72 PROBLEMS OF FERTILIZATION 



The formation of asters is postponed in some animals 

 until about the time of meeting of the germ nuclei, and 

 then it is often difficult to determine whether or not 



FIG. ii. Effects of centrifugal force on penetration of the sper- 

 matozoon in Nereis: a, b, and c show removal of varying portions of the 

 sperm head before penetration; d shows the later history of an egg into 

 which a minute portion of the spermatozoon has entered; this part has 

 produced a sperm aster and centrosome, although it represents only 

 a fraction of the apical end of the sperm head similar to the piece 

 shown in c. 



the sperm components are alone concerned in it. In 

 Crepidula (Conklin) and Unio (Lillie) there is evidence 

 that each germ nucleus causes formation of one aster 

 of the first cleavage. 



