STUDIES ON THE DYNAMICS OF MORPHOGENESIS 289 



The fifth (fig. 33), sixth, seventh (fig. 34), eighth, ninth (fig. 

 35), tenth and eleventh (fig. 36), sixteenths are practically 

 always headless or else do not live long enough to undergo regu- 

 lation. Moreover, they show an increasing degree of head- 

 lessness, as indicated by figs. 33-36; the length of the prepharyn- 

 geal region decreases in successive pieces and the size of the 

 pharynx becomes less, until in the tenth and eleventh sixteenths 

 the pharynx often fails to appear at all, i. e., these pieces do not 

 succeed in producing a prepharyngeal region or even a pharyn- 

 geal region in regulation but remain postpharyngeal. 



The twelfth sixteenth may produce either a normal or a tera- 

 topthalmic whole or a headless tail. The thirteenth and four- 

 teenth sixteenths likewise range from headless tails (fig. 37) to 

 normal wholes, but the fourteenth produces normal wholes more 

 frequently than the thirteenth. The fifteenth and the posterior 

 sixteenths rarely produce anything except normal wholes. 



In pieces smaller than sixteenths normal wholes are much less 

 commonly formed, even in the anterior and posterior regions, 

 though smaller pieces from the posterior region are capable of 

 producing normal wholes than from any other region of the body. 

 In general the very small pieces give rise either to tailless heads 

 or headless tails, or they produce biaxial heads or tails, or finally 

 they may die without regulation, a very rare occurrence in larger 

 pieces. As regards these very small pieces, however, the diffi- 

 culty of obtaining uniformity of size in isolating them is so great 

 that the series show many irregularities. We shall return to these 

 subminimal pieces later. 



The chief results from this comparative examination of pieces 

 are as follows: 



The regional factor. The capacity for the formation of wholes 

 differs in different regions of the body. In larger pieces the size 

 of the head and in smaller, the frequency of head formation 

 decrease from the anterior region to the posterior quarter of the 

 body and then suddenly increase again. The portion of the new 

 anterior end formed by regeneration increases from the anterior 

 end to the posterior quarter and then suddenly decreases. The 

 length of the posterior regenerate decreases posteriorly in wholes, 



