Book - An Introduction to the Study of Embryology 1
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Haddon An Introduction to the Study of Embryology. (1887) P. Blakiston, Son & Co., Philadelphia.
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Chapter I. Maturation And Fertilisation Of The Ovum
Introduction
Embryology is the term usually applied to the whole cycle of changes undergone by an animal in passing from an egg to the adult condition. It is, in other words, the History of its Development.
The name of embryo (or foetus, in mammalian embryology) is restricted to the unborn young. At birth the young may closely resemble the parent, or be very dissimilar ; in the latter case, it is known as a larva, and undergoes a series of changes or a metamorphosis before it attains the adult state.
Even closely allied animals may be “ born - at very different stages in their development ; the higher animals are, however, generally born at a relatively later stage than those lower in the animal scale. They are thus better fitted for the struggle for existence, and expend less energy during their development than if they had to provide for themselves.
In the higher animals the young also have the further advantage of the watchful care of their parents, a factor which must have materially influenced the evolution of the race.
Embryology may be studied under two aspects. The first, or Ontogeny, deals solely with the history of the individual, and traces the development of the animal as a whole, and of its various organs.
The second, or comparative aspect, compares the development of animals, and taking those phases which are common to all or to many, attempts therefrom to deduce or reconstruct the evolution of the animal kingdom. This study is known as Phytogeny.
The chief result of all embryological inquiry has been to demonstrate that the history of the individual recapitulates in its main features the evolution of the race, and thereby to give positive evidence in favour of the Theory of Evolution, in the general acceptance of the term.
It is very important to bear in mind that larval forms, as well as adults, have to adapt themselves to external conditions, and that they are consequently liable to be variously modified, and, within limits, to be highly specialised. These modifications often have no relation to the adult structure, and consequently can have no phylogenetic significance.
Some preliminary knowledge of Zoology and Comparative Anatomy is necessary in order to appreciate fully the phases in the development of any one animal, and it is, of course, essential in studying the general principles of Embryology, as constant reference must be made to the structure of different forms. Such a knowledge will be assumed for the readers of this book.
The Animal Kingdom is divided by zoologists into the Protozoa, or unicellular animals, and the Metazoa, or those animals composed of a number of cells so united together as to form tissues. As the ' latter alone produce ova or eggs, the science of Embryology deals solely with the Metazoa. Although there is considerable variation in the details of the classification of the Metazoa, zoologists are tolerably well agreed upon the main divisions, and in this work that classification and terminology are adopted which are in most general use in English-speaking countries.
Reproduction amongst the Protozoa consists in a direct or indirect method of cell- division, each product of such division forming a new individual (fig. I, B, c, d). This process may, or may not, be preceded by a temporary apposition or permanent fusion of two or more individuals. The conjugating individuals may either be apparently quite similar (fig. i, e), or may exhibit certain differences (fig. I, f) ; but conjugation is always effected between forms which are similarly motile - that is, ciliated individuals invariably conjugate with ciliated, and amoeboid with other amoeboid, forms. Even among those Elagellate Infusoria which pass through a comparatively complicated life-history, an individual in the flagellate stage never conjugates with another in the amoeboid condition. Active reproduction of one kind or other usually occurs after conjugation.
Some Protozoa form compound masses, but the individuals composing the colony are, with rare exceptions (Proterospongia), similar to one another, and have a practically independent existence.
Although asexual reproduction by various modes of budding and fission is known in nearly all the groups of the Metazoa, the sexual method is of invariable occurrence. The essential act of this form of reproduction consists in the fusion of a flagellate cell or spermatozoon with an amoeboid cell, the egg or ovum (figs, io and n).
Fig. 1. - Reproduction amongst Protozoa. Not drawn to scale.
A-C. Fission in an Amoeba. A. The nucleus has divided into two. B. Two contractile vacuoles have also formed and the protoplasm is dividing. C. The process is complete. [ After Howes.']
D. Fission in Paramsecium bursaria. There are two contractile vacuoles and two paranuclei, but the nucleus has not yet completely divided.
