itself, inside another secreted by the folhcle -cells, in most forms (Elasmobranch fishes. Amphibia, Reptiles, Birds). The radial striations of the uuier primary membrane disappear before the ovum is full grown. The viteUine membrane of the ripe egg is possibly the result of the fusion of both the primary and secondary membranes of an early stage.
The so-called ' chorion ' of Teleostei and the Ganoid Lepidosteus, a very thick membrane, is apparently primary. In it the radial striations are persistent. The Myxinoids, however, possess a true chorion which is provided, at the animal pole, with a number of hooks, by which the egg is attached. In Petromyzon, Teleostei, and Lepidosteus, the vitelhne membrane is perforated by a passage at the animal pole through which the spermatozoon enters. This is the micropyle.
In Mammals (Marsupiaha and PlacentaUa) there is much uncertainty as to the origm of the vitelline membrane. It varies a good deal in thickness, and is not generaUy radiate unless thick. It is a zona radiata in the rabbit (Fig. 60), mole, pig, and sheep. ChemicaUy the vitelline membrane (of Birds) is an albummoid
alhed to keratin.
The tertiary membranes are secreted by the oviduct. The innermost of these is the albumen, white of egg, or jelly. This is found in Elasmobranch fishes, Amphibia, Tortoises, and Crocodiles but not Snakes and Lizards, Ends (Fig. 17), Monotremata, Marsupials (Fig. 68), and sometimes in Placental Mammals
(rabbit). .
The white of the hen's egg is wound round the ovum m layers, spiraUy arranged. The layers are separated from one another by a thin but tough membrane, the albumen in between successive membranes being fluid. Owing to the rotation of he egg as it passes down the oviduct these layers are spiraUy tw^ted up into cords (the chalazae) on two opposite sides. The chalazae
5/1 77!.
23*.- Section tliroTigh the shell of the egg of the ostrich Cafter VAaldeyer, after Konigsborn). c, cuticle ; sp., spongy layer of stratified snbstance pierced by canals, which open internally between the bases of the conical processes of the mammillarv layer {m.) sJi.m., shell-membrane. are always placed iii the equator of the ovum so that the blastodisc is midway between them, and they lie in the long axis of the egg-shell.
The white of the egg, which has an alkaUne reaction, contauis
85-88 % water
10-13 % protein
0-7 % salts
0-5 % . . . • • dextrose
and traces of fats, soaps, lecithin, cholesterin, and lutein (to which the faint yellow colour is due).
The proteins are ovo-globulin (6-7 %), ovo-albumin (a mixture of at least two proteins), and ovo-mucoid.
The salts are sodium and potassium chloride, phosphates, and salts of calcium, magnesium, and iron.
In some birds (Insessores) the egg-white does not become opaque on boiling, but gives a transparent jelly, similar to alkali albuminate.
In Fishes and Amphibia the egg-white is a jelly, composed of mucin.
A shell is present, outside the egg-white, in Elasmobranch jfishes. Birds (Fig. 17) and Reptiles, Monotremata, and some Marsupials (Fig. 68).
In the fishes referred to the shell is horny and attached by tendril-like strings to some foreign body. It is composed of keratin.
In Birds and most Reptiles the outer layer of the shell is calcified, the iimer layer being then known as the sheU-membrane. Calcification, however, does not occiu: in some cases (Lacerta vivipara). The shell-membrane is made up of a network of fibriUae of keratin.
The calcareous layer consists - in a Bird's egg - of three sheets (Fig. 23*) : an outer dehcate porous cuticle, a middle spongy sheet, and an inner mamnullary sheet of columns whose conical ends impinge upon the sheU-membrane.
The shell contains 3-7 % of organic matter (keratin), 90 % of calcium carbonate, and small quantities of magnesium carbonate and earthy phosphates.
The colour of the shell is due to bile-pigments. In the hen's egg the shell-membrane is separable into two sheets : between these two air collects at the blunt end of the shell after the beginning of incubation, so forming the air-chamber. This air is for the chick to breathe just before hatching (Pigs. 17, 121).
The Monotremes possess a shell which in Ornithorhynchus is calcified.
Amongst Marsupials a horny shell is present in Dasyurus and Phascolarctos. In Placentaha the shell is invariably absent.
B. The Spermatozoon
In striking contrast to the large inert egg-cell, the spermatozoon is a smaU, actively-moymg body, capable of swimming towards and enteruig the ovum in fertiUzation.
