Regeneration in Polychceriis Caudatiis. 337 



Head-pieces. 



These pieces very soon began to produce new tissue at the cut 

 surface as in other Planarians. Among the 40-50 pieces in a 

 series, at the end of two weeks, the stages of posterior regeneration 

 shown in Figs. B, C and D were found with all intermediate 

 stages. A rounded mass of new tissue of considerable size forms 

 posterior to the cut surface, a — b, before the characteristic notch 

 and appendage appear. Continued regeneration adds to the 

 length of the new part while the old part decreases in width and 

 the whole piece gradually assumes the typical form. The notch, 

 at first broad and shallow, becomes deeper and narrower, and the 

 appendage longer. The new part assumes the characteristic 

 pigmentation of the adult tail-region, and a digestive region forms 

 anterior to the line of section, a — b. Regeneration of these pieces 

 was not followed longer than four weeks, when most of the pieces 

 had assumed the form shown in Fig. E, where, if one compares 

 with Figs. A and B, morphallaxis is very apparent. 



Middle-pieces. 



In these pieces posterior regeneration proceeded somewhat 

 differently. New tissue appeared along the whole of the cut 

 surface, but was so distributed as to form a median notch from a 

 very early stage. One or more appendages appeared earlier than 

 in the regeneration of head-pieces. Figs. F and G show the usual 

 amount of posterior regeneration after two weeks, and Figs. 

 H, L, M and N after four weeks. In all of these pieces the notch 

 is still much broader and more widely open than in the typical 

 form A. The multiple appendages shown in the figures were at 

 first thought to be a peculiarity connected with regeneration; but 

 examination of many normal worms showed that, though one 

 appendage is the typical structure, still all the variations observed 

 in regeneration are to be found in normal adult worms. These 

 variations are, however, far more frequent in regeneration, and 

 more frequent in middle-pieces than in head-pieces, where, as a 

 rule, only one appendage develops. These observations sug- 

 gested a comparison with the formation of the tail-region in the 

 embryo. Figs. T and X show two young worms, ten to twelve 

 days after the eggs were laid, and still in the jelly which enveloped 

 the eggs. The appendage has appeared but not the characteristic 



