574 SEGMENTATION OF OVUM CHAP. 



yolk-granules in its protoplasm, it may reach a com- 

 paratively large size (e.g. crayfish, dogfish, bird). The 

 presence of a greater or less amount of yolk in the 

 ovum results, as we know, in very considerable differ- 

 ences as regards its mode of segmentation, as well as 

 in its subsequent development. The minute eggs of 

 the lancelet and rabbit, for instance, which are each 

 only y 1 ^ mm. (about ^l v inch) in diameter, contain 

 so comparatively small an amount of food- yolk as not to 

 interfere materially with the process of segmentation : 

 such ova are called microlecithaL When the quantity 

 of food-yolk is relatively greater, the eggs may be 

 described as megalecithal : in these the yolk may become 

 accumulated towards the centre of the egg, eventually 

 leaving a layer of protoplasm comparatively free from 

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

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

 128, and 149), 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 in proportion to the amount of protoplasm, 

 the less actively can the latter divide, and the quantity 

 may be relatively so great in some parts as to prevent 

 segmentation in these parts altogether. We can there- 

 fore distinguish between holoblastic oosperms, which 

 undergo entire segmentation (e.g., 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 form 

 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 



