290 C. M. CHILD 



but the posterior regenerate is usually longer in headless tails 

 than in wholes from the same region. The amount of anterior 

 regeneration in headless pieces decreases posteriorly. 



The position of the pharynx changes from near the posterior 

 end in pieces from the anterior region to a position anterior to 

 the middle in pieces from the third quarter of the body while 

 in pieces from the fourth quarter it is in the middle or posterior 

 to it as in young animals. 



The process of change of shape in wholes changes in character 

 gradually from the extreme anterior pieces to those from the 

 third quarter, this change being associated with the decreasing 

 size of the head.^ In the posterior quarter, where the size of 

 the head again increases, the process of change of shape again 

 approaches that in the anterior pieces, but is never so extreme 

 in character, since the disproportion in size of the head is never 

 so extreme. 



In general then we may conclude that the course and rapidity 

 of the regulatory processes and the character of the results 

 differ in different regions of the body. 



The factor of size of piece. Regional differences in regulatory 

 capacity appear more and more clearly as the size of the pieces 

 decreases. The frequency of whole formation decreases and 

 the regions of whole formation become more and more sharply 

 limited to a part of the prepharyngeal region and the posterior 

 quarter of the body, with decreasing size of the piece. 



^If the contention of Morgan and others were correct, viz., that the change in 

 shape is due, at least in part, to the migration of material to form the new parts, 

 this change should be exactly opposite in character to what it is. In a piece which 

 forms a large head, e. g., figs. 20, 24, 30, the region behind the head should become 

 narrower more rapidly than other regions, since it loses so much material. Exactly 

 the reverse should be the case when the piece forms a small head, as in figs. 

 22, 25 or fig. 5. But as a matter of fact, in these cases the region posterior to the 

 head changes shape most rapidly of all. Evidently the migration hypothesis will 

 not account for these changes. 



But if we regard these changes in shape as essentially or largely due to the physi- 

 cal plasticity of the tissues and their mechanical adjustment to altered strains 

 and pressures (See Child, Studies of Regulation: also Child, '09), we have little 

 difficulty in understanding why the region posterior to the large head should be 

 the broadest and that posterior to the smallest head, the narrowest. And it is 

 easy to see also that the process of change of shape will differ in character and 

 course for every combination of parts of different size. 



