488 DEVELOPMENT OF THE CHICK. 



At the end of the third day (fig. 94). The pharynx and 

 the four pharyngeal pouches (hyomandibular slit, first, 

 second and third branchial clefts) have appeared, and the 

 first three aortic arches are established. The regions of the 

 brain are demarcated ; the eye and ear respectively are now 

 clearly indicated. The number of somites has greatly 

 increased. The chief embryonic venous spaces (cardinal 

 veins) can be easily seen and traced to their opening into 

 the sinus venosus. The circulation of the blood should 

 be carefully followed and drawn. 



At the end of the fourth or fifth day. The allantois 

 grows out of the hind-gut. The wing and leg bud out from 

 the sides and become indistinctly segmented by joints. No 

 traces of digits are as yet present. The head is now relatively 

 enormous, and the cranial flexure, or bending of the fore- 

 brain upon the hind-brain, passes through an angle of at 

 least 120. The amnion is now fully developed and encloses 

 the entire embryo in a sort of water-bath. 



The rapid development of the chick may be best ascertained 

 by sketching an embryo of, say, about 36 hours in the morning 

 and making another sketch in the late afternoon. If kept in 

 warm salt-solution during this interval, the embryo will be found 

 to have developed considerably. It must, of course, remain 

 attached to the yolk. 