While there is great variety in the form of the animal spermatozoon, two principal types may be recognized, the flageUate or tailed, and the tailless.
The Vertebrate spermatozoon is flagellate. It consists typically of two parts, a head and a tail (Fig. 24).
In the head there is at the anterior end the acrosome or perforatorium, used in perforating the surface of the ovum, and behmd this the nucleus. The nucleus is always dense and homogeneous, and highly chromatic.
The tail consists of an axial filament and a cytoplasmic envelope. Centrosomes are always present in it. Three portions may be recognized : an anterior part including the centrosomes ; this is the pars conjunctionis ; a middle part, pars principalis, as far as the end of the cytoplasmic envelope of the taU ; and a pars' terminalis, in which the axial filament
is naked. -v *
The axial filament (probably the seat of the contractihty of the tail) runs throughout the length of the tail. Anteriorly it termmates m the most anterior centrosome- referred to sometimes as the end-knob-placed immediately behmd, or even embedded m, the nucleus. Behind this are one or more other centrosomes.
The cytoplasmic envelope of the tail extends from the front end of the first to the hmd end of the second region. In the third region only the axial filament is present.
The small part interposed between the head and the tail, that is, between the hind end of the nucleus and the front end of the axial filament, and . containing only the anterior centre some, is sometimes spoken of as the neck, or middle piece. This usage cannot be justified in all cases, as the axial filament may pass right through the anterior centrosome to the nucleus (as in the Amphibian Discoglossus) . The term should therefore be dropped, or apphed to the anterior region of the tail, including aU the centrosomes Though always of the flagellate type, the form of the Vertebrate spermatozoon is variable. Thus, to take a few illustrations (Figs. 25, 26), the acrosome may be large and flattened (spoon-shaped) as in the guinea-pig, or, as is more usual, narrow and pointed (some Amphibia, Reptiles, and Elasmobranch fishes), or much reduced {Phalangista), or apparently absent (Teleostean fishes, possibly Bu-ds). Whether it is really absent or not can, however, only be stated when the origin of the spermatozoon from the spermatid has been studied in these forms. In Birds there is often a remarkable spirally-coiled membrane round the head.
The nucleus may be short and rounded (Teleostei), or short and flat (guinea-pig), or cuneiform (Phalangista), or oval
T5
Fig. 24.- Diagram of a typical vertebrate spermatozoon. H., head; a., acrosome ; n., nucleus ; T, tail; T.l, pars conjunctionis ; T.2, pars principalis ; T.3, pars terminalis; a.c, anterior centrosome ; p.c, posterior centrosome ; /, axial filament ; c, cytoplasm ; e., envelope.
{Tropidonotus), or pointed and elongated, sometimes
exces
1355
50
THE GERM-CELLS
III
sively (Urodela). The anterior end-knob, single (Fig. 25, 2, 3, 25, 1), or much enlarged, as in
centrosome may be a small 4, Fig. 26) or multiple (Fig. Urodeles especially. In Bom
17ra 2^5 -Various spermatozoa. 1. Guinea-pig (Cama) (after Meves).
4. FringUla (the chaffinch). (3 and 4 after Ballowitz.)
binator (a toad) its position near the anterior end of the nucleus is remarkable. The tail filament xs mser ted, there fore, near the front end of the head m this form Thexe ma; be (Pnalangista) one or more intermedxate es. The posterior one, at the end of the first portion of the^d, and therefore some way back, is frequently rmg- ox dxsc
Ill THE GERM-CELLS 51
shaped. Iii Urodeles it lies very far back indeed. The cytoplasm of the anterior region often presents transverse or spiral
Fig. 26. - ^Various spermatozoa. 1. Bufo (the toad) (after King). 2.
Bomhinator (a toad) (after Broman). 3. Siredon (the Axolotl). 4. Perca
(perch). 5. Eaia (skate). (4 and 6 after Ballowitz.)
markings. In the anterior and middle regions (or in the middle region only) the cytoplasm is frequently in the form of a fin, which may have a thickened undulating border, or be spirally coiled round the axial filament. Spermatozoa also vary very greatly in length, as the following
D 2
62 THE GERM-CELLS III
table will show. The lengths are given in thousandths of a millimetre.
