BILATERALITY IN CRYPTOBRANCHUS 



391 



B. Influence of the spermatozoon and of environmental factors. 

 Various observers (Roiix, '83, '85; Schultze, '00; Morgan and 

 Boring, '03) have shown that following fertihzation and previous 

 to cleavage the egg of the frog is bilaterally organized, and 

 that this bilateral symmetry foreshadows the definitive bilateral 

 symmetry of the embryo. In the fertilized egg the bilateral 



Fig. 33 Sagittal section of a beginning gastrula of Cryptobr'anchus allcgheni- 

 ensis. B, blastopore; S, segmentation cavity or blastocoele; Y, yoJk. The 

 drawing was made with the aid of a camera lucida. X 15. 



organization expresses itself superficially in the formation of a 

 gray crescent on the site of the future blastopore. Since the 

 crescent is formed on the side opposite the point of entrance of 

 the spermatozoon (Roux, '83, '85, '87, '03; Schultze, '00), Roux 

 maintained that the point of entrance of the spermatozoon 

 determines the direction of the median plane of the embr^^o. 



That the egg is not dependent upon the spermatozoon for 

 the establishment of its bilateral symmetry is shown by the 



