34 INTRODUCTORY I 



as a result of this stimulus, while in merogony we see the 

 paternal chromosomes alone so stimulated. Nor is the 

 mechanism of the process far to seek. The male cell intro- 

 duces or manufactures a centrosome, the female cell provides 

 the cytoplasm wherein the centrosome can make the necessary 

 division-apparatus. The two cells are thus mutually comple- 

 mentary. The experiments we shall have to describe in the 

 next lecture suggest that the stimulus so conveyed to the 

 egg by the spermatozoon may be of a physical or chemical 

 nature. 



5. Finally in some cases the spermatozoon produces a 

 change in the cytoplasmic structure of the egg, the original 

 radial being replaced by a bilateral symmetry. 



A very good instance of this is the formation of the grey 

 crescent in the Frog's egg, already alluded to (p. 7). 



Another is provided by the Ascidian Cynthia, in the im- 

 mature egg of which there is a peripheral yellow substance 

 surrounding a central grey yolk; the large germinal vesicle 

 is near the animal pole (Fig. 9). Upon the entrance of the sper- 

 matozoon near the vegetative pole the germinal vesicle 

 breaks down and maturation occurs. The contents of the 

 germinal vesicle are discharged as a clear substance into the cyto- 

 plasm. Meanwhile the yellow layer has moved to the vegetative 

 extremity of the egg, followed by most of the clear substance, 

 only a small portion of which remains near the animal pole 

 surrounding the female pronucleus. The animal hemisphere 

 is thus occupied by the grey yolk. The egg has still a radial 

 symmetry about its axis, but this is now replaced by a 

 bilaterality, for the yellow and clear substances move to one 

 side below the equator : this will be the posterior end, while 

 the grey yolk, thus displaced, moves into the opposite side 

 of the vegetative hemisphere. The pronuclei meet on the 

 posterior side in the clear area, but finally move, taking this 

 substance with them, into the animal hemisphere. We shall 

 see that this bilateral symmetry of the egg is identical not 

 merely with the symmetry of the bilateral cleavage but also 

 with that of the embryo. 



Other instances are known of the spermatozoon producing 



