THE VITAL IMPETUS 141 



active masses of these substances. But variations in 

 this active mass affect only the velocity of the reaction. 

 What we have to account for in our blastula experi- 

 ments is the nature of the reaction, and how can 

 velocity or even nature of reaction affect form ? If we 

 could show that the form of the crystals deposited 

 from a solution in some reaction depended on the 

 volume of the solution, the analogy would be closer, 

 though even then the difficulties in pressing it would 

 be so enormous as to render it futile to attempt to 

 entertain it. 



A chemical mechanism cannot, then, be imagined, 

 much less described, and the only other mechanism so 

 far suggested is the Roux-Weismann one, involving 

 the disintegration of the determinants supposed to be 

 present in the egg nucleus. Let us suppose (in spite 

 of the incredible difficulty in so doing) that there is 

 such a mechanism. It must usher the nuclei contain- 

 ing the determinants of the embryonic structure into 

 their places : those for the formation of the nerve- 

 centre go forward ; those for the mouth, gut, and anus 

 go backwards and downwards ; those for the arms go 

 forwards, ventrally, and posteriorly, in a very definite 

 way ; and those for the complicated skeleton are 

 distributed in a variety of directions which defy 

 description. These nuclei are, in short, moved up 

 and down, right and left, backwards and forwards, 

 and become built up into a complicated archi- 

 tecture. Suppose we prevent this. Suppose we com- 

 press the segmenting egg between glass plates so that 

 the nuclei are compelled to distribute themselves 

 in one plane only : to form a flattened disc in which 

 the only directions are right and left and anterior 

 and posterior. This has been done by Driesch and 

 others. On the Roux-Weismann original hypothesis 



