254 THE (MUCK. 



(Fig. 121, bh) the dorsal wall or roof becomes very much 

 thinner than the sides and floor. In the later stages this 

 difference becomes increasingly marked ; and before the end of 

 the third day (Figs. 113 and 114) the roof, which is now very 

 wide, becomes reduced to a single layer of epithelial cells, entirely 

 devoid of nervous matter ; a condition in which it remains 

 throughout life. This thin roof soon becomes thrown into folds, 

 which appear about the seventh clay, and rapidly increase in depth, 

 hanging down into the cavity of the medulla. Between the 

 layers of these folds a network of vessels, which early appears on 

 the outer surface of the roof, grows in to form the choroid plexus 

 of the fourth ventricle (Fig. 11G, xb). 



The division of the hind-brain into a series of vesicles, which 

 is very noticeable about the thirty-sixth hour (Fig. Ill), becomes 

 less evident as the side walls thicken, through the formation of 

 the white nervous matter ; and from the middle of the third day 

 onwards it is barely perceptible. 



The cerebellum is developed from the roof of the anterior 

 vesicle of the hind-brain, immediately behind the well-marked 

 constriction which separates the hind-brain from the mid-brain. 



It appears towards the end of the second day, as a slightly 

 marked transverse thickening of the roof of the hind-brain ; it 

 becomes more conspicuous during the third, fourth, and follow- 

 ing days (Figs. 113, 114, and 115, bl), but remains as a simple 

 transverse band until a comparatively late stage of develop- 

 ment. 



About the eighth day, the cerebellum (Fig. 116, bl) becomes 

 doubled transversely on itself; and at the same time it thickens 

 considerably, its outer surface becoming slightly folded. From 

 this time it steadily increases in thickness, and by further 

 folding of its surface becomes more complicated in structure ; 

 but up to about the sixteenth day it lies completely behind the 

 optic lobes. 



During the last few days of incubation the cerebellum 

 enlarges considerably, growing forwards over the top of the 

 mid-brain and between the optic lobes : by the time of hatching 

 it has almost met the cerebral hemispheres, and has acquired 

 the shape and proportions characteristic of the cerebellum in 

 the adult bird. 



