[134J 



OVUM 



to be formed, is scarcely granular, and in some 

 instances quite clear, consisting of a viscous 

 blastema, and as it increases separating the ger- 

 minal vesicle within from the ovarian follicle, 

 which expands proportionally. Very soon, 

 however, and in many animals indeed from 

 the first, fine opaque granules make their ap- 

 pearance, as if by precipitation or deposit, in 

 the clearer basement substance, and thus the 

 primitive yolk-substance of the ovum in all 

 animals is formed. In most instances there 

 is a time during which the ovum, consisting 

 of germinal vesicle, with a small quantity of 

 primitive yolk, exists, without any other co- 

 vering than that given to it by the ovarian 

 follicle ; but as the deposit of the finely 

 granular yolk increases, and at a very variable 

 period in different animals, the vitelline mem- 

 brane is formed round its exterior. The ad- 

 dition of this covering may be regarded as the 

 third stage in the formation of the ovum. 

 The manner of the origin of the vitelline 

 membrane has not yet been accurately ob- 

 served ; and it is probable (as will be hereafter 

 stated) that the coverings known under this 

 name may have different modes of origin ; but 

 if we restrict our attention at present to such 

 simple ova as those of Mammalia, I believe it 

 may be stated as extremely probable that the 

 so-called zona pellucida which constitutes the 

 vitelline membrane of the Mammiferous ovum, 

 takes its origin by the consodidation of the 

 superficial part of the basement substance of 

 the primitive yolk. 



It appears probable that in the large-yolked 

 ova, such as those of the bird, the vitelline 

 membrane, which we find enclosing the whole 

 mass of the yolk, owes its origin to a dif- 

 ferent source ; and I am inclined to believe 

 that in this and in many other animals the 

 membrane which we term vitelline, as being 

 the immediate investment of the yolk, is not 

 of the same nature with the zona pellucida, 

 or the simple homogeneous vesicle of the 

 smaller ova, but rather a structure of later 

 formation, which owes its origin to the fusion, 

 or amalgamation, or to some other change in 

 the outermost layer of cells which form the 

 nutritive yolk of these animals. 



In connection with this view, it is import- 

 ant to remark, that at that earlier stage of 

 formation of the bird's egg when it consists 

 entirely of formative or primitive yolk, there 

 is an approach to the formation of a zona, in 

 the existence of a very distinct, clear, and 

 consistent marginal portion of the yolk blas- 

 tema, from which the yolk granules seem to 

 retire. When the large cellular or nutritive 

 yolk is formed, this temporary zona seems to 

 disappear, and to be replaced externally by 

 the permanent vitelline membrane already 

 mentioned. 



In those animals in which the ovigerms 

 arise by development within cells so as to be 

 connected in groups (Gordiacei), and in some 

 others, the vitelline membrane, or a substitute 

 for it, seems to be formed from the earliest 

 period in a different manner from that now 

 described. 



The germinal vesicle is unimacular in ge- 



neral in the small-yolked ova, and multima- 

 cular in the large-yolked ova, and also in the 

 intermediate kinds. In the latter it is rare to 

 observe the earliest stage in which the ma- 

 cula is still single : the multiplication of the 

 maculae takes place with remarkable rapidity, 

 and apparently by a process of endogenous 

 development, or possibly by division. The 

 ultimate destination of these macula is still 

 a subject of doubt. 



3. Morphology of the ovum ; homology of 

 its parts, and relation of the ovum to other 

 organic structures. 



Should the views be correct which have 

 now been stated with regard to the relations 

 of the parts in the mature ovarian ovum, and 

 the manner in which they are formed, it will 

 be apparent that a strict homology or ana- 

 tomical correspondence can be pointed out in 

 regard only to some of the parts which are 

 recognised under similar designations, as re- 

 spectively belonging to the ova of different 

 animals. All physiologists will probably be 

 disposed to look upon the germinal vesicle 

 or ovigerm as corresponding or homologous 

 in the ova of all animals, and, notwithstanding 

 the great differences known as to its more 

 simple or multiple condition, the same view 

 may also be taken of the structure known as 

 nucleus or macula. The primitive or finely 

 granular yolk-substance, more especially that 

 which immediately surrounds the germinal 

 vesicle, and is afterwards employed in the 

 formation of the blastoderm or embryogerm, 

 seems also to have a similar origin, structure, 

 and relation in all animals. But beyond this 

 it is more difficult to trace the homological 

 correspondence ; for under the names of 

 cellular yolk-substance and vitelline mem- 

 brane it appears that there have been brought 

 together parts of which the origin, structure, 

 and relations may be dissimilar in different 

 animals. There seems at least to be sufficient 

 reason, from what is already known of the 

 varieties of the enclosing membrane, or so- 

 called vitelline membrane, to establish a dis- 

 tinction between several forms of that struc- 

 ture; as, for example, between the vitelline 

 membrane, which exists from the earliest 

 period as a pediculated sac in connection 

 with the ovarium, as in Holothuria ; that 

 which is derived from the extension of the 

 wall of the original germ-cell in grouped ova, 

 such as have been described by Meissner in 

 Gordiacei ; that which is later formed round 

 the ovum of Mammalia as a zona pellucida, 

 by the consolidation of the outer layer of the 

 primitive basement substance of the yolk ; 

 and that which in the bird and other animals 

 whose ova are similarly constituted, appears 

 to derive its origin in part, at least, from 

 coalesced cells corresponding to those of the 

 tunica granulosa of the ovarian capsule on 

 the exterior of the cellular yolk. 



With regard to the cellular yolk itself, we 

 must refrain from any attempt to establish its 

 homology till we shall be more fully ac- 

 quainted with the mode of its production ; 

 for it is still undetermined whether it arises by 

 cell formation within the primitive vitelline 



