34 



TEXT-BOOK OF EMBRYOLOGY. 



exists. The middle piece of the spermatozoon always enters the ovum with the 

 head. It has already been shown (p. 24) that one or two spermatid centro- 

 somes take part in the formation of the middle piece. Male centrosome ele- 

 ments are therefore undoubtedly carried into the ovum in the middle piece. It 



Zona pellucida 



Nucleus 

 ^7=^" Spermatozoon 



Female 

 s* pronucleus 



Head of 



spermatozoon 

 with centrosome 



Female pronucleus 



Male pronucleus 



. Chromosomes of 

 female pronucleus 



*^ Chromosomes of 

 male pronucleus 



"Centrosome 



Centrosome 



Male pronucleus 

 Female pronucleus 



Chromosome from 

 female pronucleus 



rr"I^^ Chromosome from 

 / male pronucleus 



Centrosome 



FIG. 22. Diagram of fertilization of the ovum. (The somatic number of chromosomes is 4.) 



Boveri, Bohm and von Davidoff. 



is equally well known, for some forms at least, that the centrosome of the ovum 

 disappears just after the extrusion of the second polar body. In a considerable 

 number of forms the development of the centrosome of the fertilized egg from, 

 or in close relation to the middle piece of the spermatozoon has been observed. 

 The details of the process as it occurs in the sea-urchin have been carefully 



