AYES. 267 



that the yolk-sac becomes smaller, and at last passes into the 

 body. The continuation of the somatopleure outside the boundary 

 of the body rises into folds, which grow up, and, finally meeting 

 above the body, fuse together. Their inner layers form the 

 amnion, a membrane enveloping the embryo, while their outer 

 layers (and the somatopleure prolongation in the yolk-sac region) 

 unite with the vitelline membrane. As the somato- and splanch- 

 nopleures are prolonged outside the region of the embryo, the 

 space there present between them must be a continuation of the 

 body-cavity. A flattened sac, the allantois, grows from the 

 posterior part of the alimentary canal into this space, and ex- 

 tends over the embryo. 



The yolk-sac possesses at one time a system of capillaries, and 

 functions as a respiratory organ. As the allantois develops it 

 also becomes very vascular, first assists the yolk-sac in respira- 

 tion, and then carries it on entirely. It also grows round, and 

 absorbs the albumen (' white '). 



The urinary bladder of the frog is a rudimentary allantois. 



Course of the Embryonic Circulation. The dorsal aorta sends 

 vitelline and allantow arteries from its posterior part to the yolk- 

 sac and allantois respectively. These structures return purified 

 blood to the body by vitelline and allanloic * veins. These unite 

 to form, with a vein from the gut, a trunk, the ductus venosus, 

 which traverses the liver (giving off twigs in its course), and 

 enters the now-developed postcaval. This communicates with 

 the right auricle, and its (mostly) purified blood is directed by 

 the Eustachian valve through the foramen ovalefi an aperture 

 existing at this time in the auricular septum, into the left auricle. 

 Thence it passes into the left ventricle, and so to the dorsal aorta. 

 The right auricle also receives impure blood by the Cuvierian 

 ducts. This passes into the right ventricle, whence, by the pul- 

 monary artery, it reaches the lungs and also, to some extent, the 

 dorsal aorta, by a cross branch, the ductus arteriosus. The lungs, 

 at this time functionless, return impure blood to the left auricle. 

 It will be seen that, in the embryo, the comparatively pure blood 

 of the left ventricle is derived from the right side of the heart. 

 As soon as lung-respiration commences, the foramen ovale closes, 



* = Anterior abdominal vein of Frog. 



t Its position is marked by i\\e fossa ovalis of the adult. 



