THE HIND-BRAIN. 403 



the formation in its roof of a shallow median furrow (PI. 16, fig. 

 S/?). Its cavity exhibits at the same time the indication of a 

 division into a central and two lateral parts. 



The hind-brain. The hind-brain has at first a fairly 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 portion, 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 O 

 overlaps both the optic lobes in front and the medulla behind 

 (PI. 1 6, fig. 7 a}. It exhibits in surface-views of the hardened 

 brain of stages P and O the appearance of a median con- 

 striction, and the portion of the ventricle contained in it is 

 prolonged into two lateral outgrowths (PI. 16, figs. Sc and 



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, 

 remaining united by only a very thin layer of nervous matter 

 (PI. 15, fig. 6, iv. v.). As a result of this peculiar growth in 

 the brain, the roots of the nerves of the two sides which were 

 originally in contact at the dorsal summit of the brain become 

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

 of the brain (PI. 15, figs. 6 and 7). Other changes also take 

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

 projections towards the interior (PI. 15, fig. 5<?). 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 (PI. 15, fig. 6), and 

 this latter becomes completely obliterated in the later stages. 

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

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

 (PI. 15, fig- 6). They are still very prominent at stage Q (PI. 16, 



