4 COMPARATIVE ANATOMY. 



The head of the spermatozoon, on entering the ovum, is transformed into the 

 male pronucleus, 1 which fuses with the female pronucleus to form the first 

 segmentation nucleus. 



Impregnation then consists in a material fusion of the 

 generative products of both sexes, and hence in the new 

 individual we naturally find inherited qualities. The essential 

 cause of inheritance consists in the molecular structure of the 

 nuclei of both male and female germinal cells. This structure 

 (idioplasm) is the morphological expression of the characters of 

 the species and individual. 



C D 



FIG. 2. DIAGRAMS OF THE SEGMENTATION OF THE OVUM. 



A, first stage (two segments) : RK, polar bodies. B, second stage (four segments). 

 C, further stage. D, morula stage. 



The next stages (Fig. 2) are as follows. The first segmentation 

 nucleus divides into two equal parts, each of which forms a new 

 centre for the division of the ovum into two halves. This division, 

 the beginning of the process of segmentation, takes place by 

 the formation of a furrow round the egg, which becomes deeper 

 and deeper, until the division is complete. 



The first stage in the process of segmentation is thus completed ; 

 the second takes place in exactly the same way, and results in a 



1 If the egg is to be normally developed, not more than one spermatozoon must 

 enter. The latter may either pass through a definite opening (micropyle), or else 

 bore its way through the vitellino membrane. 



