36 



THE EPITHELIAL TISSUES 



In some cases, as in the vas deferens, the cohnnnar cells may be 

 ciliated. Among the lower vertebrates this type seems to be more 

 commonly distributed, although many cases so distinguished may 

 prove to be a pseudostratified arrangement such as we have already 

 mentioned. It appears in the alimentary tract of many lower 

 forms and in the excretory ducts. In some regions, as in the 

 esophagus of many amphibians (Fig. 15), some of the columnar 

 superficial cells are ciliated, and others are transformed into goblet 

 cells filled with mucin. 



Fig. 15.- 



-Stratified ciliated columnar epithelium from the frog's esophagus, 

 the long superficial cells and several layers of small basal cells. 



Note 



Transitional Epithelium. — In this type the basal layer of cells is 

 cuboidal or low columnar in appearance in sections, while at the 

 surface of the membrane are very large cells convex on their free 

 boundary and with conca\'ities on their lower surface into which 

 underlying polyhedral cells fit. Binucleated cells commonly occur 

 in this tissue. Transitional epithelium lines the pelvis of the kidney, 

 the ureters, the bladder and part of the in-ethra of mammals, in 

 which group it appears to be well de^ eloped. Its cells seem to be 

 capable of considerable displacement; they slide by one another 

 under tension so that in a distended membrane of this type there 

 appear to be only two or three layers of cells. The surface cells 

 stretch most and the underlying cells are drawn out into a single 

 or double layer beneath them; as the tension on the membrane is 

 released the cells slide back, until the stratified appearance of a 

 number of layers is resumed. INIembranes of this type show grada- 

 tions to those reseml)ling a stratified cuboidal or stratified cohnnnar, 

 conditions connnonlv occurring in tlic urethra. (Fig. b>0.) 



