36 



TEXT-BOOK OF EMBRYOLOGY. 



into two exactly similar parts (Fig. 22, 5), one of which is contributed to 

 each daughter nucleus (Fig. 22, 6), and the cell body divides into two equal 

 parts. (For details of succeeding anaphase and telophase see p. 6.) There 

 thus result from the first division of the fertilized ovum, two cells which are 



"Zona pellucida 



' Nucleus 



;~ Spermatozoon 



~ Cytoplasm 



Head of 



spermatozoon 

 with centrosome 



. Female pronucleus 



Male pronucleus 



Male pronucleus 

 Female pronucleus 



Chromosomes of 

 iBmale pronucleus 



"^ Chromosomes of 

 male pronucieus 



Centrosome 



,' Chromosome from 

 * female pronucleus 



---.. Chromosome from 

 "/ male pronucleus 



11 



\1 '* ; ,';Y.;.viV-,- --r- Centres 



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



Boveri, Bohm and i'on Davidoff. 



apparently exactly alike and each of which contains exactly the same amount of 

 male and of female chromosome elements (Fig. 22, 6). 



The amphiaster of the fertilized ovum appears to develop as in ordinary 

 mitosis. As to the origin of the centrosomes, however, much uncertainty still 