Crocodilus 20-27
Esox 43
Homo 52-62
Boa 66
Bufo 62-91
Erinacells ..... 85
Cavia . . . . . 93
IIus 107
Eaia 215
Siredon 360-^30
Discoglossus .... 2250
The gigantic spermatozoa of the Amphibian which comes last in the Hst are not, it is hardly necessary to say, proportionately broad. It may be added that even this length is exceeded by the spermatozoa of an Ostracod Crustacean, Pontocypris monstrosa, which are 5-7 millimetres long.
The Chemistry of the Spermatozoon. The most accurate determinations of the chemical composition of the spermatozoon are those carried out on fish sperms.
In the salmon the head (nucleus) of the sperm consists of fat and nuclein and other substances. The nuclem is itself a compound of nucleic acid (C.^ H^g N^^ 0,J with a protamine known as salmm (C30 Hg^ N,^ 0^), the proportions bemg roughly 60% and 35 % respectively of the head, after removal of the fat. The remaining 5 % consists of inorganic matter (Ca3(P04)2, CaSO^) 2-5%, and an iron-containing organic material (the remainder).
In the herring the protamine known as clupein is apparently the same ; scombrin (mackerel) and sturin (sturgeon) are other protamines obtained from the sperm-heads of fishes.
The tail, in the salmon, contains heat coagulable proteids to the extent of 42 %, and fatty substances, 58 %. The latter include lecithin (50 %), fat (30 %), and cholesterin (20 %).
The metamorphosis of the spermatid into the spermatozoon. As has already been stated, four small cells, the spermatids, are produced from each primary spermocyto by the two maturation divisions. Each spermatid is then directly metamorphosed into a spermatozoon.
Fig. 27.- Metamorphosis of the spermatid into the spermatozoon in the
salamander (after Meves) 1-6, the whole cell ; 7-9, the anterior end;
10-ld, the posterior end of the head. (For explanation see text. )
The investigation of thjs process in many forms, including several Vertebrates, has shown that there is a remarkable constancy in the changes that take place. One or two examples will suffice.
As a first, let us take the salamander (Fig. 27). The spermatid, emergmg from the second maturation division, is a rounded cell, in which the chromosomes are clumped together while the centrosome, lying in the middle of the centrosphere or sphere of attraction (Idiozom), has divided into two, placed tangentially with regard to the surface of the cell (Fig. 27, 1). While the nuclear membrane is being formed round the chromosomes, the centroeomes detach themselves from the sphere (Fig. 27, 2) and adopt a radial position. When the chromosomes break up into granules, a fine filament- the axial filament of the tail- grows out from the centrosome nearest the surface (Fig. 27, 3). This is the posterior centrosome, and it soon becomes first discoidal, then ring-shaped, the axial filament passing through the ring to attach itself to the other or anterior centrosome (Fig. 27, 4). Meanwhfie the sphere- in which a spherical vacuole has been developed - moves away from the centrosomes to the opposite side of the nucleus, which is of course the anterior end (Fig. 27, 4-6). Here it becomes gradually changed into the acrosome or perforatorium. It becomes oval and an axial rod is formed in it. It is protruded from the cell, becomes pointed, and finaUy much elongated and barbed at its extremity, while the vacuole disappears (Fig. 27, 7-9). At the other, the posterior end of the cell, further changes are taking place. The anterior centrosome first attaches itself to the hinder end of the nucleus (Fig. 27, 5)- now elongated and finely granular- then enlarges and embeds itself in the nucleus (Fig. 27, 6), finally lengthening to form a long eUipsoid body. The axial filament has remained inserted into Tt (Fig. 27, 10). In the meantime, an outgrowth of cytoplasm has occurred on one side (dorsal) of the tail filament to form the fin (Fig. 27, 10). When the fin is weU developed, the posterior ring-shaped centrosome breaks into two halves. One half travels down the other (ventral) side of the tail, carrying some cytoplasm with it, and eventuaUy reaches a point near the end of the middle region (pars principaHs). The other haK remains behind and is fused with the anterior centrosome (Fig. 27, 11-13). The nucleus continues to elongate to form the sperm-head, finally becomes homogeneous, and is divested of its cytoplasmic covering.
The history of the sperm in other types, the gumea-pig for instance, is almost the same (Fig. 28).
Ill THE GERM-CELLS
In the spermatid can be seen the sphere- including some dark granules- the chromatoid accessory body, and the two centrosomes (Fig. 28, 1). These are dumb-boll-shaped, the outer or
Fig. 28. - ^Metamorphosis of the spermatid into the spermatozoon in
the guinea-pig (after Meves). 1-4 show the whole cell; 5-9 the head and
the front part of the tail ; 9 is seen in profile. In 1 the accessory chromatoid body is rendered in black. In this and the following figures the sphere
is shaded or (acrosome) stippled. In 6 and 7 the granules of von Ebner
are shown in black. (A full explanation will be found in the text.)
posterior is placed radially and bears the axial filament, the inner tangentially.
