MUSEUM OF COMPARATIVE ZOOLOGY. 25 



the internal epithelium. There are no nuclei anywhere in the external 

 epithelium of the fully developed pharynx, except near its proximal end. 

 By the use of silver nitrate, however, I have heen able to demonstrate 

 that the layer is a true epithelium. Isolated pharynges were killed 

 with hot 1 ct silver nitrate. By using the solution hot, the pharynges 

 were killed in an extended condition. A tangential section through 

 material treated in this way is represented on Plate IV. Fig. 47. I 

 have been unable by any method of staining to demonstrate the presence 

 of nuclei in these cells, the boundaries of which are so plainly brought 

 out by impregnation with silver. 



In young pharynges (Plate II. Fig. 14), where the tissues are not 

 fully differentiated, nuclei are to be seen in both the external and inter- 

 nal epithelial coverings, although no trace of them can be found later 

 on. It is not difficult to find pharynges in different stages of develop- 

 ment, since the number increases with the age of the individual. The 

 young pharynx begins as a solid bud of tissue projecting into a cavity 

 hollowed out of the mesenchyma. The cavity is lined with a layer ot 

 flattened cells, which is continuous with the cell layer covering the young 

 pharynx (Plate II.. Fig. 11). The cavity is at first closed on all sides, 

 but eventually communicates with the common pharyngeal chamber. 

 The lumen of the pharynx is formed by an infolding of its free end, 

 which projects into the cavity. Although I have not been able to trace 

 directly all the steps in the invagination, I have seen specimens where 

 the lumen was lined throughout with an epithelium, and where there 

 was as yet no connection with the intestine. The epithelium lining the 

 lumen is continuous with that covering the outer surface of the young 

 pharynx, and hence with that lining the pharyngeal cavity, and it pre- 

 sents the same histological conditions as the latter (Plate II. Fig. 11). 

 Figure 14 represents a cross section of a young pharynx somewhat 

 advanced in development, where the cellular structure of both the inner 

 and outer epithelium is still evident ; there are as yet no cilia, and no 

 traces of the longitudinal muscles. I expect to describe in another 

 paper the changes by which the mass of indifferent cells composing the 

 young pharynx is converted into the ultimate histological structures of 

 the mature pharynx. 



The outer layer of the pharynx has never been described as possessing 

 a distinctly cellular structure. Moseley ('74, p. 131), in speaking of 

 land planarians, describes "an epithelium in which no definite cell 

 structures could be observed ; but it appeared transparent, and marked 

 by vertical lines which might represent separation into cellular ele- 



