46 



OVUM. 



mals, it is uncertain what becomes of its sub- 

 stance. 



7. In the greater number of animals the 

 ovum acquires some additional parts, such as 

 albumen, external coverings (shell, chorion 

 &c.), in its descent through the passages of 

 the female parent after leaving the ovary ; in 

 some animals an external covering corre- 

 sponding with the chorion is already formed 

 in the ovary. 



8. When the spermatic substance of the 

 male parent has come in contact with the 

 ovum, so as to exert its peculiar fecundating 

 influence upon its germinal part an action 

 which in a few animals occurs before, but in 

 the greater number shortly after it has left 

 the ovary the first of the series of changes 

 which follows, and is preparatory to laying 

 the foundation of the new being, consists, in 

 almost all animals, in a peculiar process of 

 division, cleavage, or segmentation, which 

 affects either the whole yolk, or a part of it, 

 or that portion only in the vicinity of which 

 the germinal vesicle has before been situated, 

 and in which the rudiments of the embryo 

 afterwards make their appearance. This sub- 

 division proceeds continuously, till the whole 

 germinal part of the yolk is reduced by it to 

 a nearly uniform mass of corpuscles, or struc- 

 tural elements of microscopic size, which ge- 

 nerally occupy or are spread over more or 

 less of the surface of the yolk mass within the 

 vitelline membrane. 



9. The rudiments of the embryo, and its 

 accompanying organised structures in the 

 ovum, when such exist, are first formed in 

 the centre of a layer of nucleated organised 

 cells, named by Pander Blastoderm or ger- 

 minal membrane, the formation of which re- 

 sults more or less directly from the process of 

 segmentation above referred to. The dis- 

 appearance of the germinal vesicle when the 

 ovarian ovum arrives at maturity, the influence 

 of fecundation acting about the same time, 

 the process of yolk-segmentation which im- 

 mediately follows the latter change, and only 

 occurs as a consequence of fecundation, and 

 the formation of the germinal membrane or 

 blastodermic layer of cells, in which the seg- 

 mentation results, are successive phenomena of 

 change in a fruitful ovum, which undoubtedly 

 stand in some very immediate and close re- 

 lation to each other ; but the exact nature of 

 that relation is still involved in some degree 

 of obscurity. In some animals the process of 

 yolk-segmentation seems either in itself to be 

 a process of rapid cell-formation, or to be 

 accompanied by it; while in other animals 

 (chiefly those highest in the scale), the seg- 

 mentation is only the immediate prelude to 

 the generation of the true nucleated cells, 

 which afterwards constitute the blastoderm. 



10. The centre of fcetal development is co- 

 incident with the place of the germinal vesicle 

 in the mature egg, and with the centre of the 

 blastodermic layer of cells ; and it would ap- 

 pear, also, that the principal axis of the em- 

 bryo is coincident with the line of first division 

 of the germinal part of the yolk in the com- 



mencement of segmentation : but it is yet 

 undecided what part the germinal vesicle or 

 its nucleus or macula more immediately take 

 in the segmenting process, or in the origina- 

 tion of the true embryo-cells ; and in the 

 higher animals, at least, it is still uncertain 

 whether or not these embryo-cells are the 

 direct progeny or descendants of the germinal 

 vesicle, or its nucleus. The germ spot, the 

 centre of the first segmentation, and the centre 

 of embryonic formation, constitute, therefore, 

 a constant point in all animals, which may be 

 termed the germinal centre of the ovum. 



Division of the Ova of Animals into Groups, 

 Although the researches of modern ovo- 

 logists thus enable us to assert with confi- 

 dence the general similarity of structure in 

 the ova of animals, and to point out general 

 features of correspondence in the phenomena 

 which they exhibit in their first origin, pro- 

 gress to perfection, and conversion into the 

 rudiments of the new being ; yet it must be 

 admitted that very important differences are 

 also to be observed among various ova, more 

 especially when they have reached a state of 

 maturity. In what has previously been af- 

 firmed, therefore, of the similarity of animal 

 ova, it is to be understood that uniformity of 

 a general kind only is implied ; and even that 

 correspondence is demonstrated not so im- 

 mediately, in many instances, by the examina- 

 tion of the fully formed ova, as by their careful 

 comparison, at different stages of their growth, 

 and more especially in their earlier condi- 

 tions. 



The more important of these differences 

 are those which are related to the nature of 

 the germinal portion of the ovum, as com- 

 pared with the rest of its parts ; and a con- 

 sideration of these differences, in so far as 

 they have yet been observed, appears to lead 

 to a division of the ova of animals into three 

 principal groups, as follows : 



First group. In a certain number of ani- 

 mals, some of which are viviparous, others ovi- 

 parous, the ovum is for the most part of 

 proportionally small size, sometimes very mi- 

 nute, as compared with the full-grown parents, 

 and of very simple structure ; the yolk sub- 

 stance is entirely composed of elementary 

 granules, or minute and simple spherules; 

 the process of segmentation affects the whole 

 mass of the yolk, and the germinal or blasto- 

 dermic layer, resulting from that segmentation 

 extends from the first over the whole surface 

 of the ovum ; the whole of this layer con- 

 tributes at once to the formation of the rudi- 

 ments of the embryo and its accompanying 

 organised structures ; the whole yolk, or ovu- 

 lum, in fact, is germinal, or is converted into 

 the parts of the future embryo. Such is the 

 nature of the ovum in mammalia, and among 

 the Invertebrata, in most Mollusca, Entozoa, 

 Echinodermata, Acalepha?, Polypes, and a 

 considerable number of other tribes. 



Second group. In another set of animals, 

 the great majority of which are oviparous, 

 the ova are proportionally of the largest size ; 

 the yolk substance consists very obviously of 



