BILATERALITY IN CRYPTOBRANCHUS 373 



egg where the plane of first cleavage intersects the equator; 

 these spots remained perfectly distinct in the early gastrula 

 stage. To determine accurately the relation between the first 

 cleavage furrow and the median plane of the gastrula, it is 

 necessary that the egg be examined immediately after the 

 beginning of gastrulation ; for during the progress of gastrulation 

 one of the spots may, in certain cases, become involved more 

 than the other in the shifting of material toward the median 

 hne, which has been described as a phenomenon of concrescence 

 (Smith, '14). By taking observations promptly after the first 

 appearance of the dorsal hp of the blastopore, this source of 

 error may be avoided entirely. 



Fifty-five eggs were thus treated; forty-two survived to the 

 gastrula stage. In these forty-two eggs (fig. 15) there was no 

 uniformity in the direction of first cleavage with respect to the 

 median plane of the gastrula. Experience with the method here 

 employed inspires one with so much confidence in the accuracy 

 of the data obtained that the results of this experiment alone 

 might be taken as a conclusive answer to the question under 

 consideration. 



4. Direct comparison. As already stated, the biradial symme- 

 try of the cleavage pattern in the lower hemisphere enables one 

 to identify first and second cleavage furrows in the late cleavage 

 stages (figs. 26 and 27). It is sometimes possible, with the aid 

 of a good dissecting microscope, to identify the first and second 

 cleavage furrows in the region of the vegetal pole even after the 

 appearance of the blastopore (figs. 28 and 29); this enables one 

 to make a direct comparison between the direction of the first 

 cleavage furrow and the median plane of the gastrula. 



The first cleavage furrow was identified in the early gastrula 

 stage in twenty-seven eggs. In six eggs the first cleavage furrow 

 extended approximately in the median plane of the embryo, in 

 eleven eggs it was oblique to the median plane, and in ten eggs it 

 crossed the median plane approximately at right angles. The 

 vahdity of these results depends of course on a correct identifica- 

 tion of the first cleavage furrow. All cases that seemed doubtful 

 were discarded, but there remains the possibility that one might 



