EMBRYOLOGY OF CRYPTOBRANCHUS 491 



morphological axis to an angle of 44 degrees from the vertical; 

 the ventral lip of the blastopore now lies about 24 degrees beyond 

 the lower pole of the vertical axis. These changes are set forth 

 diagrammatically in figures 134 to 137. 



Two quantitative results of considerable importance are 

 brought to light through the study of these data: (a) the dorsal 

 lip of the blastopore has grown downward over the yolk cells for 

 a distance of about 42 degrees; (b) the egg has rotated in the 

 opposite direction about 37 degrees from its position at the begin- 

 ning of gastrulation, making a total rotation of 44 degrees. At 

 first, overgrowth is more rapid than rotation; at the time when 

 the blastopore has reached the form of a semicircle its dorsal lip 

 is 43 degrees below the horizontal equator. Later, rotation is 

 more rapid than overgrowth, and at the time when the blastopore 

 has become a complete circle its dorsal lip has been carried back 

 to a position 20 degrees below the horizontal equator, only 5 

 degrees lower than its original position in space. 



The changes in the upper hemisphere visible from the surface 

 during the establishment of the blastopore are remarkable, since 

 they afford clues to many important processes within. In the 

 living egg especially, because of the translucent character of 

 the upper hemisphere, one is able to get total views of many 

 phases of gastrulation, such as could not be obtained from serial 

 sections except by means of reconstructions. Except where 

 otherwise noted, the following description is based on the study 

 of the living egg. 



At the very beginning of the process of gastrulation, the nearly 

 transparent roof of the segmentation cavity is of quite uniform 



Figs. 116 to 121 Stage 11 (gastrula) of Cryptobranchus allegheniensis. Cam- 

 era drawings from preserved material. In all the figures, the upper vertical pole 

 as determined by gravity lies toward the top of the page; blp., dorsal lip of the 

 blastopore; f., fenestra (roof of the blastocoele differentiated into a window-like 

 structure); s.f., septal furrow. X 7^. 



Fig. 116 Lateral view of an early gastrula stage. The sharp differentiation 

 of the fenestra is rather precocious in this egg. 



Figs. 117 to 119 Lateral views of a characteristic series of later embryos. 



Figs. 120 and 121 Antero-ventral views showing stages in the disappearance 

 of the fenestra. Figure 120 is from the egg drawn for figure 119. 



