Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/432

414 REPRODUCTION [ANIMAU supersede what would otherwise require to be a gigantic store of yolk. Our as yet crude analyses of ova of course reveal the presence of numerous highly nutritive substances, both albuminous and fatty none, however, characteristic of the ovum alone. The general analysis of the Fowl's yolk is quoted by Hensen as follows: water 47 '2 percent., albumin stuffs 15*6, ethereal extracts 31*4, alcoholic extract 4 '8, ash 1 = 100, these results of course covering an unknown degree of complexity. Thus numerous more or less distinct albuminoid substances have been described by different authors vitellin, ichthin, ichthidin, emydin, fee., while nuclein seems to be of special importance ; the ethereal extract yields very distinct fats, not only non- nitrogenous, but nitrogenous (e.g., lecithin) ; cholesterin and many other complex products of anabolic and kata- bolic change are also present ; and so on. The subject is at present peculiarly unfit for profitable summarization, and details must be sought from the bibliography. (10) Oogenesis. Since the different modes of origin of the ova in the various groups are at once less complicated and in general outline less debateable than the corresponding process of spermatogenesis, in regard to which the state of opinion is so unsettled, a much less detailed account must here suffice, especially since a review of the different processes in any detail would involve an unwieldy com- pression of the available summaries of Balfour and others. Only a few of the more interesting aud suggestive examples can be here mentioned. The origin of the ova within the mesoderm of the Sponge, which has been already referred to, is obviously one of the simplest cases, recalling the occurrence of reproductive cells in such a sub- Metazoan form as Volvox. In regard to the Ccelentera the con- troversy over Van Beneden's hypothesis of the endodermic origin of the ova has been already mentioned ; and in connexion with this "Weismann's theory may be noted, according to which in most Hydroids the reproductive cells are only differentiated late in life, so that the actual germinal substance or mother-protoplasm is not present from the first in cellular form but in molecule groups, scattered among the somatic cells and spread perhaps over various asexual generations, to be gathered up at some favourable epoch and in the most convenient situation in the definite form of ova. In the group of Vermes the general history is that some favour- ably situated cells of the epithelial lining of the ccelom proliferate, enlarge, assume the characters of ova, and fall off into the body- cavity. A simple instance of this in Tomopteris has been already referred to (fig. 1). Sagitta furnishes a very suggestive illustration of oogenesis where one half of the primitive reproductive cell goes to form the ovary and the other half the testis. In Bonellia we find a beautiful example of the very frequent subordination of several reproductive cells to the perfecting of one. In each mass of possible ova (which arise from the division of primitive germinal cells situated round the ventral vessel just above the nerve cord) only one adjoining the stalk and therefore near the source of nutrition becomes a differentiated ovum, while the others atrophy. In Platyhelminthes the later stages of oogenesis are especially interesting, because the nutritive equipment of the ovarian ova is in many cases partially furnished by the direct absorption of some of the cells of the yolk- gland or vitellarium, which is itself probably a degenerate portion of the ovarian tract in which over-nutrition has checked repro- duction. Thus the ovum comes to be the result of a number of cells. Weismann's interesting observations on the winter-eggs of some Crustaceans afford beautiful illustration of the subordination of a large number of germinal cells to the nutrition of a few, while in Alollusca this nutrition of the ovum is otherwise effected by the direct passage of food-material from the ordinary epithelial cells of the ovarian pouches. The oogenesis of Insects, which has been the subject of so much discussion, is chiefly characterized by the very frequent presence in Fio. 7. Formation of follicular nuclei (after Will). A, B, division of nucleus in morula fashion ; C, nuclei have travelled to periphery ; D, germinal vesicle formed from residue of division. the ovarian tube of a large number of germinal cells which are wholly nutritive, and which serve to equip the minority of truly reproductive cells. Such cases inevitably raise the long-standing dispute as to the unicellular character of the ovum, some authors (e.g., Brandt) describing the ovum as a cell of the second order, formed from a cemplexof the primitive nuclei or "ooblaste/'which are combined in the terminal chamber of the ovarian tube, and which unite to form on the one hand the germinal vesicle of the ovum with enveloping protoplasm, and on the other the surrounding folli- cular cells. One of the most recent discussions of Insect oogenesis is that of Will, who maintains the origin of the follicular nuclei from a multiple division of the original primitive nucleus or ooblasr, the residue forming the germinal vesicle, a mode of origin pre iously maintained by other investigators both of oogenesis and spermato- genesis (fig. 7). In some Echinodermata, and in other groups, it has been repeatedly observed that the undifferentiated germinal cells form the elements of the follicular epithelium round the ova. A debateable but suggestive theory of oogenesis has been pro- pounded, especially by Nussbaum and Weismann, who maintain that in many cases the reproductive cells do not arise as differentia- tions of somatic cells, but are marked off from the first, in some cases even before the formation of the germ-layers. According to this ingenious hypothesis, supported, however, by little direct observation the reproductive elements would form a continuous immortal chain connecting the highest forms with their Protozoan ancestry, whose direct protoplasmic continuity has been of late also strongly insisted upon (see PKOTOZUA). Dispute has also arisen as to the origin of the follicular cells of the Tunicata, some deriving them from external juxtaposition of germinal cells, and others from migration of nuclei from within outwards, as in some J/isecta ; and a similar uncertainty prevails as to the nature and origin of certain cell-like bodies ("test-cells") which appear within the yolk. In the Craniata the ova appear in the germinal epithelium of the ovarian ridge, which is always in contact with .the stroma. They differ from the surrounding cells at first mainly in their greater size, being possessed of abundant protoplasm and a large granular nucleus. Increasing in number by division or by con- tinued differentiation of other epithelial cells, the primitive ova usually form into masses, as the result of which some atrophy and others predominate. The permanent ova once formed and defined are surrounded by a special follicle, probably resulting in most cases from superjacent epithelial cells. None of the disputes above referred to can be said to invalidate the general view of the essentially unicellular nature of the ovum (see below) ; and a discussion of the numerous speculations on the more fundamental problem of the stages of sexual differentiation is deferred to the article SEX. (11) Structure of the Ovum. While the structure of a suffi- ciently young ovum is simply that of an ordinary embry- onic cell, its protoplasm being naked and often amoeboid, and provided with a nucleus and nucleolus, the developed ovum has usually a quite characteristic appearance. Not only have its parts usually undergone considerable enlarge- ment in size, but also in details of minute structure ; the nucleus and nucleolus are, however, still recognizable as the germinal vesicle and germinal spot, while the protoplasm has usually become modified by the presence of a more or less considerable quantity of food-yolk, and by the de- velopment of an external membrane. The young amoeboid phase of so many (perhaps all) ova was first well described in the egg of Hydra (fig. 8, 6), in which the nucleus and nucleolus, the lobed pseudopodial processes of the protoplasm, and the abundant yolk spherules can be well made out. A somewhat later phase of development is well shown in the next figure of the egg of a Sea-Urchin, in which the process of encystment has begun, and the protoplasm is seen with its amooboid processes radiating through the incipient egg-membrane, while in fig. 8, d the protoplasm has no longer an amoeboid FIG. 8. a, diagram of ovum showing granular protoplasm, nucleus (germinal vesicle), and nucleolus (germinal spot) ; b, amoeboid ovum of llijdra (from Balfour after Kleinenberg) ; c, early ovum of Toxopneustes variegatus, with pseudopodia-llke processes (from Hulfour after Selenka) ; d, ovum of Toxo- pneuites lividus, more nearly ripe (from Balfour after Hertwig.) character the investing membrane or zona radiata is regularly perforated by radiating canals, of which the