372 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 



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 Monro. 



For convenience of description we may divide the 

 thalamencephalon into three regions, viz. (1) the floor, 

 (2) the sides, and (3) the roof. 



The floor becomes divided into two parts : an an- 

 terior part, giving origin to the optic nerves, in which is 

 formed the optic chiasma ; and a posterior part, which 

 becomes produced into a prominence at first incon- 

 spicuous the rudiment of the infundibulum (Fig. 39 In). 

 This comes in contact with the involution from the 

 mouth which gives rise to the pituitary body (Fig. 

 39 pt). 



In Birds, although there is a close connection be- 

 tween the pituitary body and the infundibulum, there 

 is no actual fusion of the two. In Mammalia the case 

 is different. The part of the infundibulum which lies 

 at the hinder end of the pituitary body is at first a 

 simple finger-like process of the brain (Fig. 120 inf); 

 but its end becomes swollen, and the lumen in this 

 part becomes obliterated. Its cells, originally similar to 

 those of the other parts of the nervous system, and even 

 containing differentiated nerve-fibres, partly atrophy 

 and partly assume an indifferent form, while at the 

 same time there grow in amongst them numerous 

 vascular and connective-tissue elements. The process 

 of the infundibulum thus metamorphosed becomes in- 

 separably connected with the true pituitary body, of 

 which it is usually described as the posterior lobe. 



