JS'-* 



St'X;Mi;N'TATIOX iiK (.)VUM 



\ 



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 telolecithal 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 onl)' 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 

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

 ,ent segmentation in these parts altogether. '\\'e can there- 

 lore distinguish between //<7M/(7j-//V oosperms, which.undergo 

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

 phio.xus. Frog, Rabbit), and ineroblastic oosperms, in which 

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

 I)rotoplasm is comparatively free from yolk (e.g. Crayfish, 

 Dogfish, ]-]ird), this portion, after segmentation, being known 

 as the blastodenii. 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. 119 and 138). In the case of holoblastic ova the 

 segmenting cells or hlastonieres may be equal, or nearly etjual, 

 in size (e.g., ^Vmphioxus, Rabbit) ; or if the yolk is pre- 

 sent in greater (piantities towards the lower pole, unequal, 

 {e.g. Earthworm, Frog). 



The influence of the lood-yolk in modifving the early 

 ptijcesses of development is thus evidenth very great, and 

 in order to understand these processes in their simplest fonii 

 it is necessar)' to select for our stud) an alecithal holo- 

 blastic egg, such as that of the lancelet. 



