THE CELL 



61 



occurs the first division of the fertilized ovum into two daughter 

 cells. By similar mitotic processes these two cells become four, the 

 four cells become eight, etc. This is known as segmentation of the 

 ovum. 



membrane of 

 ovum 



entering sper- 

 -" matozoon 



protoplasm of 

 ovum with 

 deutoplasm 

 granules 



female pronu- 

 cleus 



' -W \ head of sper- 

 't*-?I^i — ■ matozoon with 

 "^^^ I centrosome 



^ -- female pronucleus 



male pronucleus 



chromosome of female 

 pronucleus 



chromosome of male 

 I ' pronucleus 



"-— centrosome 



centrosome 



male pronu- 

 cleus 



female pro- 

 nucleus 



chromosome 

 -' from female 

 pronucleus 



chromosome 

 .-"i__ from male 

 " . ; pronucleus 



-• centrosome 



Fig. 12 — Diagram of Fertilization of the Ovum. (The somatic number of chro- 

 mosomes being four.) (From Bohm and von Davidoff, after Boveri.) 

 1, Ovum surrounded by spermatozoa, only one of which is in the act of penetration. 

 Toward the latter the protoplasm of the ovum sends out a process; 2, Head of spermato- 

 zoon has entered ovum, its body becoming the male centrosome, its tail having disap- 

 peared; 3, The head of spermatozoon has become the male pronucleus. Male and 

 female pronuclei approach each other. Between them is the (male) centrosome; 4, 

 The spiremes of male and female pronuclei have each formed two chromosomes. The 

 centrosome has divided; 5, Male and female chromosomes have mingled and by longitu- 

 dinal cleavage (see Mitosis, p. 54) have become eight. These become arranged in the 

 equatorial plane of the ovum. JMantle fibres extend from centrosomes to chromosomes; 

 6, Division of the ovum; two daughter cells, each containing a daughter nucleus. Each 

 daughter nucleus contains four chromosomes, two derived from each pronucleus. 



