60 PROBLEMS OF FERTILIZATION 



V. THE INTERNAL PHENOMENA OF FERTILIZATION 



The morphological study of this subject consists in 

 following the parts of the spermatozoon within the egg 

 and in determining as far as possible their relations 

 to constituent parts thereof up to the time when 

 they can no longer be separately distinguished. The 

 nuclear and cytoplasmic parts of the spermatozoon 

 have very distinct histories and will therefore be sepa- 

 rately treated. 



i. The germ nuclei. The nucleus derived from the 

 head of the spermatozoon is known as the sperm nucleus 

 or male pronucleus. It is destined to unite with the egg 

 nucleus or female pronucleus derived from the internal 

 daughter-chromosome group of the second maturation 

 division of the egg. In those cases in which the sper- 

 matozoon enters the egg prior to or during maturation 

 the sperm nucleus must await the completion of the matu- 

 ration divisions of the egg; it has therefore more time and 

 undergoes more extensive changes before union with the 

 egg nucleus than in those cases in which maturation of 

 the egg is completed before fertilization. 



Immediately after penetration the head of the sper- 

 matozoon rotates around a transverse axis through 180 

 (Figs. 7#-/, So), so that the base, which was external 

 immediately after entrance, becomes oriented toward 

 the center of the egg and the apex is directed externally 

 (Henking, Wilson, Boveri, Meves, Lillie, etc.). This 

 phenomenon is very general, and it may be universal. 

 No adequate explanation has been found for it, and its 

 significance is quite obscure. It is, however, correlated 

 with the development of the sperm aster which always 

 arises at the base of the sperm head. 



