392 BERTRAM G. SMITH 



fact that in many species of animals the egg develops bilaterality 

 in advance of fertilization, and many eggs are capable of develop- 

 ing parthenogenetically into organisms possessing the bilateral 

 symmetry characteristic of the species. Consequently, there is 

 no necessary relation between the fertilization meridian and the 

 plane of symmetry. 



The relationship which has been shown to exist in certain cases must 

 therefore depend upon a certain time relationship in the course of the 

 two processes. The influences radiating from the spermatozoon estab- 

 lish a gradient from its original excentric position, which may influence 

 the direction of the plane of symmetry in w^hich there is also a gradient, 

 if its determination is synchronous, as in the frog (Lillie, '19). 



In the egg of the frog, external influences such as gravity 

 and light acting at the time of fertilization may exert an addi- 

 tional modifying influence in determining the direction of the 

 median plane (Jenkinson, '09, Appendix A). In the egg of 

 Cryptobranchus, neither the direction of sperm entrance nor the 

 influence of gravity are factors of any appreciable importance in 

 the determination of bilaterality. 



In the frog's egg, the first cleavage furrow usually passes 

 through the entrance-point of the spermatozoon (Newport, '54; 

 Roux, '85, '87; Schultze, '00). According to Roux ('87), it is 

 the entrance-path of the spermatozoon that determines the posi- 

 tion of the gray crescent, while it is the latter part of the sperm- 

 path, the 'copulation-path,' that determines the direction of 

 first cleavage. Since the entrance-path and the copulation-path 

 do not alwaj^s lie in the same direction, we have here an explana- 

 tion of the fact that, in the egg of the frog, the first cleavage 

 furrow sometimes fails to coincide with the plane of symmetry 

 (Jenkinson, '09, pp. 248, 307 and 308). In the egg of Crypto- 

 branchus the first cleavage furrow tends to form at right angles to 

 the fertilization meridian. I have not been able to follow satis- 

 factorily the latter part of the sperm-path in Cryptobranchus, 

 but it seems likely that the condition is the same as in the egg of 

 the axolotl (Fick, '93), where the copulation-path forms at right 

 angles to the entrance-path. 



C. Relation of the first cleavage furrow to the median plane of the 

 embryo. Recent observers agree that the cleavage of the am- 



