48 THE FIRST PRINCIPLES OF HEREDITY 



to the arrangement of the yolk, four kinds of ova. We 

 had the ovum with (i) diffuse, (2) central, (3) polar, and 

 (4) predominant yolk. All the different ova develop on 

 the same principle, which is only modified according to 



A 



B 



I 



m 



IV 



-^^x 



D 



Fig. $^. — Embryogenesis of (.4) Sponge, (S) Crayfish, (C) Frog, 

 {D) Bird. (After Thomson and Haeckel.) 



I, ovum ; II, morula ; III, blastula ; IV, gastrula. 



the disposition of the yolk. First of all, we have a cell- 

 cleavage, the original mother-cell forming by repeated 

 divisions a globular mass of cells, the " morula " stage ; 

 then follows the formation of a cavity within this ball of 

 ceUs, the " blastula " stage ; thirdly, the " gastrula " stage 



