MECHANISM IN SEGKEGATION 



41 



number of chromosomes, and as a result, the original 

 twelve chromosomes have been reduced to six. 



Around the six chromosomes left in the egg, a nuclear 

 wall forms, and the chromosomes become spun out into 

 delicate fibres. Meanwhile a spermatozoon has entered 

 the egg, and out of its head another nucleus develops. The 

 two nuclei, the egg nucleus and the sperm nucleus, move 

 toward the center of the egg (Pig. 16, a), where they 

 come into contact with each other. After a time, the 

 chromatin threads begin to condense again into rods. 



Fig 16. — Eggs of Ancyracanthus within membrane. Egg with two pronuclei, a; 

 egg pronucleus with six chromosomea and sperm nucleus with six cHromoaomes, b; egg pro- 

 nucleus with six chromosomes and sperm nucleus with five chromosomes, c; union of male 

 and female pronuclei, d. (After Mulsow.) 



Six appear in the egg nucleus, and six in the male nucleus 

 (Fig. 16, b)^. A spindle develops in the protoplasm of 

 the egg around the twelve chromosomes of which six have 

 come from the father (the paternal chromosomes) and 

 six from the mother (the maternal chromosomes) (Fig. 

 16, d). Each chromosome now splits lengthwise into 

 equivalent halves, and a half moves to each pole of the 

 mitotic spindle. The spindle rotates in the cytoplasm of 

 this egg until its long axis corresponds with that of the 

 egg. As the daughter chromosomes move towards the 

 poles of the mitotic spindle the egg protoplasm constricts 



' Assuming a female producing sperm to have entered. 



