28O CHARLES RUSSELL BARDEEN. 



The most active cells in the development of the parenchyma 

 of the new part are cells which closely resemble the cells playing 

 a similar part in embryonic development (Fig. 4, c, k, /) and, 

 like them, they divide by indirect division. These cells probably 

 represent the " Stammzellen " mentioned by Keller. 1 The many 

 branched connective-tissue cells seem also to assume a spherical 

 form and to multiply by indirect division. The new connective 

 tissue (Fig. 4, ;//) resembles the embryonic (Fig. 4, /). The new 

 surface musculature may arise from the large cells shown in Fig. 

 6, f, cells which resemble the embryo myoblasts. The muscula- 

 ture of the parenchyma seems, however, to arise from cells of 

 the type shown in Fig. 4, h and /, as do also the rhabdite cells. 



The epithelium lining the intestinal branches which extend into 

 the new parts apparently multiplies by direct division (Fig. 5, c 

 and </). 



Cells bordering immediately upon the intestines multiply by 

 indirect division (Fig. 5, c). I am uncertain whether these cells 

 contribute to the supply of the intestinal epithelium, but am 

 inclined to think that they do not. Stevens (op. cit.} thinks that 

 they may. The new nervous system arises by direct exten- 

 sion of fibers and apparently also of cells from the old into 

 the new parts. Fig. 7, c, represents a growing bundle of 

 cells and fibers of this nature. Fig. /,/, shows direct nuclear 

 division in the vesicular nuclei of the cells which accompany the 

 fibers into the new parts. At present, however, no certain evidence 

 can be offered that parenchyma cells do not also become directly 

 transformed into nerve cells under the influence of the old nerve 

 cords. The majority of investigators believe that the nervous 

 system is differentiated from the parenchyma, though in direct 

 continuity with the old nervous system. 2 



The reproductive organs seem to arise from the cells of the 

 type shown in Fig. 4, e. During their development mitosis is 



1 " Die ungeschlechtliche Fortpflanzung der Siisswasser Turbellarien, Jenaische 

 Zeitschrift f. Naturwissenschaften, Vol. XXI, p. 370, 1894. 



Tlexner, Journal of Morphology, Vol. XIV., p. 337, 1898. Stevens, op. cit. 

 Morgan, " Growth and Regeneration in Planaria hignbris, Archiv fitr Entwick- 

 eh/ngsmechanik^ Vol. XIII., p. 179, 1901. E. Schultz, " Aus dem Gebiete der 

 Regeneration bei Turbellarien," Zeitschrift filr -wissenschaftliche Zoologie, Vol. 

 LXXIL, p. I, 1902. 



