MORPHOLOGY OF THE FOREBRAIN 473 



The diminution in the size of the rostral end of the pars dorsalis 

 thalami is due to two causes; first the evagination of the optic 

 vesicle from this region, and second to the fact that almost the 

 whole of the cerebral hemisphere has been evaginated from the 

 extreme rostral end of this column. 



In Ammocoetes, we are told by Tretjakoff that fibers from the 

 parapineal organ, after decussation in the superior commissure, 

 end in the pre thalamus. He therefore assumes that this struc- 

 ture (the epistriatum of Johnston, our dorso-median ridge) is a 

 correlation center for sensory impressions from the parapineal 

 organ with others from the olfactory organ. It also receives a 

 hypothalamic tract according to Johnston. Its efferent path in 

 Ammocoetes, as in Lampetra, is by way of the corpus striatum. 

 It seems very probable to me that in higher animals, in which 

 the sensory function of the pineal organs is reduced, the caudal 

 (diencephalic) part of this dorso-median ridge suffers correspond- 

 ing reduction, while the telencephalic end is preserved on account 

 of its olfactory connection, and is ultimately evaginated into the 

 cerebral hemisphere to become the primordium hippocampi. 



Johnston teaches ('02) that the dorso-median ridge lies wholly 

 within the telencephalon. Other students of cyclostomes who 

 have discussed the matter agree that it is wholly diencephalic 

 (Schilling, '07; Sterzi, '07; Kappers, '08; Tretjakoff, '09). As 

 appears from the preceding discussion, I think it clearly extends 

 across the di-telencephalic boundary, in this resembling the other 

 parts of the diencephalon of cyclostomes, which pass forward 

 without interruption into the telencephalon. 



In a recent paper Johnston ('10, p. 147) claims that the lateral 

 attachment of the velum transversum to the massive side walls 

 of the brain in several types of fishes lies far caudad of its position 

 in the mid-line, near the habenula, so that much of the border 

 which has been considered by most authors as taenia thalami is 

 really in these forms telencephalic and therefore comparable with 

 the taenia fornicis of Amphibia and higher forms. The data so 

 far available do not seem to me to support this interpretation; 

 but from the standpoint of this discussion it is not a matter of great 

 importance. For if the four longitudinal columns of which the 



