Regeneration in Polychcvrus Caudatus. 

 Normal Structure. 



341 



Before describing the process of regeneration, it seems necessary 

 to describe the normal structure, as this differs 

 essentially from that of other Planarians, and 

 has not been described in detail. 



Fig. K is a sagittal section of a whole worm 

 showing the general outline of the form and 

 the location of the different openings; x is 

 the digestive opening, r the female repro- 

 ductive opening, and p the penis. It also 

 shows the position of the cells that secrete the 

 jelly in which the eggs are laid j. 



Fig. I is a transverse section taken near the 

 anterior end of the worm (Fig. K, a — h), 

 showing the testis cells /, maturing spermat- 

 ozoa .r, mucus 7u, the parenchyma nuclei n, c/- ^- d 

 and the cilia c. Fig. 2 is a transverse section "^ 

 through the middle-region (Fig. K, c — d), 

 showing in addition, egg cells 0, the irregular 

 digestive region d, containing some food /, 

 and the digestive opening x. Fig. 3 is a 

 transverse section near the posterior end 

 (Fig. K, e — /), showing besides the foregoing 

 features, the female reproductive opening r, 

 and the jelly gland j. In these three sections, 

 certain distinctions between the dorsal and 

 ventral sides can be seen. Most of the 

 mucus lies on the dorsal side. There are 

 more nuclei on the ventral side than on the 

 dorsal, and there is a marked aggregation of 

 nuclei at the lateral edges of the ventral side. 

 By comparing Fig. 4, a piece of the dorsal 

 margin of a transverse section (similar to Fig. 

 2), with Fig. 5, a piece of the ventral margin, 

 an additional difference appears, that of the ) / X /P 

 arrangement of the muscle fibers. On the 

 dorsal side, they are more regularly arranged, 

 forming an outer circular and an inner longi- 

 tudinal layer, while on the ventral side, there 



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