1 66 BIOLOGICAL LECTURES. 



reted hydrogen, carbonic anhydride, and oxygen ; for urea, 

 nearly all inorganic compounds, the so-called internal secretions 

 of organs of the body, such as sugar, hippuric acid, ammonium 

 lactate, and all the simpler products of the cell's activity. 

 Substances of this kind leave the protoplasm because their 

 tension in the protoplasm is higher than their tension in the 

 surrounding medium. Such substances are secreted by all 

 cells. Since they are of such small molecular size, they readily 

 pass from the cell as rapidly as they are formed. They never, 

 under ordinary circumstances, accumulate in the cell. We 

 never find any great accumulation of urea, carbonic anhydride, 

 the internal secretions of such bodies as the supra-renals, or 

 free sugar in the cells themselves, unless the tension of these 

 substances in the surrounding medium is also high. Osmosis 

 is hence the commonest mechanism of secretion. It is found 

 in all cells of all kinds. 



But besides these substances of small molecular size, the 

 cell also produces substances of a much more complex kind, 

 such, for example, as the albumins, mucin, glycogen, starch, 

 proteids, fats, etc. These substances are either altogether or 

 almost incapable of osmosis. Special mechanisms must hence 

 be devised to eject such substances from the cells, if it be 

 desirable that they be ejected. To accomplish this end, special 

 devices have been developed, differing in different cells. Cells 

 which possess these special mechanisms are those generally 

 designated as "secreting" or "glandular-cells." 



The commonest method to eliminate such substances is to 

 break them up first into soluble and osmosable substances. 

 They then escape from the cell by simple osmosis. This is 

 the method by which starch and glycogen are removed. They 

 are first converted into a simple sugar. Probably the stores of 

 albuminous substances laid up in the fleshy cotyledons of the 

 leguminous plants are. removed in the same way, and trans- 

 ported in the form of these simple fragments to tissues needing 

 them. Observation shows that during germination large quan- 

 tities of the fragments of proteid molecules are set free, such as 

 asparagin, leucin, arginin, etc. In the case of muscle the complex 

 muscle substance appears to be broken down to such bodies as 



