TO THE EMBRYOLOGY OF AMPHIBIA. 7 



cleavage-line. As I have assured myself by a study of sections, 

 it is the superficial expression of the segmentation-cavity within, 

 which is seen through its very thin roof. The size of this area, 

 therefore, indicates, in every stage, the extension of the cavity 

 within, and naturally diminishes gradually with the progress of 

 development, until it vanishes entirely, with the advancing closure 

 of the blastopore (Figs. 9-14). For the sake of convenience, I 

 shall refer to this translucent area hereafter as the " area of the 

 segmentation cavity," and to the area encircled by the blastopore 

 lips as the " blastoporic area." 



In 24-48 hours after deposition, the dorsal lip of the 

 blastopore makes its first appearance close below the egg-equator. 

 Although there are many individual differences, this is seen, in 

 most cases, 10°-20° below the equator. In some rare cases, it 

 may approach to within 4° or 5° of the equator, while in other 

 cases, it rarely is as far as 25° below. These cases were found 

 mostly among eggs which had been taken out of their natural 

 frothy envelope and reared in water. According to my experiences, 

 however, these slight individual variations seem to make no serious 

 differences in the development of the future animal, and indeed 

 it seems to me that the eggs of Amphibia are endowed with a 

 strong power of adaptation and resistance to unnatural and in- 

 jurious influences. 



The encircling of the egg by the blastopore lips is generally 

 accomplished in 3-10 hours, and the complete closure of the 

 same in 15-24 hours after the first appearance of the dorsal lip, 

 varying according to the condition of the atmospheric temperature. 

 The gradual diminution in the size of the translucent area of the 

 segmentation cavity proceeds in the normally growing eggs pari 

 passu with the gradual closure of the blastopore lips, so that by 



