THE THA L A MKXCEl'HA 



siderable period this rudiment remains perfectly simple, and exhibits 

 no signs, either externally or internally, of a longitudinal constriction 

 dividing it into two lobes. 



From the above description it may be concluded that the rudi- 

 ment 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 thala- 

 mencephalon, in the primitive vertebrate stock. 



The thalamenceplialon. The thalamencephalon varies so slightly 

 in structure throughout the Vertebrate series that a general descrip- 

 tion 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. 



The cavity it contains is known as the third ventricle. Anteriorly 

 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. (1) 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 at first incon- 

 spicuous prominence the rudiment of the infundibulum (fig. 252 

 In). This comes in contact with an involution frurn the mouth, 



ait.v 



vao , r 



rfi, ft' 



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./i. cerebral hemisphere; th. optic thalamus ; in. infundibulum; pit. pineal gland; 

 ;/;. mid-brain; cb. cerebellum; md. medulla oblongata; M.r. auditory vesicle ; op. 

 optic vesicle; ol. olfactory pit; m. mouth; hr.c. branchial pouches; tit. thyroid 

 involution; r.ao. ventral aorta; Jit. ventricle of heart; ch. notochord. 



which gives rise to the pituitary body (fig. 252 pt), the development 

 of which will be dealt with separately. 



In the later stages of development the infundibulum becomes 



