386 



BERTRAM G. SMITH 



G. The bilateral symmetry manifested by the internal structure of the 

 late hlastula 



When a living egg is examined in a very late blastula stage, the 

 roof and lateral walls of the blastocoele are found to be sHghtly 

 translucent. When such an egg is immersed in water and held 

 between the observer and the source of Hght, it can usually be 

 seen to possess a very definite bilateral symmetry. On one side 



Fig. 30 Meridional section taken in the median plane of bilateral symmetry of 

 an egg of Cryptobranchus allegheniensis in a very late blastula stage (late phase 

 of stage 10). S, segmentation cavity or blastocoele; Y, yolk. The drawing was 

 made with the aid of a camera lucida. X 15. 



of the large blastocoele the roof is more translucent than on the 

 opposite side and contrasts more abruptly with the opaque 

 yolk. The condition is more clearly shown in meridional sec- 

 tions, but in order to observe the exact condition described, one 

 must be careful to obtain an egg killed when it is just ready to 

 begin gastrulation. When the plane of the section is sagittal, 

 the contrast between the two sides is usually obvious enough 

 (figs. 30 and 31). Not only has the roof of the blastocoele 



