428 AN INTRODUCTION TO ZOOLOGY 



the prosencephalon, which is therefore designated the third ventricle, 

 by wide apertures, the foramina of Munro. At first the wall of the 

 ventricle thickens slowly and fairly evenly, but soon the latero- 

 ventral region increases enormously to form the basal ganglia or 

 corpora striata, which almost obliterate the lateral ventricles. 



The next two neuromeres merge to give rise to the thalamence- 

 phalon, whose dorsal limits are the velum transversum in front and a 

 broad depression behind. On the ventral side the front end is 

 marked by the optic recess and posteriorly by an elevation in the 

 floor of the brain, the tuberculum posterius. Its cavity contributes 

 to the formation of the third ventricle. Just behind the optic recess 

 its floor thickens to form the optic chiasma, and then behind this 

 again sends down a median diverticulum, the infundibulum. During 

 later development its walls thicken greatly to form the optic thalami, 

 whose enlargement reduces the ventricle to a narrow- vertical cleft. 

 The roof remains thin, and in the region of the velum transversum, 

 together with the adjacent roof of the telencephalon, it becomes 

 modified to form the choroid plexus of the third and later also of the 

 lateral ventricles. During the third day a small tubular median 

 outgrowth arises from the hinder end of the thalamencephalon in 

 the mid-dorsal line. This is the epiphysis cerebri, which in the chick, 

 as in the .rabbit, only develops into a glandular structure. The 

 hinder limit of this part of the brain is definitely laid down somewhat 

 later by the appearance of a transverse thickening, the posterior 

 commissure. 



The mid-brain or mesencephalon comprises two neuromeres, i.e. the 

 fourth and fifth, and it comes to occupy the most anterior position in 

 the embryo topographically, indeed, it is largely owing to the great 

 growth of its roof that the cranial flexure is brought about. Its 

 anterior limits have already been noted, and posteriorly it is marked 

 off from the hind-brain dorsally by a constriction, the isthmus, 

 while ventrally its limits are ill-defined. At the end of the third day 

 little specialisation has taken place in it, and it is not until later 

 that outgrowths from its thickened dorsal region form the optic lobes, 

 and its ventro-lateral walls thicken to form the crura cerebri. These 

 various thickenings of the wall lead to the reduction of the originally 

 large cavity to a narrow lumen, the iter. 



The whole of the cavities of the neuromeres constituting the hind- 

 brain remain in wide open communication, and form one indivisible 

 fourth ventricle. Its walls, on the other hand, differentiate into 

 the metencephalon and the myelencephalon. The metencephalon 

 includes only one neuromere, and its dorsal limit can be distinguished 

 on the third day by the fact that it forms the hinder almost vertical 

 wall of the isthmus, and is much thicker than the corresponding 