The chromatoid body disappears. The sphere moves round to what will be the anterior end ; in it two portions are
56 THE GERM-CELLS III
distinguishable. A spherical body with a dense central spherule, this is derived from the dark granules of the previous stage, and an irregular body applied to the first, derived from the outer portion of the original sphere. This irregular body presently moves back to the hind end and disappears, but the spherical part becomes transformed into the acrosome (Fig. 28, 2-4). It is applied to the front end of the nucleus, and becomes lenticular (concavo-convex) (Fig. 28, 5). The central dense body then vanishes, the whole projects from the front end of the cell, being attached to the front and sides of the nucleus. Finally it becomes thin and curved (spoon-shaped) (Fig. 28, 9). The nucleus meanwhile having become homogeneous is also flattened and curved, its curvature being opposite to that of the acrosome.
The centrosomes have aU this time been passing through comphcated changes. The anterior one becomes flattened against the nucleus, the posterior hook-shaped, one hmb of the hookdirected outwards- bears the tail filament, while the other, or anterior Kmb, is at right angles to it (Fig. 28, 2).
The hinder limb of the posterior centrosome now becomes divided into a ring behind and a knob in front (Fig. 28, 4). The tail filament passes through the ring, on to the Imob, and then on to the middle of the anterior Hmb. The anterior centrosome and the anterior Hmb of the posterior centrosome then become divided, each into three knobs (Fig. 28, 5). The arrangement is therefore as foUows. A row of three knobs united by filaments next the nucleus ; each of these knobs being similarly united to one of the three knobs of the next row, also united together. The middle knob of the second row is miited to another knob, and into this is inserted the axial filament of the taU which passes through the ring. Later the ring passes backwards some Httle way; it marks the end of the first region of the tail (Fig. 28, The tail filament thickens, the posterior knob
being fused with it. . . -i i '
A curious, quite transitory, structure is the tail-sleeve ^Fie 28 4 5) This is a felt-work of fibriUae developed round the froni end of the filament to form a sort of tube. Its existence is short. Most of the cytoplasm-which has by this time passed away from the nucleus to the middle -piece-is peeled off (Fig. 28,
ih
Fig. 29. - Sperm-cells of Amphibia in tlieir cysts or follicles.
A, Section of a single seminiferous tubule from the immature part of the testis of a newt (in winter). The spermogonia [^sp.g.) are enclosed ni follicles (/".c.) ; the theca surrounding the tubule, -ex
B, "Bundle of ripe spermatozoa inside a cyst (c), from the testis of tbe Azo'lotl. B.C., Sertoli-cell in which the acrosomes of the spermatozoa are embedded.
P. 57
Ill
THE GERM-CELLS
67
6, 7), with the remains of the sphere and a number of stainable bodies - the granules of von Ebner - which always appear at this time. The remains of the cytoplasm form a sheath round the middle-piece (pars conjunctionis), and apparently a thin investment for the principal part of the tail. In the middle-piece the characteristic transverse (? annular) striations appear (Fig. 28, 8, 9).
These examples are tj^ical of spermogenesis in general. The acrosome is formed from the sphere, the tail filament grows out from the centrosome ; and even where (as in many Crustacea) the tail is absent, the two centrosomes are still present, and the posterior one becomes transformed into a ring.
The close relation between the locomotory organ of the cell and the centrosome is not peculiar to spermatozoa. In certain Protozoa there is a central corpuscle which not only -acts as an organ of cell-division, but also serves as a base of insertion for the flagella or for the axial filaments of the pseudopodia {Dimorpha, Acanihocystis).
The centrosome of the spermatozoon and the sphere - which becomes the acrosome - are both parts of the original division apparatus. They have, however, distinct functions to perform in fertihzation, for while the latter is the perforatorium, employed for ensuring the penetration of the sperm below the surface of the egg, the latter is the centre round which the primary spermsphere is formed. We shall see, nevertheless, that these distinct processes probably depend upon a property which is common to both bodies, and may be due to their community of origin.
The follick-cells of the testis. Like the ova, the male cells are associated with certain nutrient cells in the testis, known also as foUicle-cells, though they do not always form a covering for the germ-cells. Their origin from the germinal epitheUum has already been referred to.
