248 INVERTEBRATE PHYSIOLOGY 



The inorganic composition of the urine is related to that of the other 

 body fluids and to that of the external environment. Its study has become 

 an integral part of the subject of osmoregulation which, by its importance, 

 has been accorded a separate treatment in this symposium (Robertson). 



The subject to which this review is limited may properly be called the 

 quantitative physiological approach to renal function. It may be described 

 as the method of controlling the chemical composition of the blood in one 

 or more of its constituents and following the performance of the kidney 

 by analysis of serial samples of urine collected quantitatively. As has been 

 emphasized by Smith (1951), the animal must be in normal physiological 

 state during this time, having recovered from anaesthesia or shock. How 

 well these criteria can be extended to invertebrate animals may be judged 

 from the results presented below. This approach should enable the investi- 

 gator to form a critical judgment as to the role of three important mechan- 

 isms in urine formation. These mechanisms are filtration, the active trans- 

 port of materials from the filtrate back to the blood, and the active trans- 

 port of materials from blood to urine. The method should also provide a 

 wealth of detail about the normal rates and maximum excretory capacities 

 for a variety of normal and test substances. 



The terminology of the field under consideration is not in a satisfactory 

 state, nor is the terminology adopted here one carefully planned to take 

 care of future needs. Suggestions for a terminology would be welcome and 

 vigorous discussion might lead to more consistency for the future than 

 now prevails. Secretion, to at least one cytologist (Bowen, 1929), meant 

 the synthetic steps within the cell resulting in formation of a new sub- 

 stance, and the release of that substance from the cell was to him "excre- 

 tion," no matter what important function the substance might then have. 

 This limitation of the term secretion is unacceptable to physiologists, and 

 indeed would appear to be generally so because of the meaning of the root : 

 to place apart, to separate, or to sever. A more recent author (DeRobertis, 

 1948) says of secretion that it "... is the process by which cells absorb 

 substances, transform them chemically or in concentration, and expel 

 them. The products of secretion may be utilized by other cells, may stimu- 

 late or inhibit other cells, may act chemically on other substances or may 

 be eliminated from the organism. These transformations imply work done 

 by the cells, since, in chemical transformation or in fluid transfer against 

 a concentration gradient, a certain quantity of energy is always consumed. 

 In excretion nonmodified substances are expelled along favorable con- 

 centration gradients without expenditure of energy by the cell. Never- 

 theless both processes frequently are more or less intermingled and it is 

 difficult to sepapFate them clearly." It seems to the writer that we must 

 try to distinguish them clearly. 



