EMBRYOLOGY OF CRYPTOBRANCHUS 



471 



By the shifting of micromeres the biradial symmetry due to 

 the mamier of third cleavage is usually lost in the blastodisc, 

 and unless the egg has been kept under continuous observation 

 it becomes in most cases impossible to trace the first and second 

 cleavage furrows entirely through the region of micromeres. 



In preserved material, nuclei are visible from the surface in 

 some of the micromeres of this and the following stages, indicating 

 that these cells are becoming flattened out. 



As already noted, in this stage if not in the preceding one, 

 the third cleavage furrows become complete, usually joining the 

 first at some distance from the pole (figs. 91 to 96). This appar- 

 ent avoidance of the pole by the third cleavage furrow is doubt- 



3 I 



96 



Figs. 91 to 96 Lower hemispheres of eggs of Cryptobranchus allegheniensis 

 in the fifth cleavage stage. All the figures are camera drawings from preserved 

 material. Figure 93 shows a persistent sperm pit (see Part I, Smith '12). X 4f . 



Fig. 91 Lower hemisphere of the egg whose upper hemisphere is shown in 

 figure 87. 



Fig. 96 Lower hemisphere of the egg whose upper hemisphere is shown in 

 figure 85. The fourth cleavage furrows have extended further than is usual in 

 eggs of this stage. 



