SALAMANDERS AND THEIR BRAINS 13 



differentiation. The need for a comprehensive study of the develop- 

 ment of the histological structure of the brain of Amblystoma, in- 

 cluding the differentiation of the nervous elements and their fibrous 

 connections, is especially urgent in view of the very large number of 

 experimental studies on developmental mechanics which have been 

 in progress for many years and will probably continue for years to 

 come. Amblystoma has proved to be an especially favorable subject 

 for these studies, and in many of them a satisfactory interpretation 

 of the findings cannot be achieved without more complete knowledge 

 than we now possess of the development of both the nervous tissues 

 and other bodily organs. 



THE EVOLUTION OF BRAINS 



The nervous systems of all vertebrates have a common structural 

 plan, which is seen most clearly in early embryonic stages and in the 

 adults of some primitive species. But when the vertebrate phylum is 

 viewed as a whole, the nervous apparatus shows a wider range of 

 adaptive structural modifications of this common plan than is ex- 

 hibited by any other system of organs of the body. In order to under- 

 stand the significance of this remarkable plasticity and the processes 

 by which these diverse patterns of nervous organization have been 

 elaborated during the evolutionary history of the vertebrates, it is 

 necessary to find out what were the outstanding features of the 

 nervous system of the primitive ancestral form from which all higher 

 species have been derived. 



Since the immediate ancestors of the vertebrate phylum have been 

 extinct for millions of years and have left no fossil remains, our only 

 recourse in this search is to examine the most generalized living 

 species, compare them one with another and with embryonic stages, 

 and so discover their common characteristics. This has been done, 

 and we are now able to determine with a high degree of probability 

 the primitive pattern of the vertebrate nervous system. 



The most generalized living vertebrates (lampreys and hagfish) 

 have brains which most closely resemble that of the hypothetical 

 primordial vertebrate ancestor. The brains of the various groups of 

 fishes show an amazing variety of deviations from the generalized 

 pattern. The paleontological record shows that the first amphibians 

 were derived from one of the less specialized groups of fishes; and 

 there is evidence that the existing salamanders and their allies have 

 preserved until now a type of brain structure which closely resembles 



