DEVELOPMENT IN PLANARIANS. 269 



smaller cells with smaller nuclei arise and form a syncytium of 

 branched cells (Fig. 4, g). The connective tissue of the adult is 

 derived from cells of this nature (Fig. 4, z), fibrils in the processes 

 connecting the cells becoming ever more distinct. In addition 

 to these fixed connective-tissue cells there exist in the paren- 

 chyma of the adult several other types of cells. Some of these, 

 with dark protoplasm, large nuclei, and well-marked nucleoli 

 lying in clear areas, seem to be the direct, undifferentiated de- 

 scendants of the embryonic cells of the type shown in Fig. 4, e. 

 These cells divide by mitosis (Fig. 4, k and /) and seem to be the 

 chief factor in the regeneration of new parenchyma. Other cells 

 with lighter nuclei and protoplasm the granules of which 

 stain deeply in "acid" dyes may be leucocytes, as Miss Stevens 

 has aptly suggested. 1 Cells of this nature are seen at an early 

 stage and apparently are derived from cells of the type shown in 

 Fig. 4, t: Rhabdite cells begin to be differentiated even before 

 the period when the epithelium assumes a columnar structure, 

 but while this latter process is taking place the rhabdite cells ap 

 proach the basement-membrane and rhabdites are passed out into 

 the intercellular areas. In Fig. 3, c, the depression in the mem- 

 brane indicates the area through which rhabdites have been 

 passed. I have seen no clear indications of the passage of the 

 rhabdite cells themselves through the membrane. Mucus cells 

 and the gland cells belonging to the intestinal system arise at a 

 comparatively late stage. I have not been able to trace their '< 

 origin from any specific cells. 



When lining surfaces like the pharyngeal pocket the paren- 

 chyma cells assume an epithelial character. These cells have 

 an origin similar to that of the connective -tissue cells. 



Intestinal cells become clearly evident at the time when the 

 embryo assumes an ovoid form. They are at first sparingly scat- 

 tered, but at the time when septa extend into the yolk-cavity so 

 as to give rise to the permanent form of the intestinal system, 

 they begin to multiply by indirect division (Fig. 5, a). As in the 

 case of the surface epithelium, this cell multiplication finally gives 

 rise to cells extending away from the base on which they rest 



"'Notes on Regeneration in Planarialugubris" Archiv f. Enfrvickelungsmechanik 

 der Or^anismeii, XIII., p. 410, 1901. 



