42 THE HUMAN MECHANISM 



which is far from being completely understood. It would 

 seem that the cytoplasm is a mixture of a number of mate- 

 rials which differ in chemical composition and in physical 

 properties. Some are dissolved in water, making viscous 

 solutions comparable to the white of egg or to thick or thin 

 jellies. Others are known as lipins, or lipoids (from the Greek 

 lipos, "a fat"), because they resemble fats or oils in physical 

 characters and to some extent in chemical structure; they 

 do not mix, or mix only imperfectly, with the viscous aqueous 

 (that is, watery) solutions, but spread over the outer surface 

 of the cell, forming a membrane, and probably also penetrate 

 into the cytoplasm somewhat as the connective-tissue septa 

 of the gland penetrate the gland. These lipoid membranes 

 would thus separate the viscous aqueous solutions of the 

 cytoplasm into separate masses, much as the gland is divided 

 by its septa into lobes and lobules. The lipins are supposed, 

 among other functions, to control the passage of material into 

 and out of the cell. The cytoplasm also frequently contains 

 granules, one kind of which we have already seen in the 

 zymogen granules of the gland cells. 



The nucleus, on the other hand, is known to contain cer- 

 tain other compounds peculiar to itself. Some of these at 

 times are probably in an almost solid state and appear as 

 denser material within the membrane which usually bounds 

 the nucleus; at other times they undergo solution, doubt- 

 less as the result of chemical changes taking place within 

 them. There are many strong reasons for thinking that the 

 nucleus bears an important relation to the oxidations of 

 the cell. 



