EPITHELIAL TISSUES. 



93 



tion. In silver nitrate preparations, in which the boundaries of these 

 cells are brought to view, they appear as much flattened cells, 

 resembling those of squamous epithelium, with faintly granular 

 protoplasm, possessing flattened, oval, or nearly round nuclei. 

 These cells are of polyhedral shape, and appear to be united into 

 a single layer by a small amount of intercellular cement substance. 

 The borders of these cells may be quite regular or slightly wavy 

 (Fig. 55); more often they are serrated (Figs. 56, 57). According 

 to Kolossow, who has investigated these cells by means of special 

 methods devised by him, the mesothelial cells are said to be made 



Fig- 55- Mesothelium from peri- 

 cardium of rabbit. Silver nitrate prepar- 

 ation, stained in hematoxylin. 



Fig. 56. Mesothelium from mesentery 

 of rabbit. 



- Nucleus. 

 Cell boundary. 



57- Mesothelium from peritoneum of frog; X 4- 



up of two quite distinct portions: a superficial, homogeneous cell- 

 plate, beneath which is found a finely granular protoplasm contain- 

 ing the nucleus. These two portions are intimately united to form 

 a single cell. The outlines of the superficial cell-plates are figured 

 in the accompanying illustrations. The protoplasmic portion of 

 one cell unites with that of contiguous cells by means of proto- 

 plasmic branches, between which are found intercellular spaces. 

 These intercellular spaces are here and there indicated in silver 

 nitrate preparations, forming what are known as stigmata and 

 stomata, which are looked upon by certain writers as representing 



