308 C. M. CHILD 



"complete" with decreasing size, and finally the piece becomes 

 headless: similar changes also occur with respect to the posterior 

 end in other regions. In general we may say that with decreas- 

 ing size the piece gradually loses its capacity to form a whole and 

 forms a part instead. 



Secondly, changes in the method of regulation are often asso- 

 ciated with change in size of the piece. Of the two pieces with 

 anterior ends at the same level, the shorter piece shows a rela- 

 tively larger amount of regeneration in the formation of the new 

 anterior end. A comparison of fig. 19, a posterior half {iq, fig. 

 17), with fig. 22, the third quarter {im, fig. 17), illustrates this 

 point. In fig. 19 the new tissue ends just posterior to the eyes, 

 while in fig. 22 a considerable region posterior to the eyes is 

 formed from regenerated tissue. The general rule is that the 

 shorter the piece from a given region of the body, the greater the 

 relative amount of regeneration as compared with redifferentiation 

 i. e., the less the ability to reproduce the missing parts without 

 extensive outgrowth. This rule holds good only so far as simi- 

 lar structures are concerned, i. e., for heads or posterior ends 

 but not for headless or tailless pieces as compared with wholes 



Thirdly, within certain limits, decrease in the size of a piece 

 from a given region of the body is accompanied by a decrease in the 

 rapidity of regulation. This rule holds only for pieces below a cer- 

 tain size which is different for different regions. If we begin witn 

 whole animals and remove from one a small portion, from another 

 a somewhat larger portion and so on, we shall find that at first 

 the rapidity of regulation increases as the size of the regulating 

 piece decreases, i.e., as more and more is removed from it. From 

 a certain size downward, however, the relation is reversed and 

 the smaller piece regulates less rapidly than the larger. The 

 size where this reversal of relations occurs, i.e., what we may call 

 the critical size corresponds rather closely with the size where 

 teratopthalmic wholes bepin to appear. 



And finally, the death rate of the pieces varies inversely as the 

 size of the piece. Among the larger pieces there are usually no 

 deaths, except under extreme external conditions, but below a 

 certain size the death rate increases rapidly and this size differs 

 for different regions of the body. In table 1, for example, no 



