BILATERALITY IN CRYPTOBRANCHUS 389 



the first evidence that gastrulation is about to begin is the fact 

 that the cells lying just below the crevice on the dorsal side of the 

 egg, where the segmentation cavity dips downward, become 

 rounded or oval in outhne, preparatory to immigration or in- 

 vagination. 



The morphological evidence seems sufficient to connect the 

 bilateral symmetry of the late blastula (stages 8 to 10) with the 

 definitive bilateral symmetry of the gastrula, since it is very 

 improbable that sudden changes in the direction of the bilateral 

 organization of the blastula should occur after this condition is 

 well established. 



The meridian that bisects the beginning blastopore defines 

 the median plane of bilateral symmetry of the gastrula, which 

 ultimately becomes the sagittal plane of the embryo. As more 

 fully described in previous contributions (Smith, '12, II; and 

 '14), the anterior end of the embryo forms in this meridian about 

 40° from the animal pole, while the posterior end forms in the 

 region where the blastopore closes, at the vegetal pole. Conse- 

 quently, the dorsal side of the embryo forms mainly from mate- 

 rials which in the early gastrula stage lie between the beginning 

 blastopore and a point situated some distance above it, in the 

 thinner portion of the roof of the blastocoele. In the late blastula 

 and early gastrula, this dorsal region coincides with the region 

 of greatest cell activity, which is not confined to the thin portion 

 of the roof of the blastocoele, but extends below the level of the 

 blastocoele into the equatorial region on the side of the egg 

 where the blastopore begins to form. This extension or shifting 

 of the original area of excentric cellular activity is an expression 

 of the circumcrescent or epibolic phase of gastrulation. The 

 opposite and less active side of the egg, where the now thickest 

 portion of the roof of the blastocoele joins the yolk cells, is 

 ventral. Hence the differentiation which first establishes the 

 definitive bilateral symmetry of the blastula is dorsoventral in 

 direction, and constitutes a new embryonic axis secondary to the 

 principal or polar axis of the egg. In other words, radial symme- 

 try with axial differentiation gives place to bilateral symmetry 

 as soon as a new axis of differentiation is established approxi- 

 mately at right angles to the first. 



