no THE BRAIN OF THE TIGER SALAMANDER 



nature is not bound by our formal rules of logical consistency. The 

 major subdivisions of the brain were thus defined very early in 

 vertebrate phylogeny, and they retain their general characteristics 

 throughout the series, but there is no apparent limit to the range of 

 modifications which these sectors may undergo in adaptation to 

 specific physiological requirements. 



In premotile stages of Amblystoma, Coghill mapped several areas 

 in the walls of the neural tube, characterized by distinctive prolifera- 

 tion and differentiation. Adopting a modification of his scheme, I 

 gave arbitrary numbers to twenty-two such areas from the olfactory 

 bulb to the cerebellum ('37, p. 392), and their development can be 

 followed up to the adult stage. These units of the mosaic pattern of 

 the premotile embryo undergo remarkable shiftings of position dur- 

 ing larval development and an equally remarkable diversity in pat- 

 terns of differentiation and fibrous connections. When the compara- 

 tive embryologist surveys the vertebrate series as a whole, he recog- 

 nizes a striking similarity in the early stages of differentiation of the 

 neural tube of all of them. In later stages of development this similar- 

 ity gives way to wide diversity in the progress of differentiation of 

 these primordial units of structure, and practically all this divergent 

 specialization can be seen to be directed toward adaptive modifica- 

 tions of structure, correlated with differences in the action systems of 

 the several species. Some limits to the range of this modifiability are 

 set by the inherent qualities of the genetic organization so that some 

 general principles of morphological pattern can be recognized every- 

 where. Yet the structure when viewed phylogenetically is remarkably 

 plastic, and the available materials are adapted to a wide variety of 

 uses in diverse combinations and interconnections in all the different 

 phyla. The most alluring feature of these comparative studies lies in 

 our ability to sort out of this apparent confusion of detail those 

 strong threads of ancestral influence which are interwoven in ever 

 changing designs under the influence of adaptive adjustment to dif- 

 ferent modes of life. 



THE FUTURE OF MORPHOLOGY 



During the past half -century, morphology has seemed to be de- 

 clining in favor, its problems submerged in the more attractive pro- 

 grams of the experimentalists. Nevertheless, activity in this field has 

 not abated, and now there is a renaissance, the reasons for which are 



