166 LIBBIE H. HYMAN 



the head of Dero is determined at a time when the rate of me- 

 tabolism of the pieces is low, and that therefore normal heads are 

 always produced. 



Why does head formation cease at an extreme anterior level 

 in the tiibificids? To answer this question is one of the most 

 difficult problems with which we have to deal in the field of 

 morphogenesis. The dynamic factors which have thus far sufficed 

 to explain the experimental results cannot be called upon here, 

 for this result is independent of size of piece. Numerous attempts 

 to alter the regenerative capacity of Tubifex and Limnodrilus 

 have yielded negative results. The following suggestions are 

 offered as to the cause of failure of head formation in these forms. 

 In the first place, the new tissue grows out very slowly, and with 

 a relatively low rate of metabolism. It therefore is very easily 

 inhibited. Secondly, the secondary gradient runs far forward in 

 these forms, and may serve as the inhibiting factor. The cells 

 at the cut surface of a piece of Tubifex taken back of the fifteenth 

 segment are in contact with a region of high rate, which is itself 

 in contact with regions of higher rate, and so on. Unless, there- 

 fore, the cells at the surface grow out rapidly and with a very 

 high rate, and this is contrary to fact, they could not dominate 

 the regions behind. Heteromorphic tails would be expected 

 under such conditions; they actually occur in the earthworms, 

 but here the cells do not grow out fast enough, and they are 

 inhibited before they have an opportunity to produce anything. 

 It is obvious that this explanation can be tested experimentally, 

 and I intend to continue my experiments with Tubifex in the 

 hope of obtaining positive results. As to why the posterior end 

 of such pieces does not give rise to heads has already been dis- 

 cussed; the secondary gradient cannot eliminate the primary 

 gradient to that extent. 



Regarding the formation of the posterior end, a word should be 

 said. The cells -at the posterior end grow out with a high rate of 

 metabolism, but cannot become independent as long as the pri- 

 mary gradient persists. Owing to this gradient they are in 

 contact with subordinate parts, and must develop in corre'ation 

 with more anterior regions. If, however, the gradient is elimi- 



