Regeneration in Polychoerus Caudatus. 343 



K, p). The egg cells (Fig. 2, o) lie on each side of the median 

 ventral line, extending from the region of the digestive opening 

 back to the female reproductive pore. Just in front of this pore 

 lie the cells which secrete the jelly in which the eggs are laid. 



(Fig- 3. /•) 



This form has no central nervous system, no eyes or other sense 

 organs, and no excretory system. 



Regeneration. 



The regeneration of this form is as simple as its structure. The 

 worms were cut into three pieces as stated in Part I, a head-piece, 

 a middle-piece, and a tail-piece. In the regeneration of Planaria 

 simplicissima and of Planaria maculata, the old ectoderm stretches 

 over the cut surface in a thin layer, but the regenerative process in 

 Polychcerus caudatus is more like the regeneration after natural 

 fission in Planaria maculata, as described by Curtis ('02), where 

 the exposed surface simply heals over and embryonic cells migrate 

 to that region and form the new tissue. In Polychoerus, the cells 

 at the cut end secrete a cuticular substance and develop cilia. 

 Sections of most of the pieces fixed two days after being cut, show 

 short cilia at the cut end (Fig. 6, £-,) and the cells stain a little more 

 deeply at the base of the cilia. In the five day sections, the cilia 

 have reached their normal length. (Fig. 7, c.) By this time 

 there is also a decided accumulation of nuclei at the regenerating 

 end. Fig. 7 shows this, and a comparison of Fig. 7 with Fig. 6 

 clearly shows the progress of regeneration. This accumulation 

 is not due to cell division, either in the regenerating end, or the old 

 part. Cell division has been carefully looked for throughout the 

 work, and the one or two cases which might possibly be interpreted 

 as prophases of mitosis lose all significance from their rarity and 

 the entire absence of actual mitoses; neither has any evidence of 

 amitosis been discovered. Many of the nuclei in Fig. 7 have their 

 long axes pointed toward the end, and the cells, as far as their 

 outline can be made out, point in the same direction, indicating 

 a streaming of parenchyma cells toward the regenerating region. 

 In the whole worm, the parenchyma nuclei are accumulated on 

 the ventral side and especially toward the lateral edge. In Fig. 9, 

 a sagittal section some distance lateral to the median line, the 

 accumulation of nuclei on the ventral side is continuous with the 



