BILATERALITY IN CRYPTOBRANCHUS 359 



in its organization from the beginning, but arises during ovo- 

 genesis. 



In the gastrula stage, the bilateral symmetry of the embryo is 

 expressed in a perfectly obvious manner. Our problem then 

 requires us to look for the beginnings of bilateral symmetry in 

 the cleavage stages, or possibly in the fertilization stage, and leads 

 us to consider every deviation from strict radial symmetry that 

 suggests the beginning of definitive bilateral syimnetry. The 

 following features must be considered in their relation to the posi- 

 tion of the dorsal lip of the future blastopore, and as a check on 

 the results it is desirable that these same features should be 

 considered in their relation to each other : 



1. The direction of the entrance-path of the spermatozoon. 



2. The direction of the first cleavage furrow, which defines an 

 axis of biradial symmetry in the cleavage pattern of the third and 

 later cleavage stages. 



3. The excentric development of the micromeres shown by the 

 superficial cleavage pattern of the early blastula. 



4. The excentric development manifested by the internal struc- 

 ture of the early blastula. 



5. The bilateral symmetry of the superficial cleavage pattern 

 in the lower hemisphere of the late blastula. 



6. The bilateral symmetry manifested by the internal structure 

 of the late blastula. 



One desires to know the nature of the factors at work in pro- 

 ducing bilateral symmetry or determining the direction of the 

 median plane, when the egg is developing in its natural environ- 

 ment. The egg of Cryptobranchus is fertilized immediately 

 after spawning. Under the influences of light and gravity, 

 streaming movements may be induced in the cytoplasm of the 

 fertilized but unsegmented egg, which modify the results due 

 to the operation of internal factors; in the egg of the frog such 

 complications have been observed by various investigators. 

 Another circumstance that must be taken into account is 

 the well-known fact that in the amphibian egg the direction of 

 the early cleavage furrows may be changed by mechanical 

 pressure. 



