xii SEGMENTATION OF OVUM 573 



process of segmentation : such ova are called alecithal. 

 When the quantity of food-yolk is relatively greater, it may 

 become accumulated towards the centre of the egg, even- 

 tually leaving a layer of protoplasm comparatively free from 

 yolk round the periphery (centrolecitkal ova, e.g. crayfish, 

 Fig. 98) ; or, as in the case of teloledthal ova (Figs. 64, 

 128, and 148), 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. We can there- 

 fore distinguish between holoblastic oosperms, which undergo 

 entire segmentation (eg., hydroids, earthworm, mussel, 

 lancelet, 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. 383), and in the meroblastic telolecithal ovum discoid, 

 or restricted to a small germinal disc at its upper pole 

 (Figs. 128 and 148). In the case of holoblastic ova the 

 segmenting cells or blastomeres may be equal, or nearly equal, 

 in size (e.g. lancelet, rabbit) ; or if the yolk is present 

 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 



