530 THE AMNION. 



2, 2, two aortic arches, uniting posteriorly 

 to form the aorta ; 3, the auricle ; 4, the 

 opening from the auricle into the ventricle, 

 6, which is laid open ; 5, the septum rising 

 from the lowest part of the cavity of the ven- 

 tricle ; 7, the vena cava inferior : B, view 

 from behind ; 1, the trachea ; 2, the lungs ; 



3, the ventricle ; 4, 5, the large atrium cor- 

 Fcetai heart. dis, or auricle ; 6, the diaphragm ; 7, the 



aorta descendens; 8, the pneumogastric ; 9, its branches ; 10, its continu- 

 ation. (Von Bar.) 



As soon as the capillary system is fairly established, the change in 

 the character of the function of nutrition alluded to is accomplished, and 

 in those animals which depend for their development on a food yolk, it 

 is eventually entirely covered with ramifications of these vessels. The 

 blood-cells of the first order or series are evolved from the nuclei of the 

 cells which coalesced for the formation of blood-vessels. 



The development of the embryo still continuing, it assumes a form 

 Elevation of which has been aptly described as resembling that of a boat 

 the embryo. pi ace( j U p S i(J e down, the bottom of the boat rising higher and 

 higher above the surface of the germinal membrane, and lifting with it 

 that portion of the membrane to which it is attached. The two ends of 

 the boat-shaped body bend under toward one another ; the larger of the 

 two is destined to become the head of the embryo. As this elevation 

 takes place, the embryo becomes separated by a constricted space from 

 the surrounding germinal membrane, its abdominal parietes being still 

 open and in contact with the yolk. From the layer which thus lines 

 the interior of the cavity of the embryo, the intestinal canal arises as a 

 tube from the coalescence of a pair of lateral ridges, and the surrounding 

 and exterior portions of the germinal membrane, elevating themselves 

 above the constricted space, coalesce over the back of the embryo, and 

 thus inclose it in a sac. This sac constitutes the amnion, 



Theammon. . . ' 



and in this manner, by folding, the interior of the germinal 

 membrane is used as a digestive surface, the outer as one for secretion. 

 The umbilical cord obtains a sheath from the amnion, which at one end 

 is continuous with the skin of the foetus, and the other is reflected over 

 the surface of the placenta. The amnion therefore constitutes a closed 

 sac, which contains a fluid, the liquor amnii. 



The place at which the germinal membrane is constricted, so as to be 

 able to act as a digestive surface to the embryo, though linear at first, is 

 gradually narrowed down, and constitutes the umbilicus. This .con- 

 stricted part is now the omphalo-mesenteric duct, which of course com- 

 municates with the cayity of the yolk-sac, which, at this stage of devel- 



