BILATERALITY IN CRYPTOBRANCHUS 397 



To explain the maintenance of bilateral symmetry throughout 

 the subsequent development of the embryo, it is necessary to 

 suppose that later differentiations are conditioned and controlled 

 by the differentiations along the two axes already established; 

 in particular, differentiation along the mediolateral axes of the 

 body does not proceed independently, but only in subordination 

 to the more potent and regulatory anteroposterior and dorso- 

 ventral differentiation. 



It would seem simplest to assume that right and left halves of the 

 body are not at all self-differentiated, but that they are conditioned by 

 the other body axes. In this way the establishment of two axes each 

 with two distinct poles (anterior and posterior; dorsal and ventral) 

 would fully sufhce to determine the bilaterality of the organism, be- 

 cause, if we presuppose a similar interaction of analogous anlagen, those 

 lying in the third axis and giving rise to the right and left halves of the 

 body should naturally arrange themselves so that they would represent 

 mirror images of each other, when they occupy the same position in 

 relation to the two differentiated axes (Przibram, '11). 



SUMMARY 



1. The polarity *of the egg of Cryptobranchus allegheniensis 

 arises during ovogenesis and establishes approximately the direc- 

 tion of the anteroposterior axis of the embryo. 



2. The organization of the mature but unfertilized egg is 

 characterized by radial symmetry with differentiation along the 

 polar axis, but with no evidence of bilaterality. 



3. Gravity acting at right angles to the polar axis of the egg 

 during the fertilization period is without perceptible effect in 

 determining the direction of the median plane of the embryo. 



4. The direction of entrance of the spermatozoon is not a 

 controlling factor in determining the direction of the median 

 plane of the embryo. 



5. The first cleavage furrow forms approximately at right 

 angles to the direction of the entrance-path of the spermatozoon. 



6. The direction of first cleavage bears no fixed relation to the 

 direction of the median plane of the embryo. 



7. In the early blastula stages, the direction of excentric de- 

 velopment of the micromeres bears no constant relation to the 

 direction of the median plane of the embryo. 



