DEVELOPMENT IN PLANARIANS. 263 



syncytial " ectoplasm " of these cells. Cell division at this stage 

 is mainly by mitosis. In addition to the cells just described 

 many small nuclei, granular masses, and vacuoles are embedded 

 in the substance of the body wall. These bodies I take to be 

 remnants of yolk cells. Most of the small nuclei show marked 

 evidences of disorganization. Some of the large cells of the 

 body wall lie flattened against the yolk cavity but I can find no 

 evidence at this stage of a differentiation between entoderm and 

 mesoderm cells like that described by lijima for Dcndroarluin. 

 The embryo-pharynx corresponds closely in structure to that 

 described by lijima. 



As the embryo becomes distended by taking in yolk cells the 

 body wall becomes thinner and the character of the cells lying in 

 the reticular substance is altered. * The nucleus becomes more 

 densely granular and division takes place mainly by amitosis 

 (see Fig. 4, b\ The protoplasm of these cells varies in granula- 

 tion and in -staining power, but I have been able to distinguish no 

 clearly defined classes of cells. Rapid cell division gives rise to 

 a smaller variety of cell shown in Fig. 4, c and d. These cells 

 exhibit indications of amoeboid movement, as shown in Fig. 4, d. 

 In these smaller cells mitotic division prevails, though amitosis 

 is also present. 



The hitherto spherical embryo now becomes flattened dorso- 

 ventrally and elongated in the antero-posterior axis. This change 

 in form is accompanied by a more rapid cell-multiplication in the 

 ventral than in the dorsal body wall. The most rapid cell-mul- 

 tiplication takes place on the postero-ventral wall just below the 

 embryo-pharynx (see Fig. 2, B). It is from this mass of cells 

 that the permanent pharynx is developed. lijima says that in D. 

 lacteuui the permanent pharynx is developed in the region of the 

 embryo-pharynx after the disappearance of the latter. The rela- 

 tions between embryonic and permanent pharynxes in Planaria 

 inaculata were first discovered by Winterton Curtis and are de- 

 scribed by him in an article shortly to appear in the Proceedings 

 of the Boston Society of Natural History. 



At the period of the formation of the anlage of the permanent ' 

 pharynx tissue-differentiation becomes active. Before considering 

 this tissue-differentiation it may be best, however, first to go over 