E. Conjugation of Stylonychia mytilus, illustrating also the fragmentation of the nucleus.
F. Conjugation of Vorticella microstoma. Two free-swimming microzooids have attached themselves to a fixed form. They all possess a curved nucleus and a contractile vacuole. [D-F after Stein.]
c.v. contractile vacuole ; n. nucleus ; nl. paranucleus.
In a very few cases the spermatozoa are either amoeboid, as in Nematodes, some Arachnids, and Limulus, or often passive and rayed, as in most Crustacea; but in the great majority of animals the spermatozoa are flagellate and actively motile (fig. 2).
The ovum, under very rare and exceptional conditions, may develop into a new organism without previous fertilisation by a spermatozoon ; this phenomenon is known as 'parthenogenesis.
The ovum and spermatozoon unite to form the fertilised ovum or oosperm, which then undergoes rapid cell-division ; the cells thus produced remain in contact with one another, and though at first usually very similar, certain groups of cells soon take upon themselves definite characters, and thus initiate the primitive tissues.
Accepting the view that the Metazoa were derived from colonial Protozoa, it follows that every cell of the primitive Metazoa was capable of forming fresh colonies by
Fig. 2. - Spermatozoa [from various sources]. Not drawn to scale.
i. Sponge; 2. Hydroid; 3. Nematode; 4. Crayfish; 5. Snail; 6. Electric Ray ; 7. Salamander ; 8. Horse ; 9. Man. In many spermatozoa, as in Nos. 7 and 9, an extremely delicate vibratile band is present.
cell-division. Many Metazoa possess the power of asexually producing new forms by fission or by budding ; but the tissues implicated in this process must be regarded as being essentially undifferentiated in character.
Owing to the advantage derived from physiological differentiation of labour, the reproductive function came to be chiefly retained by certain cells, the remainder specialising along other lines. Those cells which pre-eminently retain the reproductive function are restricted in their position, and the tissue which they constitute (the germinal tissue) is contained within what is known as a generative organ or gland. When ripe, the germ-cells become detached, and commence a free existence.
After fertilisation, the ovum, or the embryo into which it develops, is in a few cases retained within the oviduct of the mother for a longer or a shorter period, and may temporarily even be intimately, but very rarely structurally, connected with the walls of the oviduct or uterus, as will subsequently be described.
The primitive germ-cells of animals are, practically, precisely similar to one another (fig. 175), and, when first recognisable as germ- cells, it is impossible to tell whether they will develop into ova or sperm-cells. In this connection it is suggestive to find that both the ovaries and the testes in Sagitta are developed from a single primitive germ-cell, which makes its appearance at a very early stage of development. The primitive germ-cells may more especially be said to correspond to the Protozoon ancestors of the Metazoa.
Before dealing further with the history of the germ-cells, however, it will be advisable to describe briefly their mode of origin.
The Ovum
The primitive ova usually form part of a definite epithelium, of which most of the cells, or it may be only a very small number, develop into ripe ova. The germinal epithelium is well supplied with nutritive fluid (either blood or the fluid contents of the cavity of the body), which serves for the growth of the ova. From the nutriment thus provided the ova generally store up a greater or less amount of reserve food-material, which is known as “ yolk - or “ food-yolk.-
It would be foreign to the purpose of this work to enter into a comparative account of the development of ova from primitive germinal-cells. As a general rule, certain of the cells of the germinal epithelium are directly converted into ova. In Vertebrates, the germinal epithelium is borne upon a distinct germinal ridge ; the epithelium increases in thickness, and becomes broken up into cords or trabeculae (ovarian tubes of Pfluger), which, by mutual ingrowth, lie in the stroma or mesoblastic core of the germinal ridge. Isolated masses or nests may also be formed (fig. 3).
Balfour has shown that in Elasmobranchs and other forms, in addition to the foregoing or direct origin of the ova, the protoplasm of the cells forming the nests fuses into a single mass containing the nuclei of the previously distinct ova. Various changes are undergone, but eventually a few of the nuclei segregate protoplasm round themselves to form the ova, the remainder having broken down to pabulum for the permanent ova.
