306 OUTLINES OF CHORDATE DEVELOPMENT 



the cerebral hemispheres. The cavity of the telencephalon is 

 continued into the cerebral hemispheres as the lateral ven- 

 tricles, known also as the first and second ventricles of the 

 brain. The reduced median cavity of the telencephalon re- 

 mains, forming the anterior portion of the third ventricle; the 

 openings of the lateral ventricles out of the third ventricle are 

 known as the foramina of Monro. The cerebral hemispheres 

 enlarge rapidly and soon grow out, either side of the mid-line, 

 far in front of, and above, the original extent of the telen- 

 cephalon, which remains limited to a narrow median strip dor- 

 sally and anteriorly (Fig. 124). The ventral and lateral walls 

 of the cerebral hemispheres soon become greatly thickened, as 

 the corpora striata, or basal ganglia, which finally become so 

 large that they nearly obstruct the lateral ventricles. Else- 

 where the walls of the hemispheres remain comparatively 

 thin. 



The dorsal region of the original telencephalon remains em- 

 bedded between the hemispheres. On its dorsal surface, 

 immediately in front of the velum transversum, it becomes 

 invaginated as the paraphysis. That part of its walls in front 

 of the recessus opticus, as far as the level of the foramina of 

 Monro, forms the lamina terminalis; this remains very thin 

 except in its middle where the anterior commissure develops 

 later. 



The diencephalon includes the second and third neuromeres. 

 Its antero-dorsal and antero-ventral limits are marked, re- 

 spectively, by the velum transversum and the recessus opticus. 

 Posteriorly it is marked off from the mesencephalon ventrally 

 by an elevation in the floor, the tuberculum posterius, and 

 dorsally by a broad depression of the wall (Fig. 123). 



Like the telencephalon, this also is vertically extended. Its 

 cavity forms, together with the median cavity of the telen- 

 cephalon, the third ventricle. Just behind the recessus opticus 

 the ventral and ventro-lateral walls are thickened as the optic 

 chiasma, and just back of this is a well-marked evagination, 

 the rudiment of the infundibulum. The lateral walls of the 

 diencephalon remain thin for a time, but later become greatly 



