FORMATION OF THE LAYERS OF THE BLASTODERM. 657 



In the hen's egg the cleavage is confined to the cicatricula or 

 blastoderm, and does not include the rest of the yolk. Such an 

 ovum, from the fact that the cleavage of the yolk is only partial, 



FIG. 250. 



Sections of the ovum of a rabbit, showing the formation of the blasto- 

 dermic vesicle. (E. Van Beneden.) a, b, c, d, are ova in successive stages 

 of development, z.p, Zona pellucida, ect, Ectomeres, or outer cells, ent, 

 Entomeres, or inner cells. 



receives the name of meroblastic. The human ovum, which under- 

 goes complete segmentation, is called a holoblastic ovum. 



The cells in the blastodermic vesicle become arranged into 



55 



