DIENCEPHALON 231 



systems all have a collateral discharge into the thalamus, which is 

 probably concerned with conditioning and other types of "inflection" 

 of the stable components of behavior. In the most primitive verte- 

 brates this primordial thalamic apparatus of higher synthesis and 

 conditioning is small and at a low grade of differentiation. In Amblys- 

 toma it is notably larger, but still relatively unspecialized and prob- 

 ably concerned more with regulation of central excitatory state and 

 the general physiological disposition and diathesis than with refined 

 analysis. In mammals this tissue is elaborately specialized, with pro- 

 vision for precise localization of function correlated with the simul- 

 taneous differentiation of the neopallial cortex. But even in man the 

 intrinsic functions of the thalamus are not supplanted by cortical 

 development. The thalamo-cortical connections carry two-way traf- 

 fic and all cortical activity may be profoundly affected by dience- 

 phalic influence. 



The primary subdivisions of the adult diencephalon are more 

 clearly evident in the gross preparation of the urodele brain than in 

 most other vertebrates, as illustrated in figures IB and 2. I empha- 

 sized this in the paper of 1910, and the analysis there proposed has 

 been widely accepted as applicable in all vertebrates. The epithala- 

 mus includes the habenular nuclei, the pars intercalaris, the epiphy- 

 sis, and the membranous dorsal sac and paraphysis. The dorsal 

 thalamus is an undifferentiated nucleus sensitivus, the ventral thala- 

 mus a field of motor co-ordination, and the hypothalamujs an olfacto- 

 visceral adjuster, including the pars nervosa of the hypophysis. 

 These subdivisions and their more important connections have been 

 described in previous publications (see the summary, '42, p. 204; 

 also, '10, '21a, '27, '35a, '36, '39, '396, '41, '42; Necturus, '17, '336, 

 '346, '34c, '41a; for other urodeles see the works cited on p. 11). 



DEVELOPMENT 



The larger part of the wall of the diencephalic sector of the early 

 neural tube is evaginated to form the retina and the optic nerve. The 

 details of this development, which begins at the anterior end of the 

 medullary plate, have been recorded by Adelmann ('29, '36). I have 

 described later stages of the development of these structures ('41). 

 During these stages most of the dorsal median wall of this part of the 

 neural tube remains membranous and is elaborately enlarged and 

 folded to form the epiphysial vesicle, paraphysis, and diencephalic 

 chorioid plexuses, the development of which has recently been de- 



