BILATERALITY IN CRYPTOBRANCHUS 



385 



gastrula stage. These features are usually better expressed than 

 in the eggs shown in the figures, which were chosen because the 

 distinctness of their early cleavage furrows enabled them to be 

 drawn w^ith a camera lucida. The dorsal lip of the blastopore 

 serves as a landmark for determining the true median plane of 

 bilateral symmetry of the gastrula, and in every case studied 

 the axis of excentricity and bilateraHty of the superficial cleavage 

 pattern lies in this median plane; this was clearly made out in 



Figs. 28 and 29 Surface views of the lower hemispheres of two eggs of Crypto- 

 branchus allegheniensis in the early gastrula stage, showing the biradial charac- 

 ter of the cleavage pattern which enables one to distinguish first and second 

 cleavage furrows. In figure 28 the first cleavage furrow lies approximately in 

 the median plane of the gastrula; in figure 29 it lies nearly at right angles to this 

 plane. The drawings were made with the aid of a camera lucida. X 7. 



about a dozen eggs. We conclude, therefore, that the definitive 

 bilateral symmetry of the embryo is indicated by the cleavage 

 pattern of the late blastula; the earliest stage in which bilateral 

 symmetry thus becomes unequivocally expressed is the one 

 designated stage 9. 



2. Relation to the first cleavage furrow. On account of the 

 biradial character of the cleavage pattern of the lower hemi- 

 sphere, it is often possible to distinguish the first from the second 

 cleavage furrow in the vicinity of the vegetal pole of the late blas- 

 tula. Observation of a considerable number of eggs makes it 

 certain that the direction of the first cleavage furrow bears no 

 constant relation to the axis of bilaterality in the cleavage pattern. 



