CH. IV] CLEAVAGE OF THE EGG 35 



ment. The first cleavage-plane always passes directly between 

 the separating halves of the segmentation-nucleus. 



There is an infinite number of possible planes through which 

 the first cleavage might divide the egg into equal portions. 

 What, then, determines the particular plane taken? We can 

 think of this plane as determined by external conditions, or 

 by the internal structure of the egg, or by a combination 

 of the two. In the first place, it seems probable that at the 

 first division of the segmentation-nucleus each resulting half 

 will get half of the chromatin of the male and half of the chro- 

 nuitin of the female pronucleus. The first plane of division 

 must therefore pass at right angles to the plane of apposition of 

 the two proimclei. That is to say, it will also pass through the 

 path of penetration of the spermatozoon (the male pronucleus), 

 and therefore approximately through the point at which the 

 spermatozoon has entered. This, according to Roux, is what 

 actually takes place. Moreover, since the egg has rotated as a 

 whole in the direction of the point of entrance of the sperma- 

 tozoon, the first cleavage will pass exactly through the highest 

 point of the white crescent, as seen from above. 



On the other hand, there is no direct evidence to show 

 that the two apposed pronuclei retain throughout subsequent 

 changes the position of first apposition, and there is much to 

 show that in the frog's egg, as well as in other eggs, the divid- 

 ing nucleus, or the direction of its spindle, is very susceptible 

 to modifications in the surrounding conditions. 



There is also some evidence to show that the declination of 

 the axis of the frog's egg is not necessarily determined by the 

 entrance of the spermatozoon, but by the arrangement of the 

 internal constituents of the egg itself. If, therefore, it could 

 be shown that the declination is present in unfertilized eggs, 

 and that in fertilized eggs the plane of first cleavage passes 

 more or less through the highest point of the white crescent, 

 then we should conclude that the plane of first cleavage is pre- 

 arranged in the egg. It would follow as a corollary that the 

 nuclear spindle orients itself with respect to the egg. 



There is direct evidence to show that in the newt some such 

 process as this does take place. Jordan ('93) has shown that 

 the spermatozoon may enter at any point of the surface of the 



