556 SECIMENTATION of OVuM cItAl'. 



become accumulated towards the centre of the egg, even- 

 tually leaving a layer of protoplasm comparatively free from 

 yolk round the periphery (centrokcithal ova, e.g. Crayfish, 

 Fig. 91); or, as in the case of telokcithal ova (Figs. 64, 

 119, and 138), the yolk-granules may become aggregated 

 more at the lower than at the upper pole (Frog), until 

 in the most extreme cases there is only a layer of yolkless 

 protoplasm — the germinal disc (Dogfish, Bird) — lying at the 

 upper pole of the egg. 



As yolk is an inert substance, the more of it an egg 

 contains the less actively can the latter divide, and the 

 quantity may be relatively so great in some parts as to pre- 

 vent segmentation in these parts altogether. \\& can there- 

 fore distinguish between holoblastic oosperms, which undergo 

 entire segmentation {e.g., Hydra, Earthworm, Mussel, Am- 

 phioxus. Frog, Rabbit), and meroblastic oosperms, in which 

 segmentation is limited to that part of the egg in which the 

 protoplasm is comparatively free . from yolk {e.g. Crayfish, 

 Dogfish, Bird), this portion, after segmentation, being known 

 as the blastoderm. In the centrolecithal ovum it is evident 

 that the segmentation must be superficial or peripheral 

 (p. 369), and in the meroblastic telolecitha. ovum discoid 

 or restricted to a small germinal disc at its upper pole 

 (Figs, ng and 138). In the case of holoblastic ova the 

 segmenting cells or blastomeres may be equals or nearly equal, 

 in size {e.g., Amphioxus, Rabbit) ; or if the yolk is pre- 

 sent in greater quantities towards the lower pole, unequal, 

 {e.g. Earthworm, Frog). 



The influence of the food-yolk in modifying the early 

 processes of development is thus evidently very great, and 

 in order to understand these processes in their simplest form 

 it is necessary to select for our study an alecithal holo- 

 blastic egg, such as that of the lancelet. 



