GENERAL PRINCIPLES OF MORPHOGENESIS 117 



tectum. This is followed by a flexure in the reverse direction at the 

 di-telencephalic junction (p. 212). These flexures are less obvious in 

 adult brains because they are somewhat straightened in later stages 

 and masked by growth of interstitial tissues; but the site of each of 

 them is a zone of transition between major divisions of the brain 

 with distinctive physiological characteristics. 



The foundations of the current anatomical analysis of the brain were laid by 

 Wilhelm His in terms of human embryological development. The early neural tube 

 was divided into a linear series of blocks separated by transverse planes, and a 

 longitudinal sulcus limitans on each side marks the boundary between a dorsal 

 sensory alar plate and a ventral motor basal plate. The adult derivatives of this 

 embryonic mosaic are the primary anatomical units. The nomenclature derived from 

 this analysis as officially adopted (the BNA), or modifications of it, is now almost 

 universally employed, to the great advantage of human descriptive neurology. But 

 this scheme has its limitations. Some features of it are quite inapplicable to the 

 brains of lower vertebrates; for, though the embryonic neural tube is similar in most 

 of them, its adult derivatives vary so widely in adaptation to diverse modes of life 

 that no inflexible formula is applicable. Since the brain of Amblystoma is generalized, 

 few of these difficulties arise here. 



There is difference of opinion about where the sulcus limitans ends anteriorly. 

 If the embryonic floor plate ends at the fovea isthmi (Kingsbury, '30), it is evident 

 that the basal plate extends farther forward to include the mesencephalic cerebral 

 peduncle and probably more or less of the adjoining parts of the hypothalamus and 

 ventral thalamus. The remainder of the mesencephalon and diencephalon (including 

 the retina) and the whole of the telencephalon are derived from the expanded an- 

 terior part of the alar plate and the related neural crest. The adult derivatives of 

 alar and basal plates include much tissue that is specifically neither sensory nor 

 motor; and this intercalated associational fabric crosses the boundaries of the primi- 

 tive embryonic mosaic in ways which difi^er from species to species. Each species 

 must be analyzed in terms of its own mode of life and distinctive action system. 



C. von Kupffer ('06) put special emphasis upon two deep transverse sulci in the 

 ventricular wall of the neural tube in early embryonic stages of a series of lower 

 vertebrates, including Necturus and Salamandra. These were termed sulcus intra- 

 encephalicus anterior and posterior. Study of later stages of developing urodele 

 brains shows that this emphasis was well placed, for these sulci mark the positions 

 of the two transitional sectors of the brain to which reference was made above — 

 the first, the diencephalic, and the second, the isthmic sector. 



In our specimens of Amblystoma, von Kupffer's anterior sulcus in early motile 

 stages (Harrison's stages 33-36) is a sharply defined groove, which extends dorsally 

 from the lateral optic recess in front of the chiasma ridge to the region of the velum 

 transversum. Its dorsal part is varikble, but clearly the primary course is into the 

 posterodorsal initial evagination of the hemispheric vesicle, as described b.y von 

 Kupffer and by Rudebeck ('45). This is clearly the case also in A. jeffersonianum, as 

 shown by Baker and Graves ('32) in their five stages from 5 to 17 mm. long. My 

 published references to this sulcus and its adult derivatives have been successively 

 modified, as more material was examined ('10, pp. -119, 43:2; '£7, p. 238; '33b, p. 

 240; '35a, p. 252; "38, p. 212; '38b, pp. 401, 402; '39a, p. 262). These differences in 

 interpretation are doubtless due in part to the natural variability of the specimens 

 and in part to lack of a sufficiently close series of well-preserved stages to reveal the 