Beddard finds that in Protopterus two kinds of ova are developed - (a.) The ovum is a mass of granular protoplasm, containing a germinal vesicle limited by a distinct membrane, inside of which is a peripheral layer of germinal spots. Later the protoplasm becomes vacuolated, and largely differentiates to form yolk-granules. (6.) The ovum arises from the fusion of a nest of germinal cells lying within a follicle ; not only is yolk formed within the central mass, but it is also produced within the columnar cells of. the follicular epithelium. These cells proliferate and migrate into the interior of the ovum ; eventually they disappear. The yolk of these ova appears to be largely derived from the follicular cells.
The yolk consists of highly refractive particles, which vary considerably in their appearance and structure. As a rule, the yolk elements are small vesicles, which usually contain smaller vesicles and other bodies (fig. 28, b). In Birds the whole of the yolk at first consists of these white yolk spheres ; but during the development of the egg, some of the white yolk spheres become modified to form the yellow yolk (fig. 28, A and c). In the ripe unincubated egg the yellow yolk constitutes the great mass of the “yolk,- the white yolk being restricted to a peripheral and several concentric layers, and to a central mass which extends in a constricted neck, and again widens out to form a bed, upon which the blastoderm rests (fig. 28, A, w, y ).
It not unfrequently happens (many Hydrozoa, Insects, some Vertebrates, &c.) that certain of the primitive germ-cells feed upon neighbouring germ-cells, so that the growth of the ovum
Pig. 3. - Section through a Portion of the Ovary of a Mammal. Illustrating the mode of development of the Graafian follicles. [From. Wiedersheim. ]
D. discus proligerus ; Ei. ripe ovum ; G. follicular cells of germinal epithelium ; g. bloodvessels ; K. germinal vesicle (nucleus) and germinal spot (nucleolus) ; KE. germinal epithelium ; Lf. liquor folliculi ; Mg. membrana- or tunicagranulosa or follicular epithelium ; Mp. zona pellucida ; PS. ingrowths from the germinal epithelium, ovarian tubes, by means of which some of the nests retain their connection with the epithelium ; S. cavity which appears within the Graafian follicle ; So. stroma of ovary ; Tf. theca folliculi or capsule ; U. primitive ova. When' an ovum with its surrounding cells has become separated from a nest, it- is known as a Graafian follicle.
and its store of food-yolk are made at the expense of its fellow germinal cells. In most Platyhelminths that portion of the primitive germinal epithelium which is destined to provide pabulum for the ova proper is separated from the ovary as yolk-glands, or vitellaria , and their products, yolk-cells or yolk-granules, surround the ova after they have left the ovary, and before they are enclosed within the egg-capsules. The yolk-cells may be regarded as germinal cells which have lost the power of reproduction, hut retained that of forming yolk. Either the ovum or the embryo in due course feeds upon this reserve of food.
When many ova are deposited within the same egg-capsule as in some forms of Prosobranch Gastropods (Buccinum), the more advanced embryos devour those that are imperfectly developed, so that a very limited number, sometimes only a single individual, eventually escape from one capsule.
The fusion of several germinal cells with one ovum does not correspond to the multiple conjugation of some Protozoa, as in the
Fiq. 4. - Diagrams of Ova [from, various sources after Geddes ]. Not drawn to scale.
a. Diagram of a typical ovum with a delicate egg-membrane, granular protoplasm, nucleus (germinal vesicle), and nucleolus (germinal spot), b. Amoeboid ovum of Hydra [after Kleinenberg ]. c. Early ovum of a Sea-Urchin (Toxopneustes variegatus) with pseudopodia-like processes extending into the gelatinous eggmembrane (vitelline membrane) in order to obtain nutriment from without ; afterwards they become much finer and more regular, causing the vitelline membrane to have a striated appearauce ; hence it is termed the * • Zona radiata -
- the striae are really delicate pores [after Selenka ]. d. Nearly ripe ovum of Strongylocentrotus lividus with its zona radiata [after Herticig].
formation of plasmodia ; it is merely the assimilation of several cells by one ovum, much as an Amoeba feeds upon its prey.
