THE VITAL IMPETUS 137 



But each partial blastula becomes an entire 



embryo and the same part a occupies a different 



position in each. 



Therefore any part of the blastula may become 



any part of the embryo. 



Now if a mechanism is involved, it must, according 



to our ideas of mechanism, be one which is different 



in its parts, for each part of it produces a different 



result from the others ; 



But since any part of the mechanism may produce 



any of the different results contained in the embryo, 



every one of its parts must be similar to every 



other one. 



That is, all the parts of the mechanism are the 



same, though the hypothesis requires that they 



should be different. 



We conclude, then, that a mechanism such as we 

 understand a mechanism to be in the physical sciences 

 cannot be present in the developing ovum. 



Nevertheless, an organisation, using this term as an 

 ill-defined one for the present, must exist in the ovum, 

 or the system of undifferentiated cells into which the 

 ovum divides, during the first stages of segmentation. 

 In certain animals, Ctenophores (Chun, Driesch, and 

 Morgan), and Mollusca (Crampton), for instance, 

 separation of the blastomeres in the first stages of 

 segmentation produces different results from those 

 mentioned above. In these cases the isolated blasto- 

 meres develop as partial embryos, that is, the latter are 

 incomplete in certain respects, and this incompleteness 

 corresponds, in a general way, to the incompleteness 

 of the part of the ovum undergoing development. We 

 have thus the apparently contradictory results : (i) 

 each of the first few blastomeres resulting from the first 

 divisions of the ovum is similar to the entire ovum, 



