DEVELOPMENT OF THE CHICK SECOND PEEIOD. 311 



another, and thus is the basis laid of the fourth ventricle, 

 which appears to be covered with its own peculiar medul- 

 lary and enveloping lamina. Anteriorly, the fasciculi of 

 the medulla oblongata ascend towards the corpora quadrige- 

 mina in two perpendicular laminse, which, on the fifth day, 

 become applied to one 

 another, and so cover the 

 fourth ventricle superiorly 

 and anteriorly ; thus is 

 the cerebellum, produced, 

 visible from the side as 

 an enlargement (figs. 339 

 e, 340 (I, 341 and 345 

 a 2 ), behind which the 

 fourth ventricle presents 

 itself as a deep depres- 

 sion (figs. 341 and 345, 

 <"/). The corpora quad- 

 rig emina form a simple 

 and very considerable cell, 

 which projects forwards 

 in an arched or vaulted 



Fig. 341. Embryo of the fowl, nearly 

 five lines in length, at the seventy-se- 

 cond hour of incubation (transition from 

 the third to the fourth day). The ab- 

 dominal surface is partly laid open, and 

 the parts separated; the amnion is re- 

 moved, a, corpora quadrigemina ; 6, the 

 hemispheres ; c, the nasal depression ; 

 d, the fourth ventricle, in front of which 

 lies the cerebellum, a 2 , which is now 

 more distinctly defined ; e, the ear ; /, 

 the eye, in the choroid of which, already 

 furnished with its pigment, a cleft is 

 seen ; y l g*, the four branchial clefts ; 

 h, the heart ; i, the liver ; A, the intesti- 

 nal canal, with its open vitellary duct I; 

 m, the rectum still ending in a blind sac ; 

 n, the allantois ; o, the anterior, and p, 

 the posterior, extremity ; g, q, <?, g, 

 Wolffian bodies ; ;, upper jaw ; s, under 

 jaw. 



manner, but, with the in- 

 creasing declension of 

 the head, turns always 

 more and more down- 

 wards (figs. 339 and 347 

 d, 340 e, 341 and 345 

 a, 343 B, a, 342 b, 344 

 c). The laminse, which 

 form the cerebellum, pro- 

 ceed up wards, blending in 

 the corpora quadrigemi- 

 na, under which the 

 fourth ventricle is con- 

 tinued as the aqueductus. 

 Anteriorly to the corpora 

 quadrigemina lies the 

 asymmetrical, smaller, 



middle cerebral cell (figs. 



339 and 345 c, 340 and 341 /, 343 B, before r), formed by 



