No. 2.] ORGANIZATION OF THE EGG OF UNIO. 275 



fremde Krafte geformt und an ihren Ort geschleppt worden, 

 sondern Steine und Baumeister zugleich." 



The use of the term " instinct" for such phenomena of irri- 

 tability and response as the parts of an embryo exhibit is ques- 

 tionable, and is not in any case very illuminating. While I 

 heartily agree with Zur Strassen that we must look within the 

 tgg for the chief causes of embryo formation, it appears to me 

 that he attributes too much to " social instinct " of the cells. 

 In this he resembles Hertwig, indeed, sometimes using similar 

 expressions, as where he speaks of single cells of the 4-cell 

 stage as "organisms." The four cells form but one organism ; 

 " it is neither functional economy nor social instinct that binds 

 the two halves of the q^^ together, but the constitutional bond 

 of individual organizatioji" (Whitman, '93, p. 115). 



The view that I have attempted to demonstrate is : i . That 

 the cytoplasm of the Qgg of U)iio possesses a definite organiza- 

 tion involving bilateral symmetry and certain antero-posterior 

 proportions ; 2. That this is not primarily dependent on localiza- 

 tion of specific substances, but in some way results from the 

 interaction of the idiosomes. 



I believe: i. That this particular kind of organization is 

 continued throughout the entire life cycle, and is one of the 

 main factors in axial regeneration where this occurs ; 2. That 

 in every stage of the organism it is an undivided force. It 

 therefore implies protoplasmic continuity throughout. 



I regard this as one of the most important factors in ontogeny, 

 and think it important to clearly distinguish it from the other 

 factors, such as reactions of nucleus and cytoplasm, tropisms 

 and taxic phenomena, and all relations of parts. It differs from 

 these in being the property of the cytoplasm as a whole. 



It appears to me that the problem we are considering recurs 

 in fundamentally the same form in the reorganization of a 

 fragment of a stentor or a planarian into a complete individual. 

 The materials in which the formative forces ^ are at work are 

 different in these cases, and so there are different modes 

 through which they reach their expression. But whether 

 these forces are working through adaptive cleavage, or in a 



^ I use this term in a purely descriptive sense. 



