THE SINGLE-CELLED ANIMAL BODY 39 



structural units, the cells, of which so many are included 

 in the body of any one of the complex animals. 



Protoplasm. The protoplasm, which is the essential 

 substance of the typical animal cell and hence of the 

 whole animal body, is a substance of very complex 

 chemical and physical make-up. No chemist has yet 

 been able to determine its exact chemical constitution, 

 and the microscope has so far been unable to reveal 

 certainly its physical characters. The most important 

 thing known about the chemical constitution of proto- 

 plasm is that there are always present in it certain com- 

 plex albuminous substances which are never found in 

 inorganic bodies. And it is certain that it is on the 

 presence of these substances that the power possessed 

 by protoplasm of performing the fundamental life-pro- 

 cesses depends. Protoplasm is the primitive physical 

 basis of life, but it is the presence of the complex albu- 

 minous substances in it that makes it so. 



The physical constitution of protoplasm seems to be 

 that of a viscous liquid containing many fine globules of a 

 liquid of different density and numerous larger globules 

 of a liquid of still other density. Some naturalists believe 

 the fine globules to be solid grains, while still others 

 believe that numerous fine threads of* dense protoplasm 

 lie coiled and tangled in the clearer, viscous protoplasm. 

 But the little we know of the physical structure of proto- 

 plasm thro\vs almost no light on the remarkable properties 

 of this fundamental life-substance. 



