IV. i INITIAL STRUCTURE OF THE GERM 



163 



2. AMPHIBIA. 



It will be remembered that the Frog's egg possesses, when 

 freshly laid, a symmetry which is radial about the axis, the line 

 joining the centres (animal and vegetative poles) of the pigmented 

 and unpigmented portions of the egg ; the symmetry is marked 

 internally by the arrangement of yolk and protoplasm, for the 

 latter lies most abundantly in the smaller unpigmented, the 

 former mainly in the larger pigmented portion. Since the yolk 



A 



B 



C D 



FIG. 78. Formation of the grey crescent in the Frog's egg (B. tem- 

 poraria). 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, 



is heavier than the protoplasm the white pole is turned down- 

 wards, with the axis vertical, the egg being free to rotate inside 

 its jelly membrane. Shortly after fertilization, however, the 

 radial is replaced by a bilateral symmetry (in Sana fusca and 

 R. temporarid), a grey crescent being formed on one side of the 

 egg along the border of the pigmented area by the retreat of pig- 

 ment into the interior (Fig. 78). The grey crescent eventually 

 becomes white and added to the white area. In Sana csculenta 



M 2 



