BILATERALITY IN CRYPTOBRANCHUS 377 



Seventy-seven eggs in stages 5 and 6 were marked as above 

 described; only twenty- three survived to the gastrula stage. 

 This heavy loss was due to the fact that the staining fluid was 

 inadvertently made too strong. The results for the twenty-three 

 eggs that survived are shown by the first twenty-three diagrams, 

 occupying the upper part of the page, in figure 18. 



Forty eggs in stage 7 were marked in the same manner. In 

 this stage the roof of the blastocoele is thin and translucent in a 

 region which, as a rule, is slightly excentric with respect to the 

 polar axis of the egg, and hes toward the side possessing the larger 

 micromeres. The smaller micromeres now extend further from 

 the animal pole than do the larger micromeres. Of the eggs 

 marked, twenty-five survived to the gastrula stage. The results 

 are shown by the last twenty-five diagrams, occupying the lower 

 part of the page, in figure 18. 



These experiments confirm the conclusion reached by the 

 method of orientation. Whatever the origin and significance 

 of the excentricity in the cleavage pattern of the early blastula, 

 it is clearly of no value in foreshadowing the direction of the future 

 median plane of the embryo. If it is indeed causally related to 

 the development of the definitive bilateral symmetry of the 

 embryo, then in its incipient condition this bilaterahty must be 

 an unstable thing, subject to a shifting of one of its axes of differ- 

 ential cellular activity. 



It would be interesting to apply the same tests to stages 8 and 

 9, but in these stages the excentricity in the superficial cleavage 

 pattern is not so easily recognizable, particularly in hving 

 material. 



2. Relation to the first cleavage furrow. This comparison was 

 made by identifying the first cleavage furrow in eggs that showed 

 marked excentricity in the superficial cleavage pattern of stages 

 5 to 7, inclusive, using preserved material. In these stages the 

 first cleavage furrow in the region of macromeres can be iden- 

 tified with certainty. The results for twenty-three eggs may be 

 ummarized as follows: in nine eggs the axis of excentricity 

 coincides approximately with the plane of first cleavage; in seven 

 eggs the axis of excentricity is oblique to the plane of first cleavage 



