DEVELOPMENT QF THE CHICK THIRD PEKIOD. 



327 



out (fig. 352, B). The amnion is more and more distended, 

 and at the umbilicus is brought more together, so that it 

 becomes 

 drawn out 

 into an um- 

 bilical cord, 

 in which lie 

 the pedun- 

 cle of the 

 allantois 

 and a noose 

 of the intes- 

 tine (fig. 

 V, 6) ; 



the neck ad- 

 vances in 

 its evolu- 

 tion, and 

 the lower 

 jaw-bones 

 are elonga- 

 ted and assume the fashion of a beak. The heart acquires 

 the form it possesses in after-life, the several parts having 

 approximated and become more closely conjoined : the 

 auricles are divided, and cover the ventricles, which can 

 now even from without be perceived to be double ; the 

 aortal bulb at the same time appears produced from both 

 ventricles in an arched form, arising directly over the septum, 

 and being divided into two canals, the separation between 

 which becomes visible outwardly on the seventh day ; the 

 pericardium is formed. From the aorta there now arise 

 but two vascular arches on either side, and to the right a 

 middle third arch ; this and the two anterior arches are the 

 later chief divisions of the aorta, and are filled by the stream 

 of blood transmitted from the left ventricle ; the two poste- 

 rior arches are supplied on the seventh day with blood exclu- 

 sively from the right ventricle of the heart, and are the future 

 pulmonary arteries ; the arches all terminate in the descend- 

 ing aorta. The Wolffian bodies, and the formations that take 

 place upon or in connexion with them, have many remark- 

 able relations during this period. The shut sacs of which 



Fig. 351. An embryo similar to the last, but somewhat 

 further advanced. The references are the same as in fig. 350. 



