STUDIES ON THE DYNAMICS OF MORPHOGENESIS 311 



the piece becomes less and less 'totipotent/ until a limit is 

 reached, below which it is capable of producing only a part. 

 In short, the changes in the regulatory capacity with decrease in 

 size of the piece are essentially the result of decrease in physiologic 

 correlation between parts: in other words, the shorter the piece 

 the less its dynamic activity. We shall return to this point in a 

 later paper in connection with other data. 



5. Preliminary analysis of the regional factor in regulation 



The question as to the existence of the second zooid has already 

 been considered. For the present we may limit our consideration 

 to the first zooid, i.e., the region from the anterior end to about 

 the middle of the postpharyngeal region. The regional differences 

 in this first zooid are briefly as follows: in pieces of the same length 

 below a certain limit the capacity to form a head decreases from 

 the anterior to the posterior end of the body, and conversely the 

 capacity for tail-formation decreases from the posterior end ante- 

 riorly. In pieces sufficiently small the power to form a head 

 disappears completely in the posterior one half or one third of 

 the body and the power to form a tail in the most anterior re- 

 gions behind the head; in the one case headless tails, 'in the other 

 tailless heads arise from isolated pieces. With still further de- 

 crease in the size of the pieces the regions of head and tail forma- 

 tion become still more narrowly limited. 



The position of the pharynx in pieces from different levels 

 shows that the primordium of the new prepharyngeal region is 

 longest in the anterior regions of the zooid and decreases in suc- 

 cessive pieces to the posterior end. Furthermore, the relative 

 amount of anterior regeneration is least in the most anterior 

 piece and greatest in the most posterior and the relative amount 

 of posterior regeneration is greatest in anterior and least in 

 posterior pieces. The rapidity of head formation is also greatest 

 in the most anterior, least in the most posterior pieces and the 

 rapidity of tail formation is least in the most anterior pieces, 

 but in the posterior pieces it is more or less closely correlated 

 with the rapidity of head formation. And finally, in pieces below 



