502 



BERTRAM G. SMITH 



the broad lateral bands lying in the posterior part of the egg at 

 some distance from the median line are more marked than in 

 the preceding stage. The study of sections shows that they are 

 due to the combined optical effect of the mesoderm and an 

 unusually thick region of the entoderm. The neural groove is par- 

 ticularly translucent. The greatly reduced blastocoele persists 

 in the region of the equator on the antero-ventral side of the egg; 

 the center of its external wall is marked by a pit, the vestige of 

 the fenestra. During the latter part of Stage 12 (fig. 148) the 



146 147 



Figs. 146 and 147 A living egg of Cryptobranchus allegheniensis in an early 

 neural groove stage, viewed so far as possible by transmitted light. Figure 146 

 shows the upper hemisphere, figure 147 a postero-dorsal view. X 7. 



lateral bands are obscured by the thickening of the neural plate; 

 in the central portion of the neural groove there usually appear 

 a series of translucent pits arranged at regular intervals. The 

 pit marking the site of the closing fenestra has disappeared, but 

 there usually remains a translucent area indicating a vestige of 

 the blastocoele; this area is often imperfectly separated from 

 the translucent roof of the gastrocoele. 



In this stage it is usually impossible to identify cleavage furrows 

 in the yolk plug, but the stability of the larger cells and the fact 

 that they remain longest exposed afford a means of locating 

 approximately the vegetal pole. We have seen that in the pre- 

 ceding stage the vegetal pole was situated a little above the center 

 of the area of largest macromeres; at the close of Stage 12 these 



