26 



COMPARATIVE ANATOMY 



CHAP. 



or by transfusion (follicle formations). In Insects the egg germs 

 can develop in the ovaries alternately into nutritive cells and egg 

 cells. In most Platodes the case is very complicated ; the germ 

 cells of an original germarium here fall into two more or less 

 distinct groups. The germ cells of one group (germarium) become 

 egg cells ; those of the other group (vitellarium) become nutritive or 

 yolk cells filled ivith nutritive yolk. In other Platodes only some 

 of the originally numerous germaria retain their primitive function, 

 while all the others are changed into vitellaria to supply nutritive 

 yolk. The eggs either absorb the nutritive yolk of the yolk cells 



before fertilisation, or else many yolk cells 

 are stored in an egg capsule together with 

 a few fertilised egg cells, and are used up 

 during development. 



The nueleus or the germinal vesicle 

 (vesieula germinativa) of the animal egg 

 is remarkable for its relatively great size. 

 It consists of an outer layer, in most cases 

 differentiated into a distinct membrane, 

 surrounding the light - coloured clear 

 nuclear fluid. In this lie one or more 

 solid nucleoli or germinal spots (maculae 

 germinativse), which are often connected 

 with each other and with the nuclear 

 membrane by a network of fine threads. 

 In many eggs the germinal vesicle lies, 

 throughout, in the centre of the egg ; in others it does so at least in 

 the very early stages. 



Pia. 22. — Ovanan egg of an 

 Eohinoderm, after O. Hertwlg. In 



the middle the germinal vesicle with 

 the nuclear framework and the germ- 

 inal spot. 



The Egg Yolk. 



Investigations made by means of the improved optical appliances 

 for research have lately shown that protoplasm itself (in Protozoa, 

 egg cells, and tissue cells) exhibits a fine structure. It consists of 

 very small firmer particles, arranged in the finest network of threads, 

 which form the spongioplasm, and, lying between them, clear homo- 

 geneous more fluid portions, forming the hyaloplasm. Protoplasm 

 thus constituted only in the rarest cases forms the whole of the yolk. 

 In most cases reserve nourishment in the form of fat or oil drops, 

 small plates and spheres, is found in the protoplasm, these being used 

 as food by the developing egg. These constitute, in contradistinction 

 to the formative yolk — i.e. the actual living active protoplasm of the 

 egg — an inert lifeless constituent, only serving as nutriment, the 

 deutoplasm or nutritive yolk. The quantity and arrangement of the 

 deutoplasm in the egg is of great importance, since this determines 

 the course of its first segmentation. 



It rarely happens that there is no deutoplasm in an egg. Less 



