270 



HISTOLOGY 



plasmic bridges; thus in passing through the epithelium along the inter- 

 cellular boundaries, one or two intercellular vacuoles would be encountered 

 (Fig. 266, A). Wandering cells pass readily across this epithelium, 

 between the cells, and substances in the peritoneal cavity are taken up 

 into the subserous lymphatics. It has long been thought that there are 

 permanent orifices or "stomata" between the epithelial cells (Fig. 266, B), 

 bounded either by modified protoplasm or by separate small cells, and that 

 lymphatic vessels open directly into the serous cavity through such sto- 

 mata. This is contrary to recent investigations of the nature of lymphatic 



vessels, and the existence of sto- 

 mata as permanent apertures has 

 been denied. The stomata, so fre- 

 quently found in a great variety of 

 animals may be shrinkage effects 

 caused by reagents, but their in- 

 terpretation is not clear. In any 

 case, the transfer of material 

 through the epithelium takes place 

 readily, and the substances or cells 

 which pass through may be taken 

 up freely by the closed lymphatic 

 vessels in the underlying tissue. 



In the mesentery, a thin layer of connective tissue with elastic networks 

 and interwoven bundles of white fibers fills the interval between the two 

 epithelial layers. In this connective tissue there are many lymphatic 

 and blood vessels, and nerves to the various organs. Mast cells may be 

 found along the vessels, especially in young animals (Fig. 55, p. 68) and 

 various other forms of wandering cells occur. The connective tissue layer 

 is denser in the parietal than in the visceral peritoneum. In places where 

 the peritoneum is freely movable there is a subserous layer of loose fatty 

 tissue, but there is no subserous layer in the intestine. 



FIG. 266. SEROUS MEMBRANES. 



A, Vertical section of the epithelium (after Heiden- 

 hain); B, Surface view, showing two stomata 

 (after Ludwig). 



VERMIFORM PROCESS. 



The vermiform process is a "worm-like" prolongation of the caecum. 

 Although small in size, in structure it more closely resembles the large 

 intestine, of which it is a part, than the small intestine. In embryos of 

 three and one-half to five months it is lined with villi, but with further 

 development the villi flatten out and disappear. Meanwhile the glands, 

 which are of the same type in both small and large intestines, have devel- 

 oped and are increasing in number and in length. Sometimes they pene- 



