CHAPTER II 

 THE FORM AND SUBDIVISIONS OF THE BRAIN 



GROSS STRUCTURE 



REFERENCE to figures 1-5, 85, and 86 shows that the larger 

 ; subdivisions of the human brain are readily identified in Am- 

 blystoma, though with remarkable differences in shape and relative 

 size. When this comparison is carried to further detail, the sculptur- 

 ing of the ventricular walls shown in the median section is especially 

 instructive. It is again emphasized that the application of mam- 

 malian names to the structures here revealed rarely implies exact 

 homology; these areas are to be regarded as primordia from which 

 the designated mammalian structures have been differentiated. The 

 relationships here implied have been established by several inde- 

 pendent lines of evidence: (1) The relative positions and fibrous con- 

 nections of cellular masses and the terminal connections of tracts. 

 In so far as these arrangements conform with the mammalian pat- 

 tern, they may be regarded as homologous. (2) Embryological evi- 

 dence. The early neural tubes of amphibians and mammals are simi- 

 lar, and subsequent development of both has been recorded. On the 

 basis of Coghill's observations of rates of proliferation and differen- 

 tiation in prefunctional stages, the writer ('37) gave arbitrary num- 

 bers to recognizable sectors of the neural tube in early functional 

 stages, and the subsequent development of each of these is, in broad 

 lines, similar to that of corresponding mammalian parts. (3) The 

 relationships of svipposed primordia of mammalian structures may be 

 tested by the comparative method. In an arrangement of animal 

 types which approximates the phylogenetic sequence from the most 

 generalized amphibians to man, there are many instances of progres- 

 sive differentiation of amphibian primordia by successive increments 

 up to the definitive human form. 



Many pictures of the brains of adult and larval Amblystoma and 

 other urodeles have been published, some of which I have cited 

 ('35a, p. 239). The most accurate pictures of the brain of adult A. 

 tigrinum are those of Roofe ('35), showing dorsal, ventral, and lateral 



18 



