176 PRACTICAL HISTOLOGY. . 



being sacrificed for the purpose. This serves to 

 show the epithelioid layer of the serous membrane, 

 with the differences in character of its cells in dif- 

 ferent parts, the cells being much smaller over the 

 interfascicular lymphatics than elsewhere. Amongst 

 the smaller cells, moreover, may be seen here and 

 there the minute darkly-stained angular patches 

 known as pseudostomata, which are probably merely 

 accumulations of intercellular or ground substance; 

 and also, but more rarely, the true holes or stomata 

 surrounded by a ring of small cells, and leading into 

 the lymphatic below. 



Preparation 6. Such stornata or orifices leading 

 from the serous cavities into lymphatic vessels are 

 met with occasionally in preparations from most of 

 the serous membranes in mammals. But they are 

 especially numerous and well seen in the peritoneum 

 of the frog. 



A male frog should be killed for the purpose, and 

 the intestines and stomach removed so as to expose 

 the back of the abdomen, but without cutting the 

 mesentery too near the spinal column. If the trunk 

 of the animal is now placed in a dish of salt solution, 

 and the posterior part of the peritoneum carefully 

 examined under that fluid, it will be found that it 

 does not closely cover the vertebral column, great 

 vessels, and other structures which are found at the 

 back of the abdomen, but is separated from them 

 by a large lymph-space, divided from the serous 

 cavity by a membrane. This is covered on the one 

 side by the epithelioid cells of the peritoneum ; on 

 the other by those 'of the lymphatic, and to the 

 unassisted eye appears to form a complete septum. 

 Under the microscope, however, it is seen to be 

 studded by very numerous apertures, which can be 

 seen if the membrane is removed and examined with 

 a high power in salt solution merely. But to study 

 their arrangement with reference to the epithelioid 

 cells, and also to obtain a permanent preparation, 

 the septum is to be stained with silver. With this 