An ovum is a small free cell which is characterised in the resting-stage by possessing a large clear nucleus, the germinal vesicle, and a well-marked highly refractive nucleolus, the ger
Fig. 5. - Ovum of the Cat. Highly magnified ; semi-diagrammatic. [From Quain, after Schafer.]
gs. germinal spot ; gv. germinal vesicle ; vi. vitellus, or protoplasm of ovum filled with yolk granules, round which a delicate membrane was seen ; zp. zona pellucida ( Zona radiata ) ; only a few radial pores are drawn.
minal spot ; in many cases several germinal spots occur. Pigs. 4 and 5 illustrate various kinds of ova.
The protoplasm usually has, as has just been mentioned, the power of storing up albuminoid matter as reserve food material by a differentiation of its own substance in the form of yolk-granules or spheres. The amount of food-yolk varies greatly ; in some few instances none appears to be differentiated ; often only a little is formed ; more frequently there is a considerable amount ; and in the eggs of Elasmobranchs and of Sauropsida an enormous quantity is deposited. The distribution of the yolk within the egg also varies, being either chiefly concentrated at one pole ( telolecithal ), or towards the centre {centrolecithal), or evenly distributed throughout ( alecithal ).
As the amount of protoplasm in an ovum containing much foodyolk is relatively small, the storing of the yolk-granules within its substance would naturally cause it to be distended. In those ova with a very large amount of yolk, the protoplasmic reticulum scarcely more than serves to keep the yolk-granules together
Fig. 5*. - Typicai. Cell and Nucleus of the Intestinal Epithelium of a Flesh-Maggot (asticot), treated with osmic acid vapour. [From Camay. ]f
bn. continuous band of nucleine, contracted to the centre of the nucleus, and showing numerous twists ; me. membrane of the cell ; mn. membrane of the nucleus ; pc. protoplasm of the cell, showing the radiating reticulum and the enchylema enclosed in its meshes ; pn. plasma of the nucleus, showing a reticulum and a plasmic enchylema, as distinct as those of the protoplasm.
The structure of an ovum is practically identical with that of such a tissue-cell as the above.
During development, certain cells of the embryo reconvert the food-yolk into active protoplasm.
The germinal vesicle of the unripe ovum, as Carnoy points out, has the same general structure as the ovum itself; that is, it consists of an extremely fine protoplasmic reticulum , the meshes of which are filled with a granular fluid {enchylema). The reticulum also forms a delicate nuclear membrane. But, in addition to the above, the nucleus possesses a distinctive substance, which i& variously termed nucleine , nucleoplasm , or, from its being readily stained by the action of certain reagents, chromatin. In very young ovarian ova, the chromatin occurs in the form of a very long, extremely and irregularly contorted thread or nuclear filament.
The nuclear filament is condensed in more mature ova into a single spherical mass, the germinal spot, or into a few or a large number of smaller germinal spots.
Ova may be either naked (fig. 4, A and b), or surrounded by one or more membranes (fig. 4, c, D, and fig. 5). The primary eggmembranes (vitelline membranes) are usually differentiated from the protoplasm of the ovum itself.
In Vertebrates two egg-membranes are usually present, an external delicate vitelline membrane, which is probably formed by the ovum itself, but in some cases a similar membrane may be secreted by the epithelium of the ovarian follicle. This membrane is often termed a chorion. Below the vitelline membrane a thicker membrane, perforated by innumerable fine radial pores, is differentiated out of the peripheral layer of the ovum. It is known as the. zona radiata or zona pellucida. The secondary egg-membranes are either
Fig. 6. - A Fowl -s Egg after about Thirty Hours - Incubation. Viewed from above, the upper portion of the shell being removed. [ From Kolliker after Von Baev. J
' a. shell ; 6. shell-membrane ; b'. airchamber at broad end of egg between the two layers of the shell-membrane ; c. the boundary between the outer and middle portion of the albumen ; d. the internal layer of more fluid albumen, which also extends round the yolk as a thin sheath ; e. chalaza ; v. vitellus or yolk ; av. area opaca, or that portion of the blastoderm which extends over the yolk ; the heart-shaped central portion, ao, is the vascular area of the area opaca. In the centre is the embryo surrounded by the area pellucida.
secreted by accessory generative glands, or by the glandular wall of the oviduct. When a secondary egg-membrane is impregnated with calcareous deposits, it is known as an egg-shell. The secondary egg-covering often encloses an albuminous glairy fluid - the white of egg - which serves for the protection and further nutriment of the embryo (figs. 6, 74, 75). The albumen also is secreted, either by special glands (most Invertebrates), or by the wall of the oviduct (Vertebrates).
