PROCESSES RELATED TO SEXUAL REPRODUCTION 233 



egg simultaneously. The nucleus and middle-piece of the sperma- 

 tozoon, having thus entered the egg, move very slowly toward the 

 egg nucleus, gradually rotating through an angle of almost one 

 hundred and eighty degrees, with the result that the centrioles of 

 the zygote, which develop from the middle-piece of the sper- 

 matozoon, finally occupy a position between the male and female 



Fig. 116. — Fertilization in the sea-urchin, Toxopneustes. 



A, the spermatozoon showing "head" containing nucleus, "middle-piece," and "tail" 

 or flagellum. B to F, entrance of head and middle-piece into cytoplasm of the egg, showing 

 entrance cone and the rotation of sperm-head during development of aster about the middle- 

 piece (the minute centriole in center of the middle-piece is not shown). G and H, beginning 

 of enlargement of sperm-nucleus and its movement toward the nucleus of the ovum. / and 

 J, development of amphiaster of first cleavage and union of male and female pronuclei. 

 (From Wilson, "The Cell," copyright, 1897, by The Macmillan Co., reprinted by per- 

 mission.) 



nuclei, which are often called the pronuclei. Meanwhile, the male 

 pronucleus has increased in size, and its chromosomes, which are 

 not recognizable as such in the nucleus of the spermatozoon 

 again become apparent in the haploid number seen at the close 

 of its maturation (cf. Fig. 113). 



^Yhi\e these changes are in progress, the maturation divisions 



