DYNAMICS OF MORPHOGENESIS 133 



'11 e, pp. 203-204) and it was pointed out that the formation of a 

 head, though often a teratomorphic head, was still possible in 

 pieces where the development of the pharynx was completely 

 inhibited. 



There can be no doubt that a variety of other external agents 

 which decrease metabolism will produce similar results, but since 

 the present investigation is primarily concerned with the internal 

 processes and conditions rather than with the external factors 

 which produce them, no attempt has been made thus far to extend 

 the experimentation to a great variety of substances. It has 

 seemed preferable to acquire a more extended knowledge of the 

 action of a few agents and conditions as a basis for further work. 

 Sooner or later of course it will be desirable to compare the effects 

 of a great variety of agents and such comparison will undoubtedly 

 bring to light facts of much interest, but for work of this character 

 an adequate basis is absolutely essential. 



C. THE AXIAL FACTOR IN RELATION TO ANTERIOR AND POSTERIOR 



ZOOIDS 



In the experiments described above only the axial gradient 

 within the limits of a single zooid has been considered. Within 

 these limits the gradient is more or less uniform but in the passage 

 from the posterior end of an anterior to the anterior end of a more 

 posterior zooid it shows a marked change and in general the pos- 

 terior zooids are in a somewhat different dynamic condition from 

 the anterior zooid. 



Attention was called above (p. 105) to the fact that in the regu- 

 lation of pieces under constant conditions the passage from the 

 posterior region of the anterior zooid to the anterior region of the 

 second zooid is clearly marked by a change in the regulatory 

 capacity of the pieces. Pieces of given length from the anterior 

 region of the second zooid show a much greater capacity to form 

 normal heads and so to undergo complete regulation than do the 

 pieces anterior to them in the posterior region of the first zooid : 

 in other words, the pieces from the second zooid are physiologi- 

 cally more 'anterior,' more like pieces from the region near the 

 head than those which lie just anterior to them in the body. It 



