236 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 



layer of the unfertilized egg contains a substance which is 

 needed for development but which is not available until the 

 surface layer is cytolyzed or destroyed otherwise. A further 

 discussion of these possibilities with our present knowledge of 

 the chemistry of the egg is futile. 



2. We have seen that the spermatozoon induces develop- 

 ment by two different agencies, one of which has a membrane- 

 forming effect, while the other must act somewhat like the 

 hypertonic solution in our method of artificial parthenogenesis. 

 We stated that the hypertonic solution has merely a corrective 

 effect since the membrane formation sets the whole machinery 

 of cell division into action; Boveri suggested that the spermato- 

 zoon carries in its middle piece the centrosome, the real organ 

 for cell division, into the egg. 



The idea that the centrosome is the middle piece of the 

 spermatozoon and that the carrying of this middle piece is the 

 main function of the spermatozoon in inducing development 

 does not agree with the observations. F. Lillie 1 points out that 

 the middle piece is probably not carried into the egg at all, and 

 he proves that in Nereis any piece of the spermatozoon is able 

 to give rise to centrosome and aster formation. These forma- 

 tions arise in the egg cytoplasm which is in contact with the 

 sperm fragment. The centrosome and aster formations are 

 physicochemical effects induced through the influence of the 

 sperm fragment. Such effects are also induced by the method 

 of artificial parthenogenesis. 



Morgan 2 found that supernumerary astrospheres may arise 

 if fertilized eggs are put into hypertonic sea-water, but the writer 

 is of the opinion that this happens only if the eggs remain 

 too long in the hypertonic solution. Yet it was natural to 

 consider the possibility that the second factor which the sperma- 

 tozoon must supply for development might be the centrosome; 



1 F. R. Lillie, "Studies in Fertilization," III and IV, Jour. Exper. Zool, XII, 

 413, 1912. 



2 Morgan, Archiv f. Entwicklungsmechanik, VIII, 448, 1899. 



