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 



Female 

 ^'' pronucleus 



| Head of 



M- _ . spermatozoon 



J with centrosome 



_,- Female pronucleus 



Male pronucleus 



Rr; 



V--- Centrosome 

 .^Sr~ Male pronucleus 

 ^ Female pronucleus 



' Chromosome from 

 female pronucleus 



*i 



Chromosomes of 

 ' female pronucleus 



Chromosomes of 

 male pronucleus 



' Centrosome 



fff- _ . Chromosome from 

 male pronucleus 



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



Boven, Bohm and von Davidoff. 



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

 male and oj jemale 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 



