THE MORPHOLOGY OF FERTILIZATION 85 



on the frog's egg have shown that in such cases the plane 

 of symmetry and the fertilization meridian neverthe- 

 less coincide. Roux's theory therefore does not hold 

 except for the direction of the first plane of cleavage, 

 which in the frog may not coincide with the plane of 

 symmetry. 



There is, however, no necessary relation between 

 the fertilization meridian and the plane of symmetry, 

 because polyspermic eggs develop a plane of symmetry, 

 and so also do parthenogenetic eggs. 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 radi- 

 ating from the spermatozoon establish a gradient from 

 its original eccentric position, which may influence the 

 direction of the plane of symmetry in which there is 

 also a gradient, if its determination is synchronous, as 

 in the frog. 



In the case of the annelid Nereis the observations 

 of Just (1912) show that the plane of symmetry is at 

 right angles to the fertilization meridian. Here again 

 we have obviously an interaction of two distinct gra- 

 dient processes, which, however, attain a different equi- 

 librium from the frog. The data on this subject are 

 few, owing to the difficulty of making the necessary 

 determinations. The ova of many animals are definitely 

 bilaterally symmetrical before fertilization, which has 

 therefore nothing to do with its determination. The con- 

 clusion would therefore seem to be that, when the 

 determination of symmetry and fertilization overlap, 

 the former may be affected as to its orientation by the 

 latter. 