Maturation of the Ovum. - Before or after fertilisation, certain changes, which are of considerable interest, take place in the ovum. The germinal vesicle often becomes amoeboid, and passes to one pole of the ovum, and the germinal spot disappears (fig. 7, b-d) ; in fact, both the germinal vesicle and spot disappear as such, and pass into those karyolitic figures which characterise cell-division (see p. 18).
The resulting nuclear spindle is placed vertically, the peripheral nuclear star, or “ aster,- being situated in a small protuberance from the surface of the ovum. This process is segmented off from the ovum, and a minute cell is formed, containing a portion of both the protoplasm and the nucleus of the parent-cell (fig. 7, F and l).
A. Ripe ovum with excentric germinal vesicle and spot ; B-D. Gradual metamorphosis of germinal vesicle and spot, as seen in the living egg, into two asters ; F. Formation of first polar cells and withdrawal of remaining part of nuclear spindle within the ovum ; G. Surface view of living ovum in the first polar cell ; H. Completion of second polar cell ; I. A later stage, showing the remaining internal half of the spindle in the form of two clear vesicles ; K. Ovum with two polar cells and radial striae round female pronucleus, as seen in the living egg. [B, F, H, and I, from picric acid preparations.] L. Expulsion of first polar cell.
This phenomenon is repeated, and two cells are budded off from the ovum ; these are known as the “ polar cells - (or as polar bodies, polar globules, directive bodies, &c.), from the fact that they are invariably derived from that pole of the ovum at which the epiblast or upper-layer cells will be developed; hence, also, this pole is
Fig. 8. - Formation op Polar Cells in Ovum of Elysia viridis.
The upper pole of the ovum becomes amoeboid during the formation of the polar cells. The second polar cell is in process of formation.
usually termed the upper pole of the ovum (see figs. 12 and 17). During the production of the polar cells, the ovum, especially at its upper pole, may exhibit amoeboid movements ; this is well shown in the ovum of Elysia (fig. 8).
Although the polar cells may remain attached to the developing ovum for some time, they take no share in the formation of the embryo, and are simply to be regarded as superfluous bodies.
What remains of the primitive nucleus passes towards the centre of the ovum, usually in an inactive or resting condition, being without radial striae. It is known as the female pro-nucleus.
The ovum is now in a passive condition, and ready to be fertilised. The extrusion of the polar cells, though occasionally taking place after fertilisation (ex. Elysia, fig. 8), is really to be regarded as the last term in that series of changes which occurs before impregnation, and to be, in fact, anticipatory of it.
Before following the history of the ovum further, it will be necessary to return to the sperm-cells.
The Spermatozoon. - Although we find considerable variation in certain details of structure, there is a general similarity in the appearance of the spermatozoa of animals, a head and vibratile tail being of almost universal occurrence : the most important exceptions have already been mentioned (p. 3 and fig. 2).
The primitive sperm-cells or mother-cells of the spermatozoa arise from a tissue corresponding to that which gives origin to the primitive ova (p. 242, fig. 175). The exact manner in which the spermatozoa are developed varies in different animals, and has been variously described by numerous investigators. This being the case, it will be advisable to give simply a sketch of what appear to be the most important facts in spermatogenesis , as this process is termed.
Those cells of the generative epithelium which develop into male sexual cells undergo cell-division in the ordinary manner, and may give rise to a considerable number of cells ( spermatoblasts ). Each spermatoblast is converted into a spermatozoon, and, in doing so, gives rise to a small mass of protoplasm, the so-called seminal granule , or globule, or accessory corpuscle, which appears to have no further function. Fig. 9, a-h, illustrates this process in the Rat.