In the lower Vertebrates the germ-cells commonly occur in bundles, each of which is enclosed in a wrapping of folKcle -cells, a cyst, or foUicIe. The cysts are arranged round the walls of the seminiferous tubules. In the immature tubules of the testis the cysts will be found to be small, each containing only one or two spermogonia (Fig. 29 a). But the number of the latter is
58
THE GERM-CELLS
III
soon moreased by division, and quantities of germ-cells are subsequently found in each single cyst. In each cyst all the germ-cells are usually in the same stage- whether spermogonia, spermocytes, spermatids, or spermatozoa, complete or incomplete- but in the different cysts in the same tubule different stages are found, though they are not usually very widely different in adjacent cysts (Fig. 30).
Fig 30 -Four cysts or follicles from the same seminiferous tubule of
the testis of the Axolotl. 1, contains spermatids ; 2, 3 and 4, three successive stages of the metamorphosis of the spermatid into the spermatozoon.
/., foUicle ; b.v., blood-vessel outside the tubule.
The mature spermatozoa become arranged in bmidles, and inserted, in each bundle, by their acrosomes into a smgle basal cell of the cyst (Fig. 29 b), facing the wall of the tubule. The cyst which has been much distended, then gives way, and the tails of the sperms project freely into the lumen of the tubule.
The basal cells in which the heads of the spermatozoa are embedded are apparently nutrient as well as supportmg. They are known as the cells of Sertoh, a term first apphed to the corresponding cells of the Mammahan testis.
In the Mammaha the germ-cells are also grouped m bundles.
Pia 31 - Testis of mouse. A small part of a section tlu-ongh a seminiferouV tubule in six different conditions {a-f) 1-20 stages in spermogeneiis (for further explanation see text). 8., Sertoli-cell ; th., theca.
P. 59
Ill THE GERM-CELLS 69
but are not enclosed in cysts ; further, they are disposed in several layers, and different stages in development are found in the several layers at one and the same point in a seminiferous tubule (Fig. 31).
The basal layer contains the supporting cells or cells of Sertoh, which (in the mouse) are recognizable by the presence in the nuclei of one large nucleolus and two large spherules of chromatin. In addition to these there are the spermogonia (indifferent cells), which are derived , either from the central cells of the young testis (see Fig. 13) or from the surrounding follicle-cells, or from both.
Internal to this basal layer are about three others, in each of which the germ-cells are in a different stage. For the sake of illustration the whole spermogenesis may be divided into twenty stages, as follows :
1. Indifferent cell or spermogonium.
2. Transition to spermocyte.
3. Primary spermocyte : leptotene stage.
4. Transition to synaptene stage.
5. Advanced synaptene.
6. Pachytene,
7. Pachytene to diplotene.
8. Later diplotene.
9. Commencement of ring-formation.
10. Ring-shaped (heterotypic) chromosomes formed.
11. First matviration division.
12. Secondary spermocytes.
13. Second maturation division.
14. Spermatids.
15. Later spermatids. 16.
17. Commenciag metamorphosis, with short tail filament.
18. Later stage.
19. Appearance of von Ebner's granules.
20. Peeling off of cytoplasm ; spermatozoon complete. Beginning with, for example, a stage in which the spermogonia
of the basal layer are in stage 1, the cells of the second layer m the fourth,, those of the third layer iii the tenth, and those of the fourth layer in the sixteenth stage, the progress of development
60
THE GERM-CELLS
III
in each layer may bo readily watched. As layer ii passes into stage 5, layer in shows first maturation spindles, while the spermatids of the fourth layer begin to be metamorphosed into spermatozoa, and so on. By the time that the spermatozoa of the fourth (inmost) layer are ripe and ready to drop into the tubule, the cells of the third layer have reached the spermatid stage ; those of the second are in a late prophase of the first maturation division, whUe the somewhat flattened spermogonia of the basal layer are becoming cubical and preparing to grow into spermocyiies. By the time the ripe sperms have been thrown off, the young spermocytes have detached themselves from the basal layer and he in a distinct second layer below the third and fourth layers, which are respectively in the ninth and fifteenth stages, and so the starting-point is reached once more.
