762 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



ventricle as being prolonged for the whole length of the anterior 

 lobes. 



In the embryos of all Vertebrates the cerebrum is not at 

 first divided into two lobes, so that the fact of the posterior part 

 of the cerebrum in Lepidosteus and probably other Ganoids 

 remaining permanently in the undivided condition does not 

 appear to us a sufficient ground for giving to the lobes of this 

 part of the cerebrum the special name of prothalami, as pro- 

 posed by Wilder, or for regarding them as a section of the 

 brain peculiar to Fishes. 



The thalamencephalon (///.) contains the usual parts, but is 

 is some respects peculiar. Its lateral walls, forming the optic 

 thalami, are thick, and are not sharply separated in front from 

 the basal part of the cerebrum ; between them is placed the 

 third ventricle. The thalami are of considerable extent, though 

 partially covered by the optic lobes and the posterior lobes of 

 the cerebrum. They are not, however, relatively so large as 

 in other Ganoid forms, more especially the Chondrostei and 

 Polypterns. 



On the roof of the thalamencephalon is placed a large thin- 

 walled vesicle (Plate 38, figs. 47 A and B, v.th.), which undoubtedly 

 forms the most characteristic structure connected with this part 

 of the brain. Owing to the wretched state of preservation of 

 the specimens, we have found it impossible to determine the 

 exact relations of this body to the remainder of the thalamen- 

 cephalon; but it appears to be attached to the roof of the 

 thalamencephalon by a narrow stalk only. It extends forwards 

 so as to overlap part of the cerebrum in front, and is closely 

 invested by a highly vascular layer of the pia mater. 



No mention is made by Wilder of this body ; nor is it repre- 

 sented in his figures or in those of the other anatomists who 

 have given drawings of the brain of Lcpidostens. It might at 

 first be interpreted as a highly-developed pineal gland, but a 

 comparison with the brain of the larva (vide p. 764) shews that 

 this is not the case, but that the body in question is represented 

 in the larva by a special outgrowth of the roof of the thalamen- 

 cephalon. The vesicle of the roof of the thalamencephalon is 

 therefore to be regarded as a peculiar development of the tela 

 choroidea of the third ventricle. 



