EMBRYOLOGY OF CRYPTOBRANCHUS 481 



In the living egg, the roof of the segmentation cavity appears 

 as a translucent tissue throughout a circular area about 40 degrees 

 in diameter in the region of the animal pole. This indicates a 

 decided thinning-out of the cells of this region. 



Biradiality of the cleavage pattern of the lower hemisphere 

 still enables one to distinguish in many embryos, though not 

 in all, the first and second cleavage furrows. Usually two or 

 three cells quite small in surface view occur at the vegetal pole; 

 they are quite characteristic of this and the following stage, but 

 are sometimes found in the preceding stage. At the vegetal 



109 < no 



Figs. 109 and 110 Stage 9 of Cryptobranchus allegheniensis. Equatorial 

 view and lower hemisphere of different eggs, showing cleavage pattern. Camera 

 drawings from preserved material. X 7. 



pole the cleavage furrows, both in living and preserved material, 

 are sometimes both broad and deep, forming quite noticeable 

 fissures; a similar condition is common in Amblystoma (Eycles- 

 hymer '95). In Cryptobranchus this condition is in marked con- 

 trast to the stage immediately preceding, in which the furrows 

 in this region were faint. In Necturus, on account of the varia- 

 bility of the third cleavage furrows, the biradial pattern of the 

 macromeres is not so clearly expressed as in the egg of Crypto- 

 branchus. 



Stage 10: {figs. Ill, 112 and 219). This stage, reached a day 

 or two later than Stage 9, immediately precedes the beginning 

 of gastrulation. The micromeres at the upper pole are invisible 



