484 BERTRAM G. SMITH 



tricity previously defined, hence lies in the sagittal plane of the 

 embryo. The excen.tric position of the vegetal pole within the 

 area occupied by the macromeres, and the bilateral character 

 of the cleavage in this region, are more marked in many eggs 

 taken inmediately after the beginning of gastrulation; these 

 features are usually better expressed than in the eggs shown in 

 figures 111 and 112, which were chosen because the distinctness 

 of the early cleavage furrows enabled them to be drawn with the 

 camera lucida. Schultze ('00, Taf. 11, fig. 12) has described a 

 similar bilaterality in the late cleavage of the lower hemisphere 

 of the frog's egg. 



The question of the possible relation of the excentric and bilat- 

 eral development of the lower hemisphere just described, to the 

 excentric development of the blastodisc noted in previous stages, 

 will be discussed in a later paper. 



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

 apparently thin, is not quite so translucent as in the preceding 

 stage. It is, however, noticeably more translucent on the side 

 toward the future blastopore, and on this side the transition to 

 the opaque yolk cells is more abrupt. 



During the late stages of cleavage, a tendency toward fading 

 out of the cleavage furrows in the vicinity of the vegetal pole 

 has been noted. In some individual cases this process has gone 

 so far that the earlier cleavage furrows are lost to view, even 

 when searched for with the binocular microscope. This tendency 

 may be interpreted as due to a difficulty in sustaining the holo- 

 blastic method of cleavage in an egg so heavily laden with yolk. 

 In the corresponding stages of Necturus, this tendency is even 

 more marked. My study of the cleavage pattern of the lower 

 hemisphere of the late segmentation stages of both Cryptobran- 

 chus and Necturus has been confined to preserved material, but 

 Professor Dean informs me that he has noticed this merging of 

 lower blastomeres in the late segmentation stages of the living 

 eggs of Necturus. My Necturus material is not so favorable 

 as the egg of Cryptobranchus for the study of the cleavage pattern 

 in this stage, so a detailed comparison will not be attempted. 



In the very late cleavage stages of Cryptobranchus japonicus, 

 Ishikawa ('08 and '09) describes a shallow furrow ('Scheidewand- 



