mi: S|,,;MI:N'TATION OF THE OVUM. 



In all the above types, as segmentation proceeds, the 

 protoplasm becomes more and more concentrated at the surface, 

 and finally a superficial layer of flat blastoderm cells is com- 

 pletely segmented off from the yolk below (fig. 49 D). 



In cases like those of 1'cn.uus, Eupagurus, etc., the yolk in 

 the interior is at first nearly homogeneous, but at a later period 

 it generally becomes divided up partially or completely into a 

 number of distinct spheres, which may have nuclei and therefore 

 have the value of cells. In many cases nuclei have however not 

 been demonstrated in these yolk spheres, though probably 

 present ; yet, till they have been demonstrated, some doubt 

 must remain on the nature of these yolk spheres. It is probable 

 that not all the nuclei which result from the division of the first 

 segmentation nucleus become concerned in the formation of the 

 superficial blastoderm, but that some remain in the interior of 

 the ovum to become the nuclei of the yolk spheres. 



In Myriapods (Chilognatha) a peculiar form of segmentation has been 



50. SEGMENTATION AND FORMATION OF THE BLASTODERM IN CHEUFE*. 



(After Mctschnikoff.) 



In A the ovum is divided into a number of separate segments. In B a number of 

 small cells have appeared (A/) which form a blastoderm enveloping the Urge yolk 

 spheres. In C the blastoderm has become divided into two layers. 



15. IF. 8 



