28 



TEXT-BOOK OF EMBRYOLOGY. 



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

 elements are therefore undoubtedly carried into the ovum in the middle 

 piece. It 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 egg centrosome from, 



Female 

 x^ pronucleus 



Head of 

 spermatozoon 

 with centrosome 



Female pronucleus 



Male pronucleus 



gsr Centrosome 



Male pronucleus 

 Female pronucleus 



Chromosomes of 

 female pronucleus 



Chromosomes of 

 male pronucleus 



Centrosome 



Chromosome from 

 female pronucleus 



Chromosome from 

 male pronucleus 



- Centrosome 



FIG. 15. Diagram of fertilization of the ovum. (The somatic number of chromosomes is 4.) 



Boveri, Bohm and von Damdoff. 



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

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

 described by Wilson. In cases of this type (Fig. 16) the tail of the spermato- 

 zoon remains outside the egg while the head and middle piece, almost imme- 



