B1GEL0W : NUCLEAR CYCLE OF GONIONEMUS MUKBACIIII. 383 



tions. Later the middle piece disappears and the radiations centre at 

 a point at some little distance from the sperm nucleus, but no sperm 

 centrosome could be detected. The aster divides and forms two asters, 

 which accompany the sperm nucleus in its migration through the egg. 



The egg aster disappears at the close of the second maturation division. 



Nuclear union takes place either by fusion or by apposition ; the de- 

 termining factor is believed to be the relative sizes of the nuclei at the 

 time of their union. 



Cleavage (Figs. 150-160). — In the first cleavage spindle — as seen in 

 formalin preparations — there are the full somatic number of chromo- 

 somes. 



In the second cleavage there are a reduced number of chromosomes, 

 each of which is a bivalent structure resulting from the pairing of uni- 

 valent chromosomes. 



The number of chromosomes in the third cleavage has not been ob- 

 served, but in the fourth and subsequent cleavages all nuclei have the 

 full somatic number of chromosomes. 



