Studies on Regulation. 275 



the two tails extends further posteriorly on the dorsal side than on 

 the ventral; the region between the tw^o longitudinal dotted lines 

 in Fig. 35 is not in contact with the substratum as the animal 

 creeps and at the anterior end of this space betw^een the two tails 

 the opening still persists. 



Fig. 36 is similar to Fig. 33 and indicates the relations of parts 

 in the dorso-ventral plane at the level of the transverse dotted 

 line in Fig. 35. The two tails are less widely separated than in 

 earlier stages, hence the thin membrane between them is throw^n 

 into a dorsal fold as indicated in the figure. 



The postero-ventral opening in the space between the two tails 

 was apparently connected with the pharyngeal pouch of the new 

 pharynx and may therefore be regarded as a mouth. In the 

 living animal I was unable to find any other mouth on the ventral 

 surface. 



It was my intention to keep the specimen alive as long as possi- 

 ble in order to determine whether this duplication w-as gradually 

 obliterated and then to fix the piece and study by means of sec- 

 tions. During the six w^eeks following the stage shown in Fig. 34 

 no marked changes except reduction in size occurred. At the 

 end of this time the piece was lost. It is evident that the protru- 

 sion of the mass of old pharyngeal tissue from the cut surface was 

 the condition which originally determined duplication of the end. 

 If this be admitted several questions arise at once. Of these we 

 may consider first why the region dorsal to the old pharynx does 

 not regenerate as rapidly as the regions lateral to it. The pro- 

 trusion of the old pharynx in a postero-ventral direction offers no 

 obstacle to growth in the posterior direction of the parts dorsal to 

 it. Evidently the dorsal region does regenerate, for the thin 

 membrane uniting the tw^o tails dorsal to the pharynx represents 

 the regeneration from this region. But regeneration here is less 

 rapid than in the regions lateral to the pharynx so that 

 duplication appears dorsally as well as ventrally, though to a less 

 extent. 



The factors w^hich have served in so many other cases, viz: 

 attachment to the substratum and tension consequent upon loco- 

 motion are in my opinion the chief factors concerned here. It is 

 evident that the protrusion of the mass of pharyngeal tissue in the 

 postero-ventral direction prevents the new tissue which arises 

 dorsal to it from coming into contact with the substratum. This 



