156 PROBLEMS OF FERTILIZATION 



nucleus (female pronucleus) arises and attains the same 

 size as in normally fertilized eggs. The chromosomes 

 of the first cleavage spindle then form in the usual 

 fashion and at the usual time, accompanied by dis- 

 appearance of the nuclear membrane. But, whereas 

 in the presence of a sperm nucleus cytoplasmic asters 

 accompany these changes and a spindle rapidly arises, 

 in the absence of the sperm nucleus there is absolutely 

 no sign of cytasters or evidence of spindle formation. 

 The chromosomes lie naked in the cytoplasm, surrounded 

 by a clear area. Each chromosome then splits longitu- 

 dinally in the usual fashion, but the halves do not sepa- 

 rate. At the time of the telophase of the normal first 

 cleavage there is a tendency to scattering and breaking 

 up of the chromosomes. When the normal eggs have 

 reached the two- and four-celled stages, the scattering 

 and breaking up of the chromosomes have progressed 

 much farther in the unsegmented eggs, and in the 

 course of two or three hours there remains no differ- 

 entiated nucleus or chromosomes, but only numerous 

 chromatic granules scattered throughout the cyto- 

 plasm. 



This experiment then shows that the fertilization 

 processes may be divided physiologically as well as 

 morphologically into the two main phases of the exter- 

 nal and internal phenomena. The external action is 

 adequate to produce the cortical changes alone, but not 

 the entire series of developmental events. In most eggs 

 the penetration of the spermatozoon is so rapid that it 

 accompanies or precedes the cortical changes, and it 

 is therefore difficult to ascertain whether the cortical 

 changes are dependent on penetration or not. Loeb 



