THE BRAIN I49 



After the outgrowth of the telencephalon the remainder of the fore- 

 brain forms the diencephalon ('twixt-brain, or thalamenceph- 

 alon), this including the posterior part of the dorsal and the whole of 

 the ventral zone of this region. The part below the limiting sulcus 

 becomes the hypothalamus, while the dorsal zone forms two regions 

 a more dorsal epithalamus and a larger and more ventral thalamus. 

 The floor and roof plates of the 'twixt-brain behind the velum trans- 

 versum form median structures; a ventral outgrowth, th e infundi bu- 

 lum, and dorsally a parietal organ and an epiphysis (pinealis) wMcE^' 

 will be described below (p. 155-7). 



In the mid-brain or mesgncfiphaloa, there are but slight changes. 

 The upper part of the side walls of the dorsal zone become thickened 

 into a pair of prominences — the optic lobes or corpora bigemina 



Fig. 156. — Half of model of brain of embryo pig, 15 mm. long. (Compare with fig. 

 ipSi 3-) c, cerebrum; cb, cerebellum; cs, corpus striatum; i, infundibulum; is, isthmus; 

 fi, interventricular foramen; m, mesencephalon; mo, medulla oblongata; t, thalamus. 



(in the mammals there are two pairs of such lobes — corpora quad- 

 rigemina) which protrude on the dorsal surface. The roof plate 

 remains comparatively thin, but the floor plate becomes thicker 

 and forms the cerebral pedimcle (crura cerebri). 



The isthmus is a constriction of the dorsal zone which separates 

 sharply the mesencephalon from the hind-brain. In the latter region 

 there are considerable modifications of the dorsal zone, the anterior 

 part of which develops dorsally, forming an elevation of the surface, 

 the cerebellimi or metencephalon, the extent of which varies greatly 

 in the dlflerent classes of vertebrates. The rest of the dorsal zone 

 and the whole of the ventral zone of the hind-brain form themyelen- 

 cephalon or medulla oblongata. The roof plate in the cerebellar 

 region becomes thickened by the invasion of nervous material from 

 either side, while behind the metencephalon the plate becomes greatly 



