BILATERALITY IN CRYPTOBRANCHUS 395 



of the region of micromeres. In the frog's egg, Morgan and 

 Boring ('03) have noted that the pigmented cells on the gray 

 crescent side of the egg are slightly smaller from the beginning 

 than the other pigmented cells. Eycleshymer ('98, '02, '04, 

 '15) has shown that in the blastula stages of Amblystoma, 

 Nectiirus, Rana, Acris, and Bufo there is a secondary or excentric 

 area of smaller cells, which, roughly speaking, lies within a sector 

 of the circular area occupied by the micromeres, and that this area 

 of accelerated cell division always lies on the side on which the 

 dorsal lip of the blastopore is to appear. 



The primary area of cellular activity, at the upper pole of the amphib- 

 ian egg, forms the basis of the cephalic end of the embryo. The 

 secondary area of cell activity, on the blastoporic side of the egg, forms 

 the basis of the greater portion of the posterior half of the embryo. 

 These two areas constitute an embryonic tract, from which arise at 

 least two-thirds of the embryo. The posterior end of the embryo is 

 formed by a coalescence of the lateral portions of the blastoporic mar- 

 gins (Eycleshj^mer, '£8). 



In Eycleshymer's earher writings emphasis is placed on the 

 occurrence of this area of accelerated cell division in the late 

 blastula stage, as illustrated by his figure of Amblystoma (Ey- 

 cleshymer, '98, fig. 100) ; but in his later investigations he found 

 that in several species excentric development is present in the 

 early blastula, sometimes as early as the fourth or fifth cleavage 

 stage. These two similar conditions, appearing respectively 

 early and late in the amphibian blastula, he regarded as geneti- 

 cally continuous. I have been able to confirm Eycleshymer's 

 observation concerning the early appearance of excentric develop- 

 ment in the micromeres of Necturus; but in Cryptobranchus, 

 experimental results make it necessary to distinguish between 

 the problematical significance of the excentricity of the early 

 blastula and the undoubted significance of the bilateral symmetry 

 of the late blastula. According to Lillie ('08, pp. 42 and 47), 

 the axis of excentricity in the early cleavage pattern of the 

 pigeon's egg bears no constant relation to the median plane of 

 the embryo. It is undoubtedly true that the excentric develop- 

 ment of the blastoderm which truly marks the beginning of 



