AVES BIRDS. 



heart lias become developed : so that considerable advance is 

 already made towards that disposition of the fetus and its membra- 

 nous investments represented in the next figure, to which we now 

 beg the reader's attention. 



(701.) The serous membrane (Jig. 287, c) has at the third day 

 become reflected to a considerable distance over the back of . the 

 fetus ; at one extremity investing the head with a serous covering, 

 while at the opposite it in like manner covers the tail : it is this 

 reflection of the serous layer which forms the amnion^ as will be 

 observed infig. 288, where the amniotic sac, C, is completed. 



The mucous layer, A, is now seen to line the as yet open space 

 which is to form 



the abdominal ca- F ^ 287 ' 



vity ; and by its 

 inflections gives 

 birth to the rudi- 

 ments of the ab- 

 dominal viscera. 



From the vascu- 

 lar layer, B, has 

 been developed the heart, now composed of two chambers (a, Z>), 

 and the branchial arteries (c), which join to form the aorta (w), 

 exactly as in the Menopoma (fig. 257). The allantois (p), the 

 uses of which will be described hereafter, likewise begins to make 

 its appearance.* 



(702.) At the fifth day (fig. 288) the lineaments of the viscera 

 become tolerably distinct. The sac of the amnios, c, is completed; 

 the liver ', and the lungs e, begin to show themselves ; and the 

 bag of the allantois (/?) is largely developed : still, however, the 

 heart (a, b) is that of a fish, and the aorta (ra) formed by the 

 union of the branchial arches (c) ; so we have yet to trace how, as 

 the lungs increase in size, the circulatory apparatus becomes 

 changed, and the branchial organs obliterated. 



On the third day of incubation there exist four vascular arches 

 (fig. 287, c) on each side, having a common origin from the bulb 

 (6), which obviously represents the bulbus arteriosus of Fishes and 

 Reptiles (vide figs. 259, 261) ; these encircle the neck, and join 

 on arriving in the dorsal region to form the aorta, which com- 

 mences by two roots, each made up of the union of the four bran- 



* Des Branchies et des Vaisseaux branchiaux dans les Embryons des animaux ver- 

 tebres, par Prof. Ch. Ernst v. Baer. Annales des Sciences Nat. torn. xv. 



