DEVELOPMENT Of THE CHICK THIRD PEKIOD. 



329 



hepatic vein. The vessels of the ailantois become the umbi- 

 lical vessels. 



The alterations that trans- 

 pire in the mucous layer are 

 of less moment : the or- 

 gans already formed in crease 

 in size ; the faucial cavity is 



m 



elongated as the oral cavity 

 in the bill-shaped maxillae ; 

 the esophagus extends; the 

 division into crop and mus- 

 cular stomach is distin- 

 guishable ; behind the loop 

 for the duodenum, andwhich 



1 ,1 ,-1 A'i& Uij. rxn ciiiijivu ounicv> licit 



encloses the pancreas, the ()lder e than that represented in fig. 349, 

 jejunum forms a noose of surrounded by the amnion as an am- 

 ple vesicle ; a, the amnion ; the eyes, 

 b, b, are very large ; c, c, the corpora 

 quadrigemina, now scarcely larger than 

 the hemispheres d. d ; the space be- 

 tween them is the third ventricle. 



Fig. 353. An embryo somewhat 



the same length and tenui- 

 ty, which lies completely 

 out of the abdomen within 

 the umbilical cord, where, 

 bv means of a delicate short 



tt 



conduit, it communicates with the vitellicle or yolk-sac, the 

 ductus mtello-intestinalis (fig. 352, A, ). The liver is large 

 and gorged with blood ; the trachea and lungs are entirely 

 separated from the esophagus ; the larynx makes its appear- 

 ance as a small enlargement upon the trachea. 



[ 495. The principal changes from the ninth to the eleventh 

 day are as follow : the hemispheres of the brain enlarge 

 greatly, at the cost, apparently, of the corpora quadrigemina, 

 and span the third ventricle posteriorly ; the cerebellum in- 

 creases, particularly in its middle or vermiform portion, by which 

 the fourth ventricle isnow completely hidden; in the spinal cord 

 the enlargements corresponding to the two pairs of extremi- 

 ties, become more conspicuous ; the fibrous structure of the 

 brain and spinal cord is apparent ; the eyes proceed in their 

 development, and attain still more colossal relative dimensions ; 

 the eyelids appear as a circular-shaped fold of the skin ; the 

 external organ of hearing increases in width and depth. The 

 bulbs of the feathers become apparent in certain districts, first 

 along the middle line of the back, upon the haunches, and 

 over the rump ; the joints of the extremities are more solidly 



