Chap. V.] GENERAL HISTOLOGY. 67 



the mouth. The deeper cells are rounder. These latter con- 

 stantly multiply by division, and thus force fresh layers of the 

 squamous cells to the surface, where they are shed. In the skin 

 a similar process goes on; but the external cells are more horny, 

 and have lost their nucleus. 



(b.) Scrape gently the surface of the eyeball in a dead rabbit, 

 and mount in normal salt solution. The cells are polyhedral 

 and nucleated (24, iv.). 



(c.) Scrape gently the inner surface of the roof of 

 mouth, and mount in normal salt solution. The cells are c 

 along their free edges. The nuclei are readily seen .. 



(d.) Scrape gently the inner surface of the intestinx*%' / $he ' J ( 

 frog, and mount in normal salt solution. The cells (24, v.) are 

 columnar, and some of them pointed at one end. In some 

 (mucous cells) a globule of mucus may be seen at the broader 

 end. The nuclei are nearer the deep end. 



Endothelial or epiihelioid cells are flattened squamous cells of 

 various shapes, polygonal, elongated, or irregular. They form a 

 simple membrane one layer thick. 



(e.) Kemove carefully the transparent membrane that separates 

 the pleuro-peritoneal cavity of the frog from the subvertebral 

 lymph space. Treat it for two or three minutes with a dilute 

 solution (*5 per cent.) of nitrate of silver, wash with distilled 

 water, and expose to bright light until it is stained brown. The 

 irregular endothelial calls of the peritoneum will be marked out 

 with black lines (24, vi), for the cement substance which lies be- 

 tween the cells is stained black by the silver. It may be that 

 smaller cells surrounding minute orifices (stomata) will be seen. 

 They are called germinating endothelial cells, and actively multiply 

 by division. The stomata communicate with the ccelom. 



All of these cells (a.-e.) should be measured and drawn from 

 Nature. 



3. Connective Tissue. This tissue is to be found in all parts 

 of the body. It forms the framework or scaffolding, within and 

 around which the various organs are built. Its fibres support 

 the specialised cells of these organs. 



