i6 



FERTILIZATION 



invalidated by an experiment in which the development of the 

 sperm aster was inhibited by treating eggs with ether shortly after 

 fertilization. He claimed that in these circumstances the move- 

 ments of the male pronucleus were unaffected, Wilson makes no 

 reference to this experiment in his famous textbook The Cell in 

 Development and Heredity, and the ether experiment should be 



Sperm aster 



Female pronucleus 



FIG. 5. — Path of male pronucleus from periphery of egg, labelled sperm aster, 

 towards centre of egg, the path being shown by ■ • • ■. Four alternative 

 . paths of female pronucleus to centre of egg, according to original position in 

 the unfertilized egg, are also shown. After E. L. Chambers (1939). 



treated with reserve unless it is repeated. At the same time, Conk- 

 lin's examination (1905) of fertilization in Styela partita (Stimpson) 

 shows that in some cases, at any rate, protoplasmic streaming may 

 affect the movements of the male pronucleus. As a general rule, 

 however, and until experiments of the Chambers type are done on 

 eggs other than those of echinoderms, and particularly on mam- 

 malian eggs in which there is nothing comparable to the sea- 

 urchin or frog egg sperm aster, the movement of the male pro- 

 nucleus must be assumed to be a straightforward mechanical 

 phenomenon, caused by the growth of the sperm aster (only, of 



