354 • THE THALAMEXCEPHALOK. 



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 thalamencephalon. 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 

 vmiform 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 from the mouth. 



««f ma 



I 



ht 



r.ao ,, , 



flo. 253. diafieammatic 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./t. cerebral hemisphere; th. optic thalamus ; in. infundibulum ;/>«. pineal gland; 

 mh. mid-brain; ch. cerebellum; md. medulla oblongata; au.v. auditory vesicle ; op. 

 optic vesicle; o\. olfactory pit; m. mouth; hr.c. branchial pouches; t]i. thyroid 

 involution; v.ao. ventral aorta; ht. 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 



