HISTOLOGY OF THE FROG 



109 





surface tissue whether lying on the internal or external surface of an 

 organ. 



There are, however, various types of epithelial tissue and these are 



named from their shape. 

 For example: Flattened or 

 squamous epithelium easily 

 obtained from the outermost 

 skin of the frog during the 

 time it molts, or from the 

 peritoneum, is composed of 

 cells which are broad and 

 flat with a rounded nucleus 

 near the center. 



In the mucous layer of 

 the intestine, we find what 

 is known as columnar epi- 

 thelium, because the cells 

 are shaped like columns, 

 while in many places such 

 as in the outer skin, there 

 are transitional stages be- 

 tween these two types of 

 tissues which have some of 

 the characteristic shape of both flat and columnar epithelium. 



If these cells are several layers deep they are called stratified epi- 

 thelium. 



Should they have tiny hairlike substances called cilia at their outer 

 ends they are known as ciliated epithelium. These may be almost any 

 shape columnar, cuboid or flattened. Ciliated epithelium is found in 

 the mouth, throat, parts of the peritoneal lining of the body-cavity, inner 

 lining of the oviducts, in the mouths of the ciliated funnels of the kid- 

 ney, in the ventricles of the brain, and, in very early life, even on the 

 outer surface of the body. 



2. Connective tissue (Fig. 40) serves to support and hold together 

 various parts of the body. In this type of tissue, the intercellular sub- 

 stance is quite abundant as contradistinguished from nearly all other 

 types, and it is interesting to note that nearly all of the connective tissue 

 is derived from the middle germ-layer or mesoderm ( ). 



The intercellular substance changes in many ways, remaining soft, or 

 becoming fibrous and even changing into bone. The principal types of 

 this tissue are as follows: 



White fibrous connective tissue, most widely distributed, and easily 

 obtained from the membranes connecting skin and body-wall. Under 

 the microscope it appears as a clear gelatinous substance in which many 

 fibrils are embedded. The fibrils are unbranched but have a character- 



Fig. 39. 



A, stratified epithelium from the oesophagus of the 

 rabbit, seen in section. In the lower part of the figure 

 the connective tissue and muscular layers are shown. B, 

 squamous epithelium from the mesentery of the Frog, 

 silver nitrate preparation ; El, E2, goblet cells from the 

 frog's mouth ; Dl, D2, isolated ciliated epithelium cells 

 from the frog's mouth ; D3, an isolated ciliated cell from 

 the gill of the mussel. C, columnar epithelium from the 

 intestine of the frog. (From Bourne, after a drawing 

 by Dr. E. H. Schuster.) 



