STUDIES ON THE DYNAMICS OF MORPHOGENESIS 



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clearly that the absence of new tissue is not the result of the method 

 of closure, but that the closure follows the absence of regulatory 

 growth. If the headless pieces were always of this kind, Morgan's 

 conclusions might be regarded as plausible, but the frequent oc- 

 currence of such cases as fig. 7 demonstrate its insufficiency. As a 

 matter of fact both size and region of the body are concerned in 

 these differences in the headless pieces. Of two pieces with ante- 



// 



Figs. 7-12 'Headless tails.' (figs. 7-11) and 'biaxial tails, (fig. 12). In fig. 

 9a is shown a form intermediate between the whole and the headless tail: occa- 

 sionally such pieces develop a head later, as shown in fig. 96. 



rior ends at the same level of the body the larger resembles fig. 7, 

 the smaller fig. 8, assuming of course that both are so small that 

 they do not produce heads : moreover, in pieces of equal size those 

 further anterior in the middle half or third of the body resemble 

 fig. 7, those further posterior fig. 8.^ 



^Morgan has observed that after a second removal of the anterior end such 

 headless pieces produce a head, and he regards this fact as supporting his view 

 that the failure of a head to develop after the first operation is due to the method 

 of closure. In a later paper I shall show that the method of closure has nothing 

 to do with the result in these cases. The factors which determine whether or not 

 these pieces shall form a head are to be found in the constitution of the pieces and 

 the physiological correlation of their parts. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 10, NO. 3 



