EMBRYOLOGY OF CRYPTOBRANCHUS 



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largest macromeres are the only ones exposed, and we may feel 

 quite sure that the vegetal pole lies in their midst and probably 

 very near the center of the greatly diminished yolk plug. The 

 morphological axis of the egg is thus approximately determined 

 by a line passing from the center of the yolk plug through the 

 center of the egg, and at the close of Stage 12 this axis makes an 

 angle of 52 degrees with the vertical — showing that rotation has 

 proceeded 8 degrees further than in the preceding stage (fig. 149). 



Fig. 148 Upper hemisphere of a living egg of Cryptobranchus allegheniensis 

 viewed so far as possible by transmitted light, shortly before the appearance of 

 the neural folds. X 9. 



It will be recalled that the dorsal lip of the blastopore first 

 appears, on the average, 68 degrees above the vegetal pole (fig. 

 134), and that the ventral lip is first formed about 68 degrees 

 from the vegetal pole on the opposite side (fig. 136). Hence 

 the dorsal and ventral lips are formed respectively at approxi- 

 mately equal distances from the yegetal pole, though the ventra) 

 lip is formed much later than the dorsal. We may now compute 

 the amounts of overgrowth of the dorsal and the ventral lips 

 respectively: at the close of Stage 12 the dorsal lip has overgrown 

 the yolk for an average distance of 55 degrees, and the ventral 

 lip has advanced toward it through an arc of about 55 degrees; 



