FOBE-BEAIN 



113 



the eye ; but it must here be noticed that between the mouths of the two vesicles 

 there extends across the floor of the fore-brain a well-marked recess, which is 

 known as the optic recess (fig. 149). In the posterior fold of this, the optic 

 commissure is afterwards formed, and it is an important landmark during the 

 development of the parts. 



At first the fore-brain shows the typical regions of the rest of the neural tube viz. a thin 

 ependymal roof-plate, a floor-plate of the same constitution, and in the thick lateral walls 

 a basal and alar lamina separated by a furrow (sulcus of Monro). The thalami are formed 

 from the so-called alar laminae, as are also the cerebral hemispheres, while the basal laminae 

 give rise to the hypothalami, the tuber cinereum, infundibulum, and mamillary bodies. It is 

 <loubtful whether this subdivision has the same morphological value as in the rest of the tube. 



op.st. op.it. 



FIG. 150. SECTION OF THE FORE-BRAIN OF A HUMAN EMBRYO AT THE END OF THE FIFTH OR 

 BEGINNING OF THE SIXTH WEEK. Photograph. (T. H. Bryce.) 



h, hemisphere-vesicle ; f.ni., foramen of Monro ; c.s., c.s., corpora striata; op.st., op.st., proximal 



ends of optic stalks. 



By the thickening of the lateral walls to form the thalami, the cavity of the 

 vesicle of the fore-brain is reduced to a narrow vertical cleft the third ventricle 

 of the adult brain. The thalami ultimately come in contact with one another, and 

 are joined by a grey lamina known as the intermediate mass or middle commissure. 

 The ependymal roof at first shows a longitudinal convexity. As seen in His' model 

 of the brain of a sixth week embryo (fig. 150), the roof broadens out in front where 

 it reaches on each side the margin of the foramen of Monro, while behind, it 

 runs into a mesial swelling, which is the rudiment of the pineal body or epiphysis. 

 Extending forwards from the sides of this, two curved ridges represent the peduncles 

 of the pineal body. Outside these, again, are two broader bands connected 

 behind with a semilunar ridge on the roof of the mid-brain; they become the 

 brachia of the superior corpora quadrigemina. In front of the pineal body, and 

 between the peduncular ridges, the roof becomes reduced to a simple epithelial layer 

 which covers later the under aspect of the velum interpositum (tela choroidea of the 



VOL. I. I 