The germ-cells, therefore, originating in the basal layer, are brought nearer and nearer the lumen of the tubule as the ripe sperms of the inner layer are cast ofE and fresh layers formed from below. As the spermatids undergo their metamorphosis, they become grouped into bundles ; and, in each bundle, the spermatozoa are inserted by their acrosomes into the extremity of an elongated Sertoh cell, which is retracted once more when the spermatozoa have been set free. The tails, therefore, float out into the seminiferous tubulev
The ovum and the spermatozoon are obviously different from one another in almost every respect. The former is large, inert, and, when fully ripe, as we are shortly to see, without a centrosome. It is rich in cytoplasm, contams reserve food material, arid has a structure, related in a very definite way to the structure of the embryo which is to be developed from it.
The spermatozoon, on the other hand, is motile and small, has httle cytoplasm (except hi the tail, which is of no importance in fertiUzation, smce it may be left outside the egg) but is provided with one or more centrosomes, as well as with an apparatus for entering the ovum.
We have still to examme the structure of the nuclei of the germ-cells, a structure which is the result of the pecuhar nuclear changes mvolved m maturation. This exammation will show us that in their nuclei the germ-cells are alike.
LITERATURE
B. M. Allen. The origin of the sex-cells of Chrysemys. Anal. Am. xxix, 1906.
E. Ballowitz. Untersuchungen uber die Struktur der Spermatozoen. L Arch. mikr. Ami. xxxii, 1888. III. Arch. mikr. Anal, xxxvi, 1890.
E. Ballowitz. Die merkwiirdigen, 2\ Millimeter langen Spermien des Batrachiers Discoglossus pictus. Arch. mikr. Anal. Ixiii, 1904.
J. Beard. The germ-cells. Journ. Anal, and Phys. xxxviii, 1904.
E. VAN Beneden et C. Julin. Observations sur la maturation de I'cellf chez les Chiropteres. Arch, de Biol, i, 1880.
U. Bbm. Beitrage zur Entwickelungsgeschiehte der Leibeshohle und der Genitalanlage bei den Salmoniden. Morph. Jahrb. xxxii, 1904.
I. Beoman. Ueber Bau und Entwickelung der Spermien von Bomhinator igneus. Anal. Anz. xvii, 1900.
R. BuRLiN. Chemie der Spermatozoen. Ergebn. Physiol, v, 1906.
C. A. EiGENMANN. On the precocious segregation of the sex-cells in Micrometry aggregatus. Journ. Morph. v, 1891.
0. Hammaesten. Text-book of Physiological Chemistry, trans, by J. A. Mandel. New York, 1911.
H. D. King. The egg of Bufo lentiginosus. Journ. Morph. xvii.
K. VON KoRFF. Zur Histogenese der Spermien von Phalangisla vulpina. Arch. mikr. Anal. Ix, 1902.
E. KoKSCHBLT u. K. Heider. Vergleichende Entwickelungsgeschiehte der wkbellosen Thiere. Allg. Th., Lief. L ii, Jena, 1902.
H. Lams et J. Doorme. Nouvelles recherches sur la maturation et la f6condation de I'cellf des Mammiferes. Arch, de Biol, xxiii, 1908.
F. McClendon. On the nucleo-albumin in the yolk-platelets of the frog's egg. Amer. Journ. Phys. xxv, 1909.
F. Meves. Ueber Struktur und Histogenese der Samenfaden von Salamandra. Arch. mikr. Anal. 1, 1897.
F. Meves. Ueber Struktur und Histogenese der Samenfaden des Meerschweinchens. Arch. mikr. Anal, liv, 1899.
W. Rubaschkin. Ueber das erste Auftreten und Migration der KeimzeUen bei Vogelembryonen. Anal. Hefte, l^e Abt., xxxv, 1908.
W. RuBASCHKiN. Ueber die UrgeschlechtzeUen ' bei Saugetieren Anal. Hefte, I'e Abt., xxxix, 1909.
J. SOBOTTA Ueber die Entstehung des Corpus luteum der Saugethiere. Aiiat. Hefte, 2'^ Abt., viii, 1899.
O VAN DER Stricht. La structure de I'cellf des Mammiferes. Arch, de Btol. XXI, 1905.
W. Waldeyer. Die Geschlechtzellen, in 0. Hertwig, Handhuch der JLnlwicklungslehre der Wirheltiere. Jena, 1906.
1902 ^^^"""^"^"^^l^P^^nt and inheritance. New York.
vZ'JrZ 'T^^- ^^'^^'^^^^ I'ovogen^se et I'organogendse de 1 ovaire des Mammiferes. Arch, de Biol, yiyn, \mi
