48 HISTOLOGY 



distilled water will usually bring out, after exposure to sunlight, the 

 outlines of the cells where they come in contact one with another. 

 This is of great advantage in studying some kinds whose outlines are 

 indistinct under other circumstances. The second method is that of 

 teasing. The intercellular cement substances are easily dissolved out of 

 most epithelia by the use of certain weak fixatives, as 33 per cent alcohol, 

 very weak osmic acid, and weak chromic acid (see LEE). The individual 

 cells can then be separated by gentle manipulation and examined 

 by themselves. They may be stained and permanently mounted. A 

 very useful but somewhat difficult method was used by the writers to 

 study the relations of certain epithelia without entirely separating the 

 cells. The nasal epithelium was somewhat softened and macerated in a 

 number of media and then treated with one per cent osmic acid to harden 

 and preserve it. It was then stained in several stains and carefully im- 

 bedded in paraffin ; sections were cut and mounted without being affixed 

 to the slide. The dissolving of the paraffin and addition of the balsam 

 separated the cells enough to allow one to easily see them separately 

 without having their relations to one another seriously disturbed. 



LITERATURE 



General works and papers by 



ZURSTRASSEN, O. " Uber die Mechanik der Epithelbildung," Verh, d. Deut. Zool. Gesell., 

 1903. 



THE AMPLIFICATION OF BODY SURFACE 



It is through the epithelial cells lining a body that its various material 

 relations with the world are established, especially the taking in and the 

 giving off of various substances. Several facts must be considered in 

 studying these substance exchanges. One is that only a certain amount 

 of transfer per unit of time can take place through a given unit of surface. 

 And in an organism of high specialization this limitation is increased by 

 the fact that there are many more different kinds of exchange to be per- 

 formed than in lower forms ; and usually each kind of work must have 

 its own surface of a particular character. 



The first attempts to meet these conditions consist of various gross 

 arrangements of the body surface which are purely morphological in 

 character and serve to considerably enlarge the square surface of the 

 body. When these are completed, however, there is still not enough sur- 

 face for the body functions of larger and more specialized animals to be 

 carried on. Some way is necessary whereby this surface can be made to 

 do more work without further increasing the surface morphologically. 



What does occur to bring this about is, in reality, an increase in 



