28 JOURNAI. OF COMI'ARATIXK NkI ROHXiY. 



body itself with reference to the brain, the relations to the 

 habena and the surface seem similar to those in the lizard. 



The epiphysis in Petromyzon is more complicated, but 

 obviously has a similar structure, so far as the epithelium is 

 concerned. (See Alborn, Untersuchungen iiber das (iehirn 

 der Petromyzonten, Zeitsch. f. Wis. Zoologie, xxxix.) 



The relations in Amphibia are not essentially different, 

 though the organ in adults of Anura has suffered much 

 reduction. The evidence for the sensory, and even the optic 

 nature of the organ is conclusive aside from that of palfeon- 

 tology. The embryonic condition of the epiphysis in the 

 serpents is illustrated by Fig. 5, Plate X. 



The structure of the thalamus may be conveniently stud- 

 ied in transverse sections beginning cephalad. In the cere- 

 brum, in the region of the praecommissura, the peduncular 

 fibres begin to aggregate near the ventral and median por- 

 tion, and sensory bundles arrange themselves laterad from 

 them. The latter are accompanied by fusiform cells. The 

 basal region — i. c, homologue of the pyriform lobe — con- 

 tains a dense mass of cellular elements, consisting largely 

 of small, dark, spidery cells of the olfactory type. On 

 entering the thalamus the peduncles acquire a circular sec- 

 tion, and the aesthesodic fibres arch over the bundle median- 

 ly. The median nidulus of the thalamus which occupies 

 the sides of the third ventricle arches over them. 



In sections somewhat farther caudad the habena is seen 

 perched, as it were, on the summit of the diencephalon, 

 and well circumscribed ventrally from Meynert's nidulus. 

 The definite contours of the peduncles are soon lost as we 

 proceed caudad. 



The optic tracts are everywhere separated from the thal- 

 amus by the lenticular gelatinous band forming the substantia 

 negra, entad to which is the nidulus, already described, con- 

 sisting of slender multipolar or pyramidal cells lying trans- 

 versely and sending processes toward the tract and cephalo- 

 medianly (Fig. 3b, Plate XII). The dorsal region of the 



