ARTIFICIAL PARTHENOGENESIS AND OXIDATIONS 125 



In naturally fertilized eggs a distinct aster (cleavage aster) appears 

 at one pole of the nucleus, its rays centering in a clear area which repre- 

 sents a diffuse centrosome. This area divides and the two halves 

 move apart until they come to lie at opposite sides of the nucleus and 

 form the poles of a typical amphiaster which is developed in the 

 nuclear region. Meanwhile the chromatin assumes the form of a 

 spireme, which breaks up into 36 chromosomes that arrange themselves 

 about the equator of this amphiaster to form a nuclear spindle. In 

 the chemically fertilized eggs a nuclear spindle arises in a similar way 

 and the chromatin assumes the form of a spireme preparatory to 

 breaking up into chromosomes, but, instead of 36, only 18 of these 

 latter bodies appear. The subsequent changes are identical in both 

 kinds of eggs. The chromosomes split longitudinally and each half 

 moves along the spindle fibres toward its respective pole. As they 

 approach the poles the chromosomes sw r ell up and eventually fuse 

 together to form a single nucleus in the region occupied by each of the 

 diffuse centrosomes. Meanwhile a cell wall develops between the two 

 nuclei dividing the cytoplasm into two, and finally the spindle fibres 

 disappear. The succeeding processes of development, both internal 

 and external, are similar in both naturally and chemically fertilized 

 eggs, with the exception that at each succeeding division only 18 chro- 

 mosomes appear in the latter instead of the normal number, 36. 1 



We had mentioned that the eggs which formed a membrane 

 upon butyric-acid treatment begin to divide and may go through 

 a series of divisions if the temperature is sufficiently low. 

 According to Hindle, at room temperature only a monaster is 

 formed with the nucleus as a center, while at a low temperature 

 a typical amphiaster is formed. The subsequent changes in the 

 latter case are the same as with the treatment with hypertonic 

 sea-water. 



1 Hindle, op. cit. 



