Fig. 1. — Schema of the stages of fertilization of the ovum, to indicate the 

 «,„, respective parts played by contributions from the ovum and spermatozoa to 

 the fertilized ovum or new individual (after Boveri). 



(1) Entrance of spermatozoon into ovum. (2) Head of spermatozoon enlarging to form 

 the male pronucleus, the neck-piece forming the centrosome of the cell. (3) Division 

 of the centrosome ; male pronucleus undergoing further enlargement. (4) Further 

 enlargement of male pronucleus, until in (5) the male and female pronuclei are 

 approximately equal. (6) Formation of equivalent chromosomes in male and female 

 pronuclei. (7) Formation of nuclear spindle, equivalent chromosomes of paternal 

 and maternal origin becoming distributed, so as eventually to pass into the two 

 primary blastomeres of the segmenting ovum. (8) The two primary blastomeres with 

 their nuclei, each receiving thus equivalent portions of chromatin from both parents. 



