272 C. M. Child, 



applied itself to the dorsal surface of the first (Fig. 28). But the 

 margins of the body bent across the posterior end adhered so much 

 more closely that the regenerating tails were not subjected to the 

 characteristic tension. Even when one or both of the* posterior 

 ends underwent temporary contraction and the new tails were 

 stretched the contraction took place in a curve parallel with the 

 curved margin of each half as indicated by the direction of the 

 arrows below^ Fig. 28. It is easy to see that this peculiar form of 

 contraction follows from the course of the longitudinal muscles in 

 the curved posterior parts. 



From this description of the movements it is clear that the ten- 

 sion to which the regenerating tissue on the originally posterior 

 surfaces is subjected is slight compared with that in the typical 

 case of posterior regeneration. The incurved lateral margins of 

 the old part perform the function which in typical cases is per- 

 formed by the new tissue on the posterior cut surface. And sec- 

 ondly, since all muscular contraction of these posterior ends follows 

 the curve indicated by the arrows in Fig. 28 it is clear that any 

 tension to which the new tissue may be subjected in consequence 

 of attachment during such contraction is approximately perpen- 

 dicular to the cut surfaces. Attachment of one of the new tails 

 was sometimes observed under these conditions and actual stretch- 

 ing perpendicular to the cut surface was visible. 



If we compare the amount of regeneration in this specimen with 

 that in other pieces in which- the cut surfaces had united we find 

 that there is a marked difference. Figs. 28 and 30, both drawn 

 to the same scale, represent two pieces of the same series at the 

 same time after the operations. In the case shown in Fig. 30 

 the cut surfaces united fully, and the new tissue was subjected to 

 the typical longitudinal tension and posterior regeneration occurred 

 in the typical manner and amount. In the other case the new tissue 

 on the (originally) posterior cut surfaces could not be used in the 

 typical manner, hence was subjected to slight tension only. The 

 amount of regeneration in this case is only a small fraction of the 

 amount in Fig. 30. This case constitutes almost an experimental 

 demonstration of the correctness of the conclusions reached in pre- 

 vious papers (Child, '04b, '04c), viz: that the mechanical conditions 

 are important factors in determining the amount of regeneration. 



Another feature of importance in this case is the direction of 

 regeneration from the posterior cut surfaces. On each side the 



