DEVELOPMENT OF THE CHICK SECOND PEKIOD. 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 laminae, 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 d, 341 and 345 

 a 2 ), behind which the 

 fourth ventricle presents 

 itself as a deep depres- 

 sion (figs. 341 and 345, 

 ?). The corpora quad- 

 rigemina form a simple 

 and very considerable cell, 

 which projects forwards 

 in an arched or vaulted 

 manner, but, with the in- Fig. 341. Embryo of the fowl, nearly 

 creasing declension of ^ ve lines m length, at the seventy-se- 

 the head turns alwavs con d hour of incubation (transition from 



more and more down- f e . thi f d to f the . fourth tl *?): The ab- 



, an A* nominal surface 1S partly laid open, and 



as (ngs. 66$ and 347 the parts separated; the amnion is re- 



d, 340 e, 341 and 345 moved, a, corpora quadrigemina ; b, the 



a, 343 B, a, 342 b, 344 hemispheres; c, the nasal depression; 



e). The laminae, which *' the fourth ventricle > in front of which 



f , *i,^ v, n " es ^e cerebellum, a-, which is now 



form the cerebellum, pro- more distinctly defined ; e, the ear;/, 



ceed upwards, blending in the eye, in the choroid of which, already 



the corpora quadrigemi- furnished with its pigment, a cleft is 



na, under which the seen ff l ff 4 > the four branchial clefts ; 



fourth ventricle is con- ^' ^ e ^ eart 5 *> the h'ver ; k, the intesti- 

 nal canal, with its open vitellary duct I ; 



tinned as the aqueductus. ^ the rectum stm ei f cling in a b ^ lind sac ; 



Anteriorly to the corpora n , the allantois ; o, the anterior, and p, 

 quadrigemina lies the the posterior, extremity ; g, q, g, g, 

 asymmetrical, smaller, ) y olffian bodies ; r, upper jaw ; s, under 

 middle cerebral cell (figs. ^ aw ' 

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



