II 



CLEAVAGE 



63 



effected by means of a noose of hair tied round the egg in the 

 first furrow (Fig. 13). 



It is highly probable, however, that even then due attention 

 would have to be paid to the position of the first furrow with 

 regard to the grey crescent. For Brachet has shown that if 

 one blastomere be killed, the fate of the other depends upon 

 the angle made by the first furrow with the symmetry plane. 

 When the planes coincide, the survivor becomes a right or left 

 half-embryo; when the angle is 90 the survivor (when it 



a 



i 



FIG. 13. Three stages in the production of a double monster by 

 strong median constriction of the Newt's egg. (After Spemann, 1903.) 



a. Beginning of gastrulation ; there is a separate lip in each half. 



b. I. and r. Med., Medullary folds of left and right embryos ; *, point 

 where the medullary grooves separate ; Bl, blastopore. c. The double- 

 headed larva. 



includes the grey crescent) becomes a postero-dorsal half- 

 embryo ; while when the angle is between and 90 the 

 survivor develops into an embryo which is defective on the 

 right or left, anteriorly or posteriorly as the case may be. 



This experiment demonstrates in the most convincing way 

 the closer dependence of the embryonic symmetry upon the 

 plane of egg-symmetry than on the first furrow. 



In the Frog's egg, therefore, the factors that determine the 

 symmetry of the embryo must be distinct from those that 



