The cavity of the abdomen is therefore at first open to the vesicula umbilicalis or yelk, but this changes as the abdominal parietes begin to close in; in man and the mammalia merely a part of it, as above mentioned, forms the intestinal canal, whereas, in oviparous animals the whole of the yelk-bag enters the abdominal cavity, and serves for an early nutriment to the young animal. Another change connected with the serous or outer layer of the germinal membrane is the formation of the _amnion_. The foetal rudiment which from its shape has been called _carina_, now begins to be enveloped by a membrane of exceeding tenuity, forming a double covering upon it; the one which immediately invests the foetus is considered to form the future epidermis; the other, or outer fold, forms a loose sac around it, containing the liquor amnii. Whilst these changes are taking place in the serous layer of the germinal membrane, and whilst the intestinal canal, &c. are forming on the anterior surface of the embryo, which is turned towards the ovum, by means of the inner or mucous layer, equally important changes are now observed in the middle or vascular layer. "In forming this fold," says Dr. A. Thomson, "the mucous layer is reflected farthest inwards; the serous layer advances least, and the space between them, occupied by the vascular layer, is filled up by a dilated part of this layer, the rudiment of the heart." (_Op. cit._ p. 301.) Whilst this rudimentary trace of the vascular system is making its appearance, minute vessels are seen ramifying over the vesicula umbilicalis, forming, according to Baer's observations, a reticular anastomosis, which unites into two vessels the vasa omphalo-meseraica. (_British and Foreign Med. Rev._ No. 1.) These may be demonstrated with great ease in the chick: the cicatricula increases in extent; it becomes vascular, and at length forms a heart-shaped net-work of delicate vessels, which unite into two trunks, terminating one on each side of the abdomen. [Illustration: _b_ Is a portion of the convexity of the amnion, upon which, at _a_ is the fundus of the diminutive human allantois. _c_ The duct of the vesicula umbilicalis, dividing into two intestinal portions; and besides this duct are two vessels which are distributed upon the vesicula umbilicalis, and form a reticular anastomosis with each other. _From Baer._]