STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 765 



The original embryonic fore-brain is divided in both embryos 

 into a cerebrum (ce.) in front and a thalamencephalon (th.) behind. 

 In the younger embryo the cerebrum is a single lobe, as it is 

 in the brains of all Vertebrate embryos ; but in the older larva 

 it is anteriorly (Plate 37, fig. 46 A) completely divided into 

 two hemispheres. The roof of the undivided posterior part of 

 the cerebrum is extremely thin (Plate 37, fig. 46 B). Near the 

 posterior border of the base of the cerebrum there is a great 

 development of nervous fibres, which may probably be regarded 

 as in part equivalent to the anterior commissure (Plate 37, figs. 

 44, 45 a.c.). 



Even in the oldest of the two brains the olfactory lobes are 

 very slightly developed, constituting, however, small lateral and 

 ventral prominences of the front end of the hemispheres. From 

 each of them there springs a long olfactory nerve, extending for 

 the whole length of the rostrum to the olfactory sack. 



The thalamencephalon presents a very curious structure, and 

 is relatively a more important part of the brain than in the 

 embryo of any other form which we know of. Its roof, instead 

 of being, as usual, compressed antero-posteriorly 1 , so as to be 

 almost concealed between the cerebral hemispheres and the optic 

 Jpbes (mid-brain), projects on the surface for a length quite equal 

 to that of the cerebral hemispheres (Plate 37, figs. 44 and 45, th.}. 

 In the median line the roof of the thalamencephalon is thin 

 and folded ; at its posterior border is placed the opening of 

 the small pineal gland. This body is a papilliform process of 

 the nervous matter of the roof of this part of the brain, and 

 instead of being directed forwards, as in most Vertebrate types, 

 tends somewhat backwards, and rests on the mid-brain behind 

 (Plate 37, figs. 44, 45, and 46 C and D, /.). The roof of the 

 thalamencephalon immediately in front of the pineal gland forms 

 a sort of vesicle, the sides of which extend laterally as a pair 

 of lobes, shewn in transverse sections in Plate 37, figs. 46 C and 

 D, as th.L This vesicle becomes, we cannot doubt, the vesicle 

 on the roof of the thalamencephalon which we have described in 

 the adult brain. Immediately in front of the pineal gland the 

 roof of the thalamencephalon contains a transverse commissure 



1 Vide F. M. Balfour, Comparative Embryology, Vol. II. figs. 248 and 250. 



