10 THE BRAIN OF THE TIGER SALAMANDER 



(cyclostomes, primitive ganoid fishes, urodeles), are correlated with 

 a histological texture of the brain which is characteristic and prob- 

 ably primitive (chap. iii). 



The external configuration of the urodele brain also is generalized, 

 much as in a human embryo of about 6 weeks. In the next chapter 

 special attention is directed to this comparison to assist the reader in 

 identifying familiar parts of the human brain as they are seen in the 

 simplified amphibian arrangement. 



In our comparison of the amphibian brain with the human, two 

 features are given especial emphasis, both of which are correlated 

 with differences in the mode of life of the animals in question, that is, 

 with the contrast between the amphibian simplicity of behavior with 

 stereotyped total patterns of action predominating and the human 

 complexity of movement in unpredictable patterns. The correlated 

 structural differences are, first, in Amblystoma the more generalized 

 histological texture to which reference has just been made, and no- 

 tably the apparent paucity of provision for well-defined localization 

 of function in the brain; and, second, the preponderant influence of 

 motor patterns rather than sensory patterns in shaping the course of 

 differentiation from fishlike to quadrupedal methods of locomotion 

 and somatic behavior in general. 



SOURCES AND MATERIAL 



The material studied comprises gross dissections and serial sections 

 of about five hundred specimens of Amblystoma from early em- 

 bryonic to adult stages. About half these brains were prepared by the 

 Golgi method and the remainder by various other histological pro- 

 cedures. Most of these are A. tigrinum, some are A. maculatum 

 (punctatum), and a few are A. jeffersonianum. In early develop- 

 mental stages some specific differences have been noted in the em- 

 bryological papers of 1937-41, but no systematic comparative study 

 has been made. The late larval and adult brains under consideration 

 in this book are of A. tigrinum. In former papers there are comments 

 on this material and the methods of preparation ('25, p. 436; '35a, 

 p. 240; '42, p. 193). 



In the study of these sections the cytological methods of Nissl and 

 others are less revealing than in more highly differentiated brains 

 because of the unspecialized structure of the nervous elements. Some 

 modifications of the method of Weigert which decolorize the tissue 

 sufficiently to show the myelinated fibers and also the arrangement 



