2 C. M. CHILD. 



or quite to zero, cut surfaces at this level showing no regeneration 

 beyond the union of ectoderm and entoderm over the cut mus- 

 cular layer. 



The influence of temperature in determining the level at which 

 the regenerative power is reduced to zero will be discussed in 

 another section. As regards the amount of regeneration, the 

 effect of position is similar to its effect on rapidity. Not only is 

 there a decrease in the rapidity of regeneration toward the aboral 

 ends, but the total amount of oral or aboral regeneration also de- 

 creases in the same manner. In certain cases, as in Series 54, 

 55 and 35, the difference in size of pieces may counteract the 

 effect of position in later stages of regeneration, so that short 

 pieces show slightly less oral regeneration than long pieces with 

 oral ends at the same level (Series 54^ and 55/2), or the amount 

 of oral regeneration on long aboral pieces may equal that on 

 much shorter pieces from a region nearer the oral end (Series 35, 

 A and B). But aside from exceptions of this kind the rapidity 

 and the amount of regeneration depend upon the level in the 

 parent body from which the cut surface is taken, both being 

 greatest at the oral end and decreasing aborally. 



The existence of a difference in regenerative power at different 

 levels of the body being established, the question as to its nature 

 and significance next requires consideration. I believe that it may 

 be due in part to a difference in the general reactive capacity of 

 the tissues in the different regions, i. e., to a decrease in the re- 

 active power of the tissues with increasing distance from the oral 

 end. That such a difference does exist is indicated by various 

 facts in the normal anatomy and physiology of the animal. 

 We must consider first the anatomical features which bear upon 

 this point. There is a marked decrease in the thickness of the 

 body-wall and especially of the muscular layer, toward the aboral 

 end ; the number of mesenteries also decreases toward the aboral 

 end, until only a single pair remains ; all new mesenteries appear 

 first at the oral end and extend gradually aborally, thus indicating 

 that growth in circumference begins orally. The physiological 

 differences of different regions which have been noted are : the 

 greater sensitiveness of the oral region to tactile and other stim- 

 uli, and the greater contractility of the muscles of this region 

 when stimulated. 



