110 THE BRAIN OF THE TIGER SALAMANDER 



such as metamerism, germ layers, and so on, were formalized, and the 

 tendency was to regard them as stable and immutable factors in 

 morphogenesis. The problems were thus simplified in terms of mis- 

 leading logical categories, resulting in a static, rather than a dynamic, 

 analysis of development and evolution. These formal and rigid con- 

 cepts have too often retarded, rather than facilitated, a true under- 

 standing of the structure. 



The dialectic of some current "form-analytic" programs of re- 

 search on morphogenesis of the brain seems to postulate a predeter- 

 mined primordial pattern of the neural tube, which is preserved 

 throughout all stages of differentiation and which can be recognized 

 by the arrangement in space of cellular areas and their relations to 

 one another independently of their functional connections or of any 

 dynamic agencies in morphogenesis other than cellular proliferation. 

 The result is an oversimplified, logically consistent, morphological 

 schema uncontaminated by functional or other complicating factors; 

 but this has little interest for the working anatomists and physiolo- 

 gists, for it has no obvious relation with the structural forms with 

 which they are practically working. Nerve fibers are quite as impor- 

 tant as cell bodies in cerebral organization and as elements in cerebral 

 forms. By what right does the morphologist ignore them in his study 

 of form? We have ample evidence that the growth of nerve fibers and 

 the migration of cells may be determined by functional requirements, 

 which differ from species to species in correlation with different 

 modes of life. Are forms which have physiological meaning of no sig- 

 nificance in morphology, and can they safely be ignored by morphol- 

 ogy.^ 



My paper of 1933 cites several illustrations from my own experi- 

 ence and that of others of the seductive influence of rigid categories of 

 morphological concepts, which simplify analysis by neglect of other 

 significant factors in morphogenesis. In modern morphology the 

 search is for genetic relationships, and homologies are defined in 

 terms of such relations. In all phylogenetic study we must constantly 

 keep in the foreground of attention two main classes of morphoge- 

 netic agencies. These are, first, the conservative factor of stable 

 genetic organization and, second, the more labile influence of the 

 specific functional requirements. Both factors are always present, 

 and one important task of the morphologist is the analysis of his 

 material so as to reveal the parts played by each of them. A sound 

 and fruitful morphology will take both into account. For the practical 



