HISTOLOGY OF THE FROG 85 



Stratified epithelium is distinguished from simple by the 

 fact that its component cells are arranged in several layers or 

 strata, it is commonly said like a heap of tiles; but the simile 

 is not exact, for the tiles at the bottom of the heap would have 

 to differ from those at the top to make the comparison good. 

 Stratified epithelium is found in the epidermis or outer skin, 

 the cornea, and other parts of the frog. In the epidermis there 

 are several layers of epithelial cells ; the lowest are more or 

 less columnar in form, arranged in a definite row, and have 

 abundant cytoplasm in the cell-body : this layer is commonly 

 known as the stratum malpighi. The cells of the middle 

 layer become progressively more flattened and polygonal in 

 outline, and the outermost layer consists of quite flat trans- 

 parent scales, in which the nucleus is still present, but the 

 cell-body has been converted into a horny substance. The 

 skin of man is formed of a somewhat similar stratified 

 epithelium, with a more abundant horny outer layer ; this 

 extends into the mouth, and it is easy to scrape some of the 

 superficial layer from the roof of one's mouth, and observe the 

 horny scales of which it is composed. The epithelium lining 

 the urinary bladder of the frog (and of mammals) presents a 

 variety intermediate between columnar and stratified epi- 

 thelium. There are three layers of cells, but they do not 

 over-lap one another, nor are those of the outer layer horny 

 and flattened ; they are dovetailed together by their wedge- 

 .shaped and serrated upper and lower ends, those of the 

 outermost layer, lining the cavity of the bladder, being almost 

 columnar in form. This is known as transitional epithelium. 



Both in the epidermis and in the alimentary canal of the 

 frog there are numerous glands of simple structure ; and the 

 liver and pancreas are large glands of more complicated 

 structure. The essential part of these glands is the epithelium 

 lining their cavities, its cells being modified to form what 

 is known as glandular epithelium. There are also many 

 uni-cellular glands amongst the columnar and ciliated cells in 

 the buccal cavity, and amongst the stratified cells of the 

 epidermis. Excellent examples of these single gland-cells 

 are to be found amongst the ciliated cells scraped from 

 the palate. Each has the form of a modified columnar 

 cell with a broad free extremity and a narrow fine-pointed 

 internal extremity. The goblet or chalice-cell, as it is 



