ACTION OF THE SPERMATOZOON UPON THE EGG 241 



if two spermatozoa enter the egg, not only two but three or 

 four astrospheres are formed. When we cause the segmentation 

 of an unfertilized egg by treating it with hypertonic sea-water, 

 two astrospheres are formed (unless the eggs have been too 

 long exposed), and the division of the nucleus takes its regular 

 course. But if the eggs are left too long in the hypertonic 

 solution, they divide into more than two cells at once when they 

 are put back into normal sea-water. According to Morgan, 

 this is owing to the formation of more than two astrospheres. 

 Such eggs do not develop into normal larvae. 



Why is it, then, that only those eggs develop into vigorous 

 larvae in which the first division leads to the formation of only 

 two cells ? To this question Boveri gives the following answer. 

 In normal nuclear division, each chromosome splits length- 

 wise into two similar pieces, one of which goes into each of two 

 astrospheres, and into the new nucleus; so that, therefore, after 

 the division is accomplished, each of the two daughter nuclei 

 contains, quantitatively and qualitatively, the same nuclear 

 material. But if two spermatozoa enter an egg, then three or 

 four astrospheres are formed, and, correspondingly, three or 

 four daughter cells. But since each chromosome of the mother 

 nucleus divides only into two parts, it is naturally impossible 

 that in this case each daughter nucleus will contain a half of 

 each chromosome of the mother nucleus. Boveri and many 

 other authors assume, and with good reason, that the different 

 chromosomes of the nucleus are physiologically dissimilar. 

 It will therefore be apparent that equivalent and fully potent 

 daughter nuclei will accordingly result only from regular nuclear 

 division with two astrospheres; and that when three or more 

 astrospheres are present the single daughter nuclei will not con- 

 tain the full number and, as a rule, not qualitatively the same 

 nuclear material. 



This hypothesis, then, would also explain why, under cer- 

 tain conditions, the superposition of artificial parthenogenesis 



