When the whole of the vascular membrane (b, fig. 141) was spread out, its figure ap peared to have been that of a cone, of which the apex was the umbilical chord, and the base the terminal vessel above-mentioned. Three vessels could be distinguished diverging from the umbilical chord and ramifying over it. Two of these trunks contained coagulated blood, and were the immediate continuations of the terminal or marginal vessel: the third was smaller, empty, and evidently the arterial trunk. Besides the extremely numerous rami fications dispersed over this membrane, it dif fered from the chorion in being of a yellowish tint. The amnios (c, 141) was reflected from the umbilical chord, and formed, as usual, the immediate investment of the foetus.
The umbilical chord measured two lines in length and one in diameter. It was found to contain the three vessels above-mentioned, with a small loop of intestine; and from the ex tremity of the latter a filamentary process was continued to the vascular membrane. The margins of the umbilicus or abdominal open ing were very strong, offering much resistance to their division. On tracing the contents of the chord into the abdomen, the two larger vessels with coagulated blood were found to unite ; the common trunk then passed back wards beneath the duodenum, and after being joined by the mesenteric vein, went to the under surface of the liver, where it penetrated that viscus: this was consequently an omphalo mesenteric or vitelline vein. The artery was a branch of the mesenteric. The membrane, therefore, upon which they ramified, answered to the vitellicle, i. e. the vascular and mu cous layers of the germinal membrane, which spreads over the yolk in oviparous animals, and which constitutes the umbilical vesicle of the embryo of ordinary Mammalia. The fila mentary pedicle which connected this mem brane to the intestine was given off near the end of the ileum, and not continued from the ccecum, the rudiment of which was very evi dent half a line below the origin of the pedicle. (See the foetus in fig. 141.) The small intestine above the pedicle was disposed in five folds. The first from the stomach or duodenum curved over the vitel line vein, and the remaining folds were dis posed around both the vitelline vessels. From the ccecum, which was given off from the re turning portion of the umbilical loop of the intestine, the large intestine passed backwards to the spine, and was then bent, at a right angle, to go straight down to the anus. The stomach did not present any appearance of the sacculated structure so remarkable in the adult, but had the simple form of a carnivorous stomach. The liver consisted of two equal and symme trically disposed lobes. The vena port was
formed by the union of the vitelline with the mesenteric, and doubtless the other usual veins, which were, however, too small to be distinctly perceived. The diaphragm was perfectly formed.
The vena cava inferior was joined, above the diaphragm, by the left superior cava, just at its termination in a large right auricle. The ventricles of the heart were completely joined together, and bore the same proportions to each other as in the adult,—a perfection of structure which is not observed in the embryos of ordi nary Mammalia at a corresponding period of development. The pulmonary artery and aorta were of nearly the same proportionate size as in the adult: the divisions of the pul monary artery to the lungs were at least double the size of those observable in the embryo of a sheep three inches in length. The ductus arteriosus, on the contrary, was remarkably small. The aorta, prior to forming the de scending trunk, dilated into a bulb, from which the carotid and subclavian arteries were given off.
The lungs were of equal size with the heart, being about a line in length, and nearly the same in breadth : they were of a spongy tex ture and of a red colour, like the veins, from the quantity of blood they contained. This precocious development of the thoracic viscera is an evident provision for the early or prema ture exercise of the lungs as respiratory organs in this animal : and on account of the simple condition of the alimentary canal, the chest at this period exceeds the abdomen in size.
The kidneys had the same form and situ ation as in the adult. The supra-renal glands were half the size of the kidneys.
The testes were situated below the kidneys, and were one-half larger than those glands, the superiority of size depending on their large epididymis, with the adherent remains of the Wolffian body. They continue within the ab domen for six weeks after uterine birth.
At a later period of uterine development, when the fmtus, measured in a straight line L from the mouth to the root of the tail, is ten lines in length, the urachus expands into a small allantois (d, fig. 141), of a flattened pyriform figure,and finely wrinkled external surface. This bag insinuates itself between the amnios and chorion, carrying along with it two small hypo gastric arteries and an umbilical vein, but not establishing by their means an organized and vascular surface of the chorion by which a placental attachment is formed between the ovum and the womb. The allantois depends freely from the end of the umbilical chord, and has no connexion at any part of its cir cumference with the adjoining membrane. Its office is apparently that of a receptacle of urine.