XII.] THE HIND BRAIN. 367 



The central nervous system. 



The development of the spinal cord in Mammals 

 differs in no important respects from that of the chick, 

 and we have nothing to add to the account we have 

 already given of its general development and histoge- 

 nesis in that animal. The development of the brain 

 however will be described at greater length, and some 

 additional facts relative to the development of the 

 Avian brain will be mentioned. 



The first differentiation of the brain takes place in 

 Mammalia before the closure of the medullary folds, 

 and results as in the chick in the formation of the three 

 cerebral vesicles, the fore-, mid- and hind-brain (Fig. 

 106, B). A cranial flexure precisely resembling that of 

 the chick soon makes its appearance. 



The hind brain early becomes divided into two 

 regions, the rudimentary medulla oblongata and cere- 

 bellum. 



The posterior section, the medulla, undergoes changes 

 of a somewhat complicated character. In the first place 

 its roof becomes 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, consisting of a single row of 

 flattened cells (Fig. 40). 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. 



