DEVELOPMENT IN PLANARIANS. 



267 



The surface epithelium is the simplest of the tissues to follow, 

 owing to the sharp line of demarcation existing between it and 

 the underlying tissues. Up to the time of the formation of the 

 anlage of the permanent pharynx the surface membrane is beset 

 but sparsely with cells. Fig. 3, a, shows one of these cells in 

 cross-section. During the period of assumption of adult struc- 

 ture by the embryo these cells multiply rapidly by amitosis, as 

 shown in Fig. 3, b. The cells increase in thickness, Fig. 3, d. 



FIG. 3. Sections illustrating various stages in the development of the surface 

 epithelium. Magnification, 540 diameters. 



a. From the embryo shown in Fig. I, A. 



b. From an embryo in which the pharyngeal anlage is being formed. 



c. Dorsal epithelium near the pharyngeal region of an embryo in which the 

 nervous system is beginning to appear. 



d. Epithelium from the ventral surface of the tail region of the same embryo. 



e. Dorsal epithelial cells from an adult individual. 



f. Ventral epithelial cell from an adult individual. 



g. Epithelium covering a cut surface of an adult individual twelve hours after 

 injury. 



In b and c evidences of direct nuclear division may be seen. 



Finally the membrane becomes covered externally with cells so 

 crowded together that the long axis of each cell lies at right 

 angles to the surface of the body, Fig. 3, c. This crowding 

 first takes place near the center of the dorsal and ventral sur- 

 faces, where bodily extension is least rapid, and only later at the 

 growing extremities of the worm (compare c and d, Fig. 3). The 

 dorsal epithelium at no time is ciliated (Fig. 3, c\ the ventral 

 epithelium acquires cilia at an early stage of development (Fig. 

 3, d}. Although it is commonly stated in the text-books that 

 fresh-water planarians are completely covered with cilia when 



