ISO THE HIND-Br.AIN. 



(PL XV. fig. Sh). Its cavity exhibits at tlie same time the indi- 

 cation of a division into a central and two lateral parts. 



TJie hind-brain. The hind-brain has at first a ftiirly uniform 

 structure, but by the close of stage I, the anterior part becomes 

 distinguished from the remainder by the fact, that its roof does 

 not become thin as does that of the posterior part. This anterior, 

 and at first very insignificant poi^tion, forms the rudiment of 

 the cerebellum. Its cavity is quite simple and is continued 

 uninterruptedly into that of the remainder of the hind-brain. 

 The cerebellum assumes in the course of development a 

 greater and greater prominence, and eventually at the close 

 of stage Q overlaps both the optic lobes in front and the me- 

 dulla behind (PI. XV. fig. 7a). It exhibits in surface-views of 

 the liardened brain of stages P and Q the appearance of a 

 median constriction, and the portion of the ventricle contained 

 in it is prolonged into t\fO lateral outgrowths (PI. XV. fig. 8c 

 and 8d, ch). 



The posterior section of the hind-brain which forms the me- 

 dulla undergoes changes of a somewhat complicated character. 

 In the first place its roof becomes in front very much extended 

 and thinned out. At the raphe, where the two lateral halves 

 of the brain originally united, a separation, as it were, takes 

 place, and the two sides of the brain become pushed apart, re- 

 maining united by only a very thin layer of nervous matter 

 (PL XIV. fig. 6, iv. v.). As a result of this peculiar growth in 

 the brain, the roots of the nerves of the tv\ro sides which were 

 originally in contact at the dorsal summit of the brain become 

 carried away from one another, and appear to arise at the sides 

 of the brain (PL xiv. fig. 6 and 7). Other changes also take 

 place in the wails of the brain. Each lateral wall presents two 

 projections towards the interior (PL xiv. fig. 5a). The ventral 

 of these vanish, and the dorsal approximate so as nearly to 

 divide the cavity of the hind-brain, or fourth ventricle, into a 

 large dorsal and a small ventral channel (PL xiv. fig. 6), and 

 this latter becomes con:ipletely obliterated in the later stages. 

 The dorsal pair, while approximating, also become more promi- 

 nent, aud stretch into the dorsal moiety of the fourth ventricle 

 (PL XIV. fig. G). They are still very prominent at stage Q (PL 

 fig. XV. Sd,ft}, and correspond in position with the fasciculi 



