EMBRYOLOGY OF CRYPTOBRANCHUS 



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pole of a germinal disc or blastodisc consisting of two layers, a 

 very thin peripheral layer of yolk-free cytoplasm which has been 

 called the cytodisc, and underlying this a thick lenticular layer of 

 mingled cytoplasm and dense fine yolk which has been called the 

 yolk disc. At any given level the egg is radially symmetrical 

 about the axis of polarity. In general the egg has progressed from 

 an alecithal through an isolecithal to a telolecithal stage. 



Fig. 26 Meridional section through an ovarian egg of an adult Cryptobranchus 

 allegheniensis killed Sept. 6, 1910, showing organization just before the germinal 

 vesicle reaches the surface. X 20. 



The establishment of polarity, with axial differentiation, is an 

 event of great morphogenetic importance, since the formative 

 materials for the embryo are being segregated in the vicinity of the 

 animal pole. Through later changes in the distribution of this 

 material the animal pole comes to mark the anterior, the vegetal 

 pole the posterior end of the future animal; hence the establish- 

 ment of polarity defines the principal axis of the embryo. 



