8 VERTEBRATE ZOOLOGY 



glossus, and possibly that of the echinoderms. The most characteris- 

 tic stage in certain annelid larvae has also three metameres. The 

 higher vertebrates have in the process of their evolution gradually 

 appropriated to the head, one after the other, the adjacent body 

 metameres. 



The Backward Retreat of the Lower Functions. — ^Accompanying 

 the concentration of the chief nervous elements in the head there is 

 a progressive series of changes in the opposite direction, consisting of 

 a steady retreat toward the posterior end of the respiratory, digestive, 

 excretory, and reproductive functions. These functions are, in the 

 more primitive vertebrates, present in the metameres just back of the 

 head, but in the higher forms they are steadily pushed back into the 

 posterior metameres by becoming atrophied in the anterior metameres 

 and by the development of new sets of more or less equivalent 

 organs further back. An excellent example of this type of process is 

 seen in the evolution of the kidneys.' In the most primitive verte- 

 brates the embryonic kidney is located well toward the anterior part 

 of the coelom, and in the hag-fishes this anterior kidney, or pronephros, 

 functions in the adult. In vertebrates of the fish and amphibian 

 grades the kidney of the adult is a mesonephros, located farther back 

 in the body cavity; while in the land vertebrates the functional kid- 

 ney is a metanephros, situated still farther toward the posterior end. 



The farther these organs of lower dynamic activity retreat from 

 the dominant anterior end the more readily they appear to dif- 

 ferentiate and the more specialized and condensed they become, just 

 as though they were steadily growing out from under an influence 

 that tends to suppress their full developmental possibilities. The evo- 

 lution of the urogenital organs of the vertebrates affords an interest- 

 ing example of this. In the lowest vertebrates the gonads and their 

 ducts are simple, are located far forward in the body cavity, and are 

 to a large extent separate from the urinary elements. In the highest 

 vertebrates, however, these organs have retreated far to the poste- 

 rior end of the primary axis and have been intimately involved with 

 the urinary system. 



A PHYSIOLOGICAL INTERPRETATION OF THE FUNDAMENTAL 

 STRUCTURAL PLAN OF VERTEBRATES 



Professor C. M. Child has developed what appears to the writer 

 to be the most scientific and far-reaching dynamic interpretation of 