Instead of becoming distinct, the spermatoblasts or incipient spermatozoa may remain aggregated together ( spermosphere or sperm-morula), and surround a central non-nucleated protoplasmic mass (the sperm-Uastophore ), as in the case of the Snail and Earthworm (fig. 9, o-s).
In Elasmobranchs (fig. 9, i-n) the nucleus of the sperm-cell (sometimes called the spermatocyst) alone divides, forming a number of daughter-nuclei, the remains of the parent-nucleus still persisting. The protoplasm of the cell differentiates into the tails of the spermatozoa, while the daughter-nuclei constitute the main portion of their heads. The ripe spermatozoa are liberated by the rupture of the wall of the sperm-cell, leaving behind the parent-nucleus and a small remnant of unused protoplasm. This latter is merely an abbreviated variation of the former process, and the residual nucleus and protoplasm clearly correspond to the accessory corpuscle or to the sperm-blastophore in the preceding forms.
The nucleus of each daughter sperm-cell constitutes the head of a spermatozoon ; it is surrounded by an extremely delicate film, which is produced from one end into a fine flagellum, and sometimes also into an almost imperceptible undulating membrane ; these are formed by the protoplasm of the spermatoblast. Every spermatozoon is thus a true morphological cell.
Kolliker, however, maintains that the entire mammalian spermatozoon is simply a free nucleus.
Fertilisation of the Ovum
It is needless to recount the various ways by which spermatozoa may reach ova ; suffice it to say, that either within the female or in the surrounding water a
A-H. Isolated sperm-cells of the Rat, showing the development of the spermatozoon, and the gradual transformation of the nucleus into the spermatozoon head. In G the seminal granule is being cast off. [ After H. H. Brown.]
I-M. Sperm-cells of an Blasmobranch. The nucleus of each cell divides into a large number of daughter-nuclei, each one of which is converted into the rodlike head of a spermatozoon.
N. Transverse section of a ripe cell, showing the bundle of spermatozoa and the passive nucleus. [I-N after Semper .]
O-S. Spermatogenesis in the Earthworm : O. young sperm-cell ; P. the same divided into four ; Q. spermatosphere with the central sperm-blastophore • R. a later stage ; S. nearly mature spermatozoa. [After Blomfield .]
spermatozoon comes into contact with an ovum, and either penetrates any membrane which may surround it, or passes through an aperture (micropyle) left in the egg-membrane.
When the spermatozoon is approaching the actual surface of an ovum, a process from the latter sometimes rises up to meet it, and a fusion is effected (fig. io, a-d, and fig. n, a). The head of the spermatozoon penetrates the ovum, while the tail, after vibrating feebly, is absorbed.
The head, or rather the nucleus of the spermatozoon, is converted into an aster or star, and is now known as the male pro
Fxo. io Fertilisation of Ovum of a Star-fish (Asterias glacialis). [ From Geddes after FoL]
In A-D the spermatozoa are represented as imbedded within the mucilaginous coat of the ovum. In A a small prominence is rising from the surface of the ovum towards the nearest spermatozoon ; in B they have nearly met, and in G they have met. D. The spermatozoon has penetrated the ovum, and a vitelline membrane with a crater-like opening has been formed, which prevents the entrance of other spermatozoa. H. ovum showing polar cells and approach of the male and female pro-nuclei ; the protoplasm is radially striated round the former.
E. F. G. later stages in the coalescence of the two nuclei.
nucleus. It travels towards the female pro-nucleus, which, it will be remembered, is situated in the centre of the ripe ovum (fig. io, h). The female pro-nucleus becomes somewhat amoeboid, and fusion occurs between the two elements, thus forming a new nucleus (fig. io, e-g). While this is taking place, the ovum itself often exhibits amoeboid movements (fig. n, a).
Fig. ii. - Fertilisation of Ovum of Elysia viridis.
A. ovum sending up a protuberance to meet the spermatozoon ; B. approach of male pro-nucleus to meet the female pro-nucleus ; F.PN. female pro-nucleus ;
M.PN. male pro-nucleus; S. spermatozoon.
