70 SELECTION AND PREPARATION OF ORGANISMS 



this organ is extremely active metabolically. Nerve tissue, such as rat 

 brain, may be desirable because the metabolic rates are very high. Under 

 proper conditions, almost any animal tissue will continue to perform 

 its functions for a while after removal from the animal. Since osmotic 

 relationships are extremely important, the tissues are often bathed in a 

 solution of various salts having approximately the same composition as 

 the liquid which normally bathes them. 



Microorganisms: An experiment can be made more convenient by 

 eliminating other activities that proceed simultaneously, a purpose 

 achieved by the use of a part of an animal or plant. The limit in simplifi- 

 cation is reached when single-celled organisms, or microorganisms, are 

 used. Here there can be no interaction between cells as in the higher 

 organisms. 



Algae, fungi, bacteria, and protozoa of various kinds may be very 

 desirable experimental organisms. The contribution of CMorella to photo- 

 synthesis was mentioned earlier. The single green cells grow rapidly 

 in artificial nutrient solutions. Among the fungi, ordinary baker's yeast is 

 undoubtedly the most convenient. Any grocery store sells yeast cakes 

 which are pure cultures of Saccharomyces cereviseae. In any of the 

 larger cities, the same yeast is available by the pound. If the yeast cake 

 contains more starch and other substances than is desirable, the cells are 

 washed by suspending them in water or a buffer solution and then cen- 

 trifuging at low speed. The cells will settle, leaving most of the starch and 

 the soluble materials in the liquid. 



Amoeba, Paramecium, and several other single-celled animals have 

 been used. One which has become increasingly popular in the past sev- 

 eral years is Tetrahymena pyriformis. It is a tough little animal that 

 grows well under artificial conditions and offers some decided advantages 

 in experiments on metabolism. 



Microorganisms are raised in the laboratory in "pure culture"; that is, 

 the culture contains the one desired organism and no other species. A 

 pure culture of CMorella contains the minute green spheres character- 

 istic of this species and presumably no bacteria, no protozoa, and no 

 other species of algae. The original separation of a pure culture is a 

 tedious task because microorganisms almost never exist singly in nature. 

 Once a pure culture is achieved, it must be maintained in such a way 

 that it will not become contaminated with the foreign organisms which 

 are present everywhere. 



The techniques by which microorganisms are raised and maintained 

 are so complex that they can only be learned first hand. Every biologist 



