268 



CHARLES RUSSELL BARDEEN. 



young, I have seen no specimens of Planaria maculata at any 

 stage of development in which the dorsal epithelium was ciliated. 

 For the sake of comparison a dorsal and a ventral epithelial cell 

 from an adult individual are shown in c and f of Fig. 3. These 

 seem to be slightly smaller than the embryo cells. Although 

 the possibility cannot be excluded that cells are added to the 

 surface epithelium from the parenchyma, I have found no evi- 

 dence of this. 



The cells of the parenchyma offer a most difficult and uncer- 

 tain subject for study. Cells of the type shown in Fig. 4, e, are 



FIG. 4. Sections illustrating the development of the parenchyma. Magnification. 

 540 diameters. 



a. One of the cells surrounding the yolk cavity of the embryo shown in Fig. 

 I, A. 



b. Corresponding cell of a slightly older embryo. 



c. Type of cell prevailing in the parenchyma of an embryo immediately before 

 ihe stage when the pharyngeal anlage appears. 



d. Similar cell showing evidence of amceboid movement. 



e. Cells illustrating the reduction in size which takes place during the tissue dif- 

 ferentiation that occurs at the time when the pharyngeal anlage appears. 



f. Closely packed cells seen in the anlage of the permanent pharynx. 



g. Connective-tissue cells of body wall during this stage. 

 h. Early rhabdite cell. 



/. Connective-tissue cells of adult individual. 



j. Large parenchymal cell in resting stage. 



k, I. Large parenchymal cells undergoing division. 



m. Cells from anlage of head thirty- eight hours after isolation of the piece. 



those most commonly met with at the stage preceding the de- 

 velopment of the permanent pharynx. In certain regions, such 

 as the pharyngeal anlage, these cells may be very closely packed 

 together (Fig. 4,/), but if we assume them to form a syncytium 

 we must consider it a syncytium in which the cell territories are 

 fairly well marked. The individual cells may be irregular in 

 shape or distinctly elongated. From cells of this character 



