430 THE THALAMENCEPHALON. 



rudiment of the cerebral hemispheres is contained in the 

 original fore-brain. In spite however of their great importance 

 in all the Craniata, it is probable that the hemispheres were 

 either not present as distinct structures, or only imperfectly 

 separated from the thalamencephalon, in the primitive vertebrate 

 stock. 



The thalamencephalon. The thalamencephalon varies so 

 slightly in structure throughout the Vertebrate series that a 

 general description will suffice for all the types. 



It forms at first a simple vesicle, the walls of which are of 

 a nearly uniform thickness and formed of the usual spindle- 

 shaped cells. 



md. 



FIG. 253. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A 

 LARVA OF PETROMYZON. 



The larva had been hatched three days, and was 4*8 mm. in length. The optic 

 and auditory vesicles are supposed to be seen through the tissues. 



c.h. cerebral hemisphere ; th. optic thalamus; in. infundibulum ; pn. pineal gland ; 

 mb. mid-brain ; cb. cerebellum ; md. medulla oblongata ; au.v. auditory vesicle ; op. 

 optic vesicle; ol. olfactory pit; m. mouth; br.c. branchial pouches; th. thyroid 

 involution; v.ao. ventral aorta; ht. ventricle of heart ; ch. notochord. 



The cavity it contains is known as the third ventricle. An- 

 teriorly it opens widely into the cerebral rudiment, and posteriorly 

 into the ventricle of the mid-brain. The opening into the 

 cerebral rudiment becomes the foramen of Munro. 



For convenience of description I shall divide it into three 

 regions, viz. (i) the floor, (2) the sides, and (3) the roof. 



The floor becomes divided into two parts, an anterior part, 

 giving origin to the optic nerves, in which is formed the optic 

 chiasma ; and a posterior part, which becomes produced into an 



