DEVELOPMENT OF TIIE CHICK THIHD PERIOD. 



329 



Fig. 353. An embryo somewhat 

 older than that represented in fig. 349, 

 surrounded by the amnion as an am- 

 ple vesicle ; , the amnion ; the eyes, 

 b, 6, 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. 



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

 lical vessel*. 



The alterations thattrans- 

 pire in the mucous layer are. 

 38 moment: the or- 

 ready formed increase 

 in size ; the faucial cavity is 

 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 

 encloses the pancreas, the 

 jejunum forms a noose of 

 the same length and tenui- 

 < it'll lies completely 

 out of the abdomen within 

 the umbilical cord, where, 

 by means of a delicate short 

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

 ducfi'-"! vitello-intestinali* (fig. 352, A, 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. 



[ 49.5. 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 isnowcompletely hidden; in the spinal cord 

 the enlargements corresponding to the two pairs of extremi- 

 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 

 rnal 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 



