276 C. M. Child. 



was clearly seen to be the case by observation of the specimen 

 during movement. Only the regions lateral to the pharynx could 

 be used in the characteristic manner. The result of the duplica- 

 tion in the complex of functional conditions is the duplication in 

 structure. The thin membrane dorsal to the pharynx elongates 

 only as the tension exerted upon the two tails is transferred to it 

 and so remains behind these parts in growth. 



But it may appear at first glance that the loss of the old pharynx 

 a few days later alters conditions and that after this there is noth- 

 ing to prevent the median region from coming into contact with 

 the substratum and elongating as rapidly as other parts. This 

 is not the case, however, for as long as the two tails are used in the 

 typical manner (Figs. 34 and 35) the median connecting region 

 cannot come into contact with the substratum but must remain as 

 a dorsal fold (Fig. 36). Indeed the fold is frequently more marked 

 than before the loss of the old pharynx since the two tails often lie 

 nearer together than was possible when they were separated by 

 the old pharynx. Thus, even after the loss of the old pharyngeal 

 tissue the median region is subjected to tension only as the lateral 

 parts are strongly stretched and so maintains the relations with 

 other parts which were originally determined by the presence of 

 the mass of pharyngeal tissue. 



Incidentally this case shows very clearly that the terminal por- 

 tions are formed first in the regeneration of the posterior end. The 

 postero-ventral "mouth" between the two tails which was at first 

 situated at the level of the cut surface is carried posteriorly as 

 regeneration occurs and at the stage of Fig. 34 lies far behind the 

 cut surface. Occasionally when the animal drew back suddenly 

 from some object which its head had touched the two tails became 

 more or less extended laterally each forming an angle of about 

 45° with the longitudinal axis. Under these conditions the median 

 region was sometimes in contact with the substratum, but since 

 this position was not often taken and was never maintained 

 for more than a short time no appreciable effect could be 

 expected. 



This case is interesting chiefly because it constitutes valuable 

 evidence in favor of the view that the functional conditions and 

 among them the tension to which parts are subjected are important 

 "formative" factors. 



