SO-CALLED FORMATIVE SUBSTANCES. 175 



positively that the writer's position does not at all involve a de- 

 nial of the clearly demonstrated fact that these regions are differ- 

 ent or have become different from others in the egg or embryo, 

 nor does he underestimate the value of the observations which 

 have directed attention to this fact. But to say that these regions 

 contain specific formative substances is to say too much, for the 

 visible differences do not necessarily stand in direct relation to 

 their formative activity. The same " kind" of protoplasm is cer- 

 tainly capable of widely diverse formative reactions under differ- 

 ent conditions and the determining conditions are demonstrated in 

 many cases to be dynamic rather than substantial. It seems more 

 nearly correct therefore to maintain that particular lines of activity 

 have been initiated in these formative regions in consequence of 

 past or present conditions and may continue even after isolation 

 of the regions. That different substances exist in the egg and 

 embryo is also clearly demonstrated, but to assert that these are 

 formative or that their presence indicates the existence of particu- 

 lar formative substances is quite another matter, and as the writer 

 believes, not justified by the facts. Should we not therefore be 

 content in view of the facts to designate these so-called "forma- 

 tive" or " morphoplasmic substances" merely as cytoplasmic 

 differentiations, leaving the question as to the nature and sig- 

 nificance of the differentiation open ? 



FORMATIVE SUBSTANCES IN REGULATION. 



The attempt has recently been made by Morgan ' to account 

 for certain phenomena of regulation, especially those connected 

 with polarity by postulating the existence of formative substances. 

 This hypothesis is selected for discussion since it is the latest 

 attempt to interpret formative processes in this manner. Other 

 similar hypotheses are, however, open to most of the same 

 objections. In addition to the objections discussed above to 

 which the idea of formative substances in general is open there 

 are certain other objections which apply specially to the applica- 

 tion of this idea to the phenomena of form-regulation. 



It is of interest to note first that these formative substances are 

 quite different from those of the embryologists. Here for exam- 



1 Science, XX., December, 1904. Joitrn. Exp. ZooL, I., 1904, and II., 1905. 