The fertilised ovum is a very different body from the primitive ovum, as it consists of a portion of the original protoplasm and nucleus of the latter reinforced by those of another cell, which is usually derived from a different animal. The new nucleus is called the segmentation nucleus , and it may be well to adopt Balfour -s name of oosperm for the fertilised ovum.
There is some doubt whether the male pro-nucleus has the full value of a true nucleus, and this has led Flemming to define fertilisation as the union of “the chromatin of a male with that of a female nuclear body.- Yan Beneden has recently shown that the essential act of fertilisation consists in the grouping together (or probably, more accurately, of the fusion) of the chromatin or germ-plasma of the nucleus of the spermatozoon with that of the nucleus of the ovum. During the first division of the oosperm, and in all the succeeding phases of segmentation, each new cell receives an equal share of the paternal and maternal chromatin. (See chap. ii. p. 19, and fig. 13.)
The fertilisation of an ovum by a spermatozoon is paralleled by the permanent conjugation Cf such Protozoa as Vorticella and many Monads. In each case the phenomenon is followed by rapid cell-division - the resulting cell-units remaining separate in the Protozoa, whereas they group themselves together so as to form an aggregate of a higher series in the Metazoa.
Significance of the Maturation and Fertilisation of the Ovum
There have been numerous speculations concerning the significance of the polar-cells. The view now generally accepted is that first propounded by Minot, and subsequently (but independently) proposed by Balfour, which suggests that the polar-cells represent what may be regarded as the male element of the primitive germinal cell, the sexes not being supposed to be differentiated in the latter. The ovum is thus preparing itself for the reception of a vigorous element derived from a different source. Similarly, the accessory corpuscle, or its equivalent, is regarded as the female portion of the primitive sperm-cell, the remaining nuclear matter and protoplasm being used up in the manufacture of unisexual (male) cells. The mature ovum, being unisexual, is free to conjugate with a male cell. The two are mutually complemental, and after union constitute a single perfect unit.
The relations between the male and female elements may, according to this view, be thus tabulated
Indifferent germinal cells, which eventually specialise into
ovum (oospore), | sperm-cell (spermospore).
Each by cell- division develops into
(oosphere), | (sperm-morula) spermosphere, *
which is composed of A. a passive element,
polar-cells, j sperm-blastophore (seminal globules or
granules) ;
B. an active sexual element,
mature ovum, I spermatoblasts, which are directly
I converted into spermatozoa.
The union of the latter constitutes the fertilised ovum (oosperm).
Minot proposed the common term of thelyblast for a mature ovum and for a spermblastophore, and arsenoblasts for the polar-cells and spermatozoa. Sexual reproduction would thus consist in the union of a thelyblast from one source with an arsenoblast from another source.
In Mammals the sperm-cell gives rise to spermoblasts, each of which gives off a seminal globule, the remainder differentiating into a spermatozoon.
Unfortunately, tlie terms employed in describing the various stages in the development of the generative elements are not used in a synonymous sense by the various investigators and writers on the subject ; those in the most general use have been here adopted.
A more simple view is that the extrusion of the polar-cells prevents the parthenogenetic development of the egg, merely by eliminating a considerable quantity of nuclear matter. The researches of Van Beneden on Ascaris have demonstrated in a quantitive manner the amount of chromatin thus lost ; the precise amount for Ascaris being three-fourths of that present in the nucleus of the ovarian ovum.
The spermatozoon supplies a sufficient amount of new chromatin to enable the embryo to develop. According to this view, there is no essential distinction between the chromatin of the male as opposed to that of the female germ-cell.
At the end of this work will be found a summary of Weismann -s and Geddes - conclusions respecting the significance of the maturation and fertilisation of the ovum.
The next series of changes undergone by the oosperm is that known as segmentation. The unicellular oosperm divides, by ordinary cell-division, into a large number of cell-units. The resulting mass is a multicellular organism, whose “ life - consists of the sum-total of the activities of its component cells. It is thus an individual of a higher order than a Protozoon, and one possessing an infinitely greater capacity for progressive evolution.
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