i6 



FIRST LECTURE. 



sues? According to our opinion it is permissible; still, 

 we readily admit, the opposite view may be defended. 

 Other tissues, such as the epithelium or the epidermis (Fig. 

 20), present the cellular elements in close conjunction. At 

 the same time, even the first examination teaches us that our 

 cells are not loosely crowded together ; they are intimately 

 united ; they are plastered or cemented together. This 



Fig. 21. — Capillary vessel from 

 the mesenlerium of the Guinea- 

 pig, after the action of the ni- 

 trate of silver solution ; a. vascu- 

 lar cells : l>, their nuclei. 



Fig. 22. — Cells of the enam- 

 el organ of a four months' hu- 

 man embryo. 



substance, which is of very frequent occurrence in a minimal 

 thin layer, is called either the tissue cement or the intercellu- 

 lar substance. If a portion of such tissue is placed for a short 

 time in a very dilute solution of nitrate of silver and then 

 exposed to the light, the tissue cement becomes black. 

 This excellent accessory is nowadays very frequently used. 

 In this manner we years ago recognized that the finest blood- 



