64 



CLEAVAGE 



II 



determine the plane of the first and therefore of subsequent 

 furrows, or as we may put it, the symmetry of segmentation, 

 and in fact we know, with some degree of precision, the nature 

 of these factors and the causes of their divergence. 



The unfertilized egg has a radial symmetry about its axis : 

 as a result of fertilization it becomes bilateral, since the grey 

 crescent appears on the side of the egg opposite the point of 

 entrance of the spermatozoon. It appears that the position 

 of the grey crescent is determined not by the actual point 

 of entrance of the spermatozoon, but by the position of the 

 whole of the first part of the sperm-path, the entrance funnel. 

 We also know that there is a considerable tendency for the 

 median plane of the embryo to coincide with the plane of 



FIG. 14. Roux's diagrams to show the relation of the sperm-path 

 (Pig.} to the first furrow in the Frog's egg. In A the furrow includes the 

 sperm-path, in B it is parallel to it, in c it is parallel to the inner portion 

 of the path (copulation path), in D it includes only the very last portion 

 of the copulation path. (From Korschelt and Heider, after Roux.) 



embryonic symmetry, for the dorsal lip of the blastopore to 

 be formed on the side of the grey crescent. We may assume 

 then that the entrance of the spermatozoon imposes a bilateral 

 structure upon the cytoplasm, a structure which persists as 

 the bilaterality of the embryo. 



In the second part of its path the sperm -nucleus moves to 

 meet the female-nucleus. This second part may but need 

 not lie in the same meridional plane as the first part (Fig. 14). 

 When the first part is directed towards the egg-axis, and when 

 the female pronucleus lies in the egg-axis, then both parts do 

 lie in the same meridional plane, but when, as may happen, the 

 female pronucleus is not in the egg-axis, or the sperm entrance 

 path is not directed towards the axis, or possibly on account 

 of other disturbances (gravity, &c.), then the meridional planes 



