12 THE BRAIN OF THE TIGER SALAMANDER 



excellent illustrations the early stages of the neural plate and neural 

 tube. Baker ('27) illustrated dorsal and ventral views of the open 

 neural plate, and Baker and Graves ('32) described six models of 

 the brain of A. jeffersonianum from 3 to 17 mm. in length. Burr 

 ('22) described briefly the early development of the cerebral hemi- 

 spheres. 



Successive stages of the brain of A. punctatum have been illus- 

 trated by Coghill and others (some of which I have cited, '37, p. 391, 

 and '38, p. 208), and at the Wistar Institute there are other models 

 of the brains of physiologically tested specimens. Coghill's papers 

 include a wealth of observation on the development of the mecha- 

 nisms of the action system, and these were summarized in his London 

 lectures, published in 1929. His reports were supplemented by a series 

 of papers which I published from 1937 to 1941, but these fragmentary 

 observations (of the younger stages particularly) were based on in- 

 adequate material and are useful only as preliminary orientation for 

 a more systematic investigation. In Coghill's papers there are ac- 

 curate projections of all mitotic figures and neuroblasts of the central 

 nervous system in nonmotile, early flexure, coil, and early swimming 

 stages and the arrangement of developing nerve fibers of the brain in 

 the last-mentioned stage (Coghill, '30, Paper IX, fig. 4). On the basis 

 of these data he divided the embryonic brain in front of the isthmus 

 into sixteen regions, each of which is a center of active and char- 

 acteristic differentiation. These regions are readily identified in our 

 reduced silver preparations of these and later stages. Using a modifi- 

 cation of this analysis, I have distinguished and numbered twenty- 

 two such regions in the cerebrum and cerebellum ('37, p. 392), and 

 the development of each of these can be followed through to the 

 adult stage. In my papers of 1937-39 some salient features of these 

 changes are recorded; but this account is incomplete, and more 

 thorough study is urgently needed. In the present work some details 

 only of this development are given in various contexts as listed in the 

 Index under "Embryology." 



The most detailed description of the development of the urodele 

 brain is the paper by Sumi ('26) on Hynobius. Soderberg ('22) gave a 

 brief description of the development of the forebrain of Triturus 

 (Triton) and a more detailed account of that of the frog, and Rude- 

 beck ('45) has added important observations. 



The successive changes in the superficial form of the brain can be 

 interpreted only in the light of the internal processes of growth and 



