STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 763 



How far this vesicle has a homologue in the brains of -other 

 Ganoids is not certain, since negative evidence on this subject is 

 all but valueless. It is possible that a vesicular sack covering 

 over the third ventricle of the Sturgeon described by Stannius 1 , 

 and stated by him to be wholly formed of the membranes of the 

 brain, is really the homologue of our vesicle. 



Wiedersheim 2 has recently described in Protopterus a body 

 which is undoubtedly homologous with our vesicle, which he 

 describes in the following way : 



" Dorsalwarts ist das Zwischenhirn durch ein tiefes, von 

 Hirnschlitz eingenommenes Thai von Vorderhirn abgesetzt ; 

 dasselbe ist jedoch durch eine hautige, mit der Pia mater zusam- 

 menhangende Kuppel oder Kapsel uberbruckt." 



This " Kuppel " has precisely the same relations and a very 

 similar appearance to our vesicle. The true pineal gland is 

 placed behind it. It appears to us possible that the body found 

 by Huxley 3 in Ceratodus, which he holds to be the pineal gland, 

 is in reality this vesicle. It is moreover possible that what has 

 usually been regarded as the pineal gland in Petromyzon may 

 in reality be the homologue of the vesicle we have found in 

 Lepidosteus. 



We have no observations on the pineal gland of the adult, 

 but must refer the reader for the structure and relations of this 

 body to the embryological section. 



The infundibulum (Plate 38, fig. 47 A, in.} is very elongated. 

 Immediately in front of it is placed the optic chiasma (Plate 38, 

 figs. 47 A and C, op.c/i.} from which the optic fibres can be traced 

 passing along the sides of the optic thalami and to the optic 

 lobes, very much as in Muller's figure of the brain of Po- 

 lypterus. 



On the sides of the infundibulum are placed two promi- 

 nent bodies, the lobi inferiores (/.#/.), each of which contains a 

 cavity continuous with the prolongation of the third ventricle 



1 " Ueb. d. Gehirn des Stors," Muller's Archii\ 1843, and Lehrbuch d. z-ergl. Anat. 

 d. Wirbdthiere. Cattie, Archives tie Biologic, Vol. in. 1882, has recently described 

 in Acipcnscr sturio a vesicle on the roof of the thalamencephalon, whose cavity is 

 continuous with the third ventricle. This vesicle is clearly homologous with that in 

 Lepidostetis. (June 28, 1882.) 



2 R. Wiedersheim, Morphol. SfuJitn, 1880, p. 71. 



a " On Certitihtm h'orstirij' kc., /'roc. tool. Soc. ^76. 



492 



