No. 2.] ORGANIZATION OF THE EGG OF UNIO. 27 1 



I dwell on this because I am especially concerned here with 

 a problem of localization, vis. : What determines the position 

 of the first cleavage plane ? and this, as I have attempted to 

 show, is equivalent to asking, What determines the general 

 proportions of the embryo of Unio ? With Driesch I recog- 

 nize that the conditions of this problem are found in a mass 

 of protoplasm, the parts of which cannot be shown to be 

 different. 



Now how does Driesch conceive of this mechanism ? In 

 the first place, he attempts to explain the polarity of the egg 

 by assuming polarity of the protoplasmic parts or elements, 

 without considering further the nature of these ; and as he 

 cannot find the causes of the bilateral structure of the embryo 

 in external conditions, he endows the parts with bilaterality 

 also, so that each is doubly heteropolar. The problem of the 

 polarity and bilaterality of the egg is thus transferred to its 

 parts, so that the organization of the whole might be pictured 

 as due to a "magnetization of the parts." But these postu- 

 lates are still inadequate to explain the localization of onto- 

 genetic processes ; why the stomodaeum, for instance, arises in 

 a certain part, or why the intestine of the pluteus becomes 

 divided into three parts of definite proportions. How is this 

 condition satisfied in Driesch's system ? 



" Let us first examine the simpler case of differentiation in 

 a system with but one heteropolar axis, for instance, the intes- 

 tine of the echinid gastrula, or the stem of Tiibularia. Here 

 we can represent the process in * causal ' form to ourselves 

 by assuming that one of the end points of the axis is the seat 

 of incitive (auslosender) ' Fernkrafte ' ; this spot, which is dis- 

 tinguished as something different from other points of the 

 system by the very fact that it is the end of the axis, produces, 

 therefore, an effect { Wirkt also) ; that on which the effect 

 is produced must be able to answer to the cause, as in all 

 cases where effects are produced ; and in this manner a kind 

 of causal harmony is postulated by this conception of the 

 matter. 



" In what way, now, does this particular designated place of 

 the axis affect the system .'' Let us explain this first in the 



