80 



EXTERNAL FACTORS 



III. i 



The most important first result of fertilization is, however, 

 the replacement of the radial by a bilateral symmetry. About 

 two or three hours after insemination a certain portion of the 

 border of the pigmented area, crescentic in shape and extending 

 over about half its periphery, becomes grey by retreat of the 

 pigment into the interior (Roux). The egg can now be divided 

 into similar halves by only one plane, the plane of bilateral 

 symmetry, which includes the axis and the middle of this grey 

 crescent (Fig. 43). 



A 



B 



D 



FIG. 43. Formation of the grey crescent in the Frog's egg (/?. tern- 

 porariu). A, B from the side; c, D from the vegetative pole. In A, c 

 there is no crescent, in B, D a part of the border of the pigmented area 

 has become grey. 



The middle of the grey crescent is always diametrically 



opposite to the point of entry of the sperm (Roux and Schulze) ; 

 the crescent has hence been held by Roux to be directly caused 

 by that entrance. 



The plane of symmetry, as we shall see in another connexion, 

 becomes in most cases the sagittal plane of the embryo, since the 

 dorsal lip of the blastopore arises in the region of the grey 



