OVUM. 



Fig. 31. 



JOB. 



Diagrammatic representation of the three principal 

 kinds of ovarian ova, in sections (tlie relation, of 

 size not, however, being maintained). 



A. Mammiferous ovarian ovum ; z. zona pellucida 

 or vitelline membrane of some authors ; v. granular 

 yolk substance; g. germinal vesicle, with its nucleus 

 or germinal spot ; A', young ovum of a molluscous 

 animal. 



B. Young ovarian ovum of a bird ; v. m. vitelline 

 membrane ; v. large yolk corpuscles of two kinds 

 viz. those of the yellow external part, and those of 

 the cavity ; g. germinal vesicle without spot or 

 nucleus, but granular diffused maculae ; v. d. small 

 granular yolk substance forming the vitelline disc. 



c. Ovarian ovum of a batrachian reptile ; v. m. 

 vitelline membrane ; v. d. vitelline disc, or forma- 

 tive yolk-substance deeply coloured, and surrounding 

 the large granular yolk substance, in one nearly 

 entirely, in the other, c', only half round the ovum ; 

 a. germinal vesicle. 



two kinds of organised particles, viz., of 

 smaller granules nearly similar to those which 

 form the whole yolk in the last group, and of 

 larger cells, usually non-nucleated, and fat vesi- 

 cles, which constitute the greater part of the 

 mass. The first or granular part of the yolk 

 constitutes a thin disc, limited to one region 

 of the surface, viz., the upper side in the 

 vicinity of the place occupied by the germinal 

 vesicle, while the cellular substance of the 

 yolk forms the larger spherical mass of the 

 egg. Besides these two, there are also various 

 intermediate forms, which seem to be stages 

 of transition between the other kinds of struc- 



lural elements. The germinal vesicle in the 

 ripe ovarian ovulum, is situated in the centre 

 of the granular disc, and after its disappear- 

 ance, the process of segmentation is limited to 

 that disc. The germinal or blastodermic mem- 

 brane, or layer of cells, extends, therefore," at 

 first, no farther than this granular disc, and 

 consequently it is only a very small part of 

 the ovum which directly contributes to lay 

 the first foundations of the embryo, or its 

 accessory parts ; while the larger mass of 

 cellular yolk comes only secondarily to take a 

 part in the process of embryo nourishment. 

 Hence, in such ova, the distinction may be 

 broadly drawn between the germinal or for- 

 mative, and the nutritive parts of the yolk. 

 Such is the invariable relation of the parts of 

 the ovum to development in the whole class 

 of birds, with some differences in scaly rep- 

 tiles, in cartilaginous fishes, and perhaps 

 also in cephalopodous mollusca, ami a few 

 other invertebrata. 



Third group. In another group of animals 

 the structure and relations of the parts of the 

 ovum are different from, but in some degree 

 also intermediate between, those of the two 

 groups previously described. In this one the 

 yolk, or ovulum, may be stated to be of 

 middle size ; its structural elements appear to 

 be of two kinds, viz., the smaller germinal 

 or formative granules, and the larger, or 

 nutritive corpuscles ; but these last are in less 

 quantity, are subject to considerable variety, 

 and exhibit less of the cellular structure 

 which characterises the ova of the previous 

 group. The germinal layer occupies a larger 

 portion of the surface of the yolk than in the 

 large-yolked ova (second group), but in ge- 

 neral less than in the small-yolked ova (first 

 group), and its extent is subject to consider- 

 able variety ; in some, covering not more than 

 a half, in others, extending nearly over the 

 whole surface of the yolk. The segmentation 

 is co-extensive with the germinal part, and 

 more or less of the yolk, therefore, contributes 

 at the first to form the primitive parts of the 

 embryo. Such is the condition of the ovum 

 in the Scaleless Reptiles or Amphibia, and 

 Osseous Fishes. The ova of the higher Crus- 

 tacea, Arachnida, Insecta, and some other 

 Invertebrata may perhaps be included in the 

 same group. 



It will be perceived that in the three groups 

 now mentioned, a distinction has been drawn 

 between a part of the yolk, which is imme- 

 diately employed in the formation of the em- 

 bryo, ami another, which is only remotely 

 connected with that process. In the bird's 

 egg, it has been stated that the latter part of 

 the yolk is in large quantity, and that in the 

 minute mammiferous ovum the first part only 

 exists, and that in batrachia the two kinds of 

 yolk substance are more nearly equally ba- 

 lanced. This difference among the ova of 

 animals has been long known to physiologists 

 in a general way ; but its true nature, as con- 

 nected with a difference of structure of the 

 two kinds of yolk substance, and their relation 

 to the earliest development of the embryo, has 



