2/8 C. M. CHILD. 



It is not difficult to show that this increase in area of the new 

 tissue is due simply to the internal water-pressure. If the aboral 

 end of the piece be prevented from closing or be punctured from 

 time to time and thus the accumulation of water in the enteron 

 be prevented, the spreading of the oral end and the increase in 

 area of the new tissue does not occur. The cut surfaces remain 

 rolled inward and approximated as in Fig. 3, while the new 

 tissue merely fills the spaces between them, but does not increase 

 farther. A similar experiment affords the same result for the 



I 



aboral end. In such cases the end retains the form acquired by 

 inrolling of the margins (see Child, '03^, Figs. 3-5), differing 

 from the stages figured only in that the crevices between the in- 

 rolled cut surfaces are closed by the new tissue. 



Experiments of this kind were performed many times and 

 always with the same result. Under conditions to be described 

 later certain other regenerative processes may occur as usual in 

 pieces which are kept open aborally, but in no case does the 

 spreading apart of the cut surfaces at the oral end occur and in 

 no case does the disc assume its typical form. 



The increase in the area of the new tissue when under tension 

 is not merely a stretching ; as was noted above, attempts to 

 stretch the tissue artifically always resulted in rupture. At the 

 end of this extension of the new tissue its area is many times as 

 great as at first and, what is more important, its thickness is as 

 great as, or greater than before. Actual growth has without 

 doubt occurred during the extension, indeed the conclusion is 

 justified that the increase in area of the new tissue could not have 

 occurred without growth ; the attempts at artificial stretching 

 leave little room for any other conclusion. If this conclusion is 



* 



correct, then the growth of this new tissue is dependent on the 

 tension to which it is subjected and not on any factor or condi- 

 tion existing in the tissue itself, except of course the power of 

 growth. Unless the tissue is subjected to tension, growth ceases 

 with the closure of the small spaces between the folds of the 

 inrolled ends ; when it is subjected to tension, within certain lim- 

 its, it grows, /. e., increases in quantity, number of cells, etc. Its 

 increasing thickness and power of resistance to tension as differ- 

 entiation occurs limit its growth, so that increase in area does 

 not go beyond a certain point (Fig. 4). 



