THE INTERNAL INFLUENCES THAT DETERMINE 

 THE RELATIVE SIZE OF DOUBLE STRUC- 

 TURES IN PLANARIA LUGUBRIS. 



T. H. MORGAN. 



In a previous paper 1 I attempted to determine what internal 

 factors regulate in planarians the limit of size of each of two 

 heads when such are present. Two conditions appeared to have 

 an influence on the result : (i) The width of the region connect- 

 ing the part to the rest of the organism, (2) the length of the 

 new part. By the following experiments I have attempted to 

 gain the further insight into the conditions : 



In one series the planarians were split lengthwise into unequal 

 parts, as shown in Fig. I ; or else the head was first cut off, and 

 then the posterior piece was split lengthwise, as shown in Fig. 2. 

 Under both of these conditions the head on the smaller piece is 

 much smaller than that on the larger part. The purpose of the 

 experiment was to see if by abundant feeding the size of the smaller 

 head could be brought up to that of the larger ; or whether its 

 size is determined by the width of the region connecting the 

 small head with the larger piece. If the longitudinal cut does 

 not extend as far posteriorly as the old pharynx, a new pharynx 

 does not, as a rule, come into the smaller part. If, however, the 

 cut is extended posteriorly as far as, or beyond, the old pharynx, 

 a new one may come into the small part. The same end can be 

 reached by cutting off the worm in front of the pharynx as 

 shown in Fig. 3, and then splitting the posterior piece lengthwise. 

 The side piece will be, of course, shorter in the last case. 



The results show that the size that the new smaller head at- 

 tains is determined largely by the presence or absence of a new 

 pharynx in the small part. 



The following cases will serve to illustrate some of the ways 

 in which the regeneration takes place. A narrow piece was split 

 off in one case as shown in Fig. I . A small head developed at 



i Roux's Archiv, XIII., 1901. 



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