THE FERTILIZATION OF THE OVUM. 2J 



little smaller than those of the egg cell, and they frequently 

 remain slightly smaller as long as they can be distinguished. 

 In cases where there is no appreciable difference in size 

 between the two pronuclei, the one may be distinguished from 

 the other, as long as they remain distinct, by the position of 

 the polar bodies which lie directly over the female pronucleus. 

 After the two pronuclei have met, the two asters begin to 

 move apart. They continue to separate, moving around the 

 appressed nuclei, until they lie at opposite poles, Fig. 7. The 

 sperm aster now lies on the outer side of the sperm nucleus, 

 and the egg aster on the outer side of the egg nucleus ; it is 

 thus seen that the position which the asters occupy relative to 



FIG. 8. The halves of the male and FIG. 9. The halves of the female 

 female asters about to unite at the aster at the poles of the pronuclei ; the 

 poles of the pronuclei. male aster still undivided. 



the pronuclei, is just the reverse of that which obtained previ- 

 ous to the meeting of the two pronuclei. It must also be 

 allowed, I think, that the function now exercised by the asters 

 must be the reverse of that which prevailed before the pro- 

 nuclei met. Then, by active contraction, they drew the two 

 pronuclei together; now, by active expansion, they diverge, move 

 to opposite poles, and press the pronuclei together. This func- 

 tional alternation of contraction and expansion, or perhaps 

 better, attraction and repulsion, is manifest not only during 

 fertilization, but throughout the entire process of cleavage. 

 The contraction can be seen in the way in which the asters 



