2 5O REG EN ERA TION 



assumption that the internal symmetry may develop in response to a 

 symmetrical change in shape of the piece as a whole, which is partly 

 the outcome of purely physical factors. At present, however, I see 

 no other probable inference from the facts. 



If we suppose a bilateral structure is present in the fertilized egg, 

 and that it corresponds to the first plane of cleavage, a change of the 

 sort that we have just sketched above may be supposed to take place 

 when the blastomeres are separated. The stimulus is found in the 

 new spherical form assumed by the isolated blastomere, and we may 

 imagine the change to take place, in the way indicated, by virtue 

 of the old bilaterality that is present, the change beginning at the 

 side originally in contact with the other half. 



There are several facts which seem to indicate that a change in the 

 axial relations of the egg is very easily brought about before any 

 definite organs have appeared. The fact that the point of entrance 

 of the spermatozoon in the egg of the frog 1 and of the sea-urchin 2 

 may determine the first plane of cleavage points to this conclusion. 

 The fact that, in the frog, and also in the triton, the median plane of 

 the embryo corresponds sometimes to the first, sometimes to the 

 second plane of cleavage, and sometimes to neither one, shows that 

 the bilaterality of the embryo-structure may or may not coincide with 

 the plane of cleavage. In the fish also there seems to be no corre- 

 spondence between the planes of cleavage and those of the embryo, 

 so that different factors may determine the two. We should not be 

 justified in concluding from this evidence that a bilateral structure is 

 absent, but rather that it is of such a sort as to be independent of 

 the cleavage, and that it can be also easily changed. It is prob- 

 able that the kind of organization that we must suppose to exist in the 

 egg is of a very simple sort, and capable of easy readjustment. There 

 is certainly no evidence in favor of the view that the organization of 

 the egg need be anything like the organization of the embryo that 

 comes from the egg, although the organization of the egg may be 

 perfectly definite in its character. Until we know more of the nature 

 of this organization, it is useless to speculate further as to how it can 

 be altered. 



Another question of much importance in connection with our 

 present topic is the part played by the individual cells in the 

 early development of the whole egg, or of any part of the egg. 

 Hertwig ('93) thinks that the development is brought about by the 

 action of the individual cells on each other. Driesch, when he states 

 that the fate of a blastomere is a function of its position in the whole, 

 does not commit himself definitely one way or the other so far as the 

 cell as a unit is concerned. Whitman and others have urged the 



1 According to Roux. 2 According to E. B. Wilson. 



