DIENCEPHALON 245 



lar, and here the connection of these fibers specifically with the com- 

 pact nucleus magnocellularis is unmistakable ('41&). This, accord- 

 ingly, is one source of activation of the hypophysial tract, the course 

 of which in Ameiurus has been described by Palay ('45). His observa- 

 tions also indicate that in these fishes the nucleus magnocellularis 

 itself is a source of endocrine secretion, as is clear also from the work 

 of Scharrer and Scharrer ('40). 



A second specific source of excitation of the nucleus magnocel- 

 lularis is the small number of fibers which separate from the optic 

 tracts near the chiasma and arborize in the area where these cells 

 are most abundant (p. 221), thus providing visual control of en- 

 docrine secretion. This, however, is minimal in Amblystoma, and the 

 very extensive hypophysial innervation must be concerned in the 

 main with other functions. 



The very numerous unmyelinated axons of the cells of the nucleus 

 magnocellularis are diflficult to follow, for they descend in dispersed 

 arrangement, penetrate the chiasma ridge, and then converge into 

 the compact tr. hypophysius in the thin floor of the infundibulum. 

 Their further course is clear. We have many brilliant Golgi impregna- 

 tions of their distribution, some of which have been illustrated ('42, 

 figs. 55-65). In the pars nervosa of the hypophysis they form a very 

 dense neuropil, are less abundant in the pars intermedia, and a few of 

 them penetrate the pars distalis. They terminate in tiny end-bulbs 

 resting upon the epithelial cells. The pars nervosa is a rather thin 

 sheet of convoluted epithelium which forms the posterodorsal wall of 

 the wide infundibulum. These lobules take the form of irregular villi, 

 in the axis of each of which is a capillary loop. 



CONCLUSION 



In most lower vertebrates the hypothalamus is relatively much 

 larger than in the higher groups, and this seems to be correlated with 

 the dominance of the olfactory system in the organization of the fore- 

 brain of the lower forms. All other functional systems have extensive 

 representation here, and in this respect the hypothalamus resembles 

 the habenular complex, but with the difference that in the latter the 

 afferents converge into a very small compact area with one major 

 efferent path, the f . retroflexus, while the much larger hypothalamus 

 is diversified in structure and has a wider range of distribution of its 

 efferent fibers. Though both these complexes are evidently concerned 



