DEVELOPMENT OF THE EMBRYO. 719 



embryo to the Chorion; and its extent bears a pretty close correspondence 

 with the extent of surface, through which the Chorion comes into vascular 

 connection with the Decidua. Thus, in the Carnivora, whose Placenta ex- 

 tends like a band around the whole Ovum, the Allantois also lines the whole 

 inner surface of the Chorion, except where the Umbilical Vesicle comes in 

 contact with it. On the other hand, in Man and the Quadrumana, whose 

 Placenta is restricted to one spot, the Allantois is small, and conveys the 

 foetal vessels to one portion only of the Chorion. When these vessels have 

 reached the Chorion, they ramify in its substance, and send filaments into its 

 villi; and in proportion as these villi form that connection with the uterine 

 structure, which has been already described, do the vessels increase in size. 

 They then pass directly from the Fo3tus to the Chorion; and the Allantois, 

 being no longer of any use, shrivels up, and remains as a minute vesicle, only 

 to be detected by careful examination. The same thing happens in regard to 

 the Umbilical vesicle, from which the entire contents have been by this time 

 exhausted; and from henceforth the Fcetus is completely dependent for the 

 materials of its growth, upon the supply it receives through the Placenta, 

 which is conducted to it by the vessels of the Umbilical Cord. This state of 

 things is represented in the preceding diagram. The Allantois has a corre- 

 spondence in situation with the Urinary Bladder ; but it is only the lower 

 part of it, pinched off, as it were, from the rest, that remains as such. The 

 duct by which it is connected with the abdomen gradually shrivels; and a 

 vestige of this is permanent, forming the Urachus or suspensory ligament of 

 the Bladder, by which it is connected with the Umbilicus. Before this takes 

 place, however, the Allantois is the receptacle for the secretion of the Corpora 

 Wolffiana, and of the true Kidneys, when they are formed. 



942. It will be seen from the preceding diagram, that the Umbilical Cord 

 receives an investment from the Amnion, which forms a kind of tubular 

 sheath around it ; it is continuous at the Umbilicus with the integument of 

 the fetus; and at the point where the cord enters the Placenta, it is reflected 

 over its internal or foetal surface. The Amnion (which thus forms a shut 

 sac, like that of the Pleura, Arachnoid, &c.) contains a fluid known as the 

 liquor amnii; this consists of water holding in solution a small quantity of 

 albumen and saline matter, and resembling, therefore, very diluted serum. 

 During the first two months of gestation, the Amnion and the inner surface 

 of the Chorion (which is really the reflected layer of the Amnion, just as the 

 lining of the abdominal cavity is formed by the peritoneum) are separated by a 

 gelatinous-looking substance ; which may perhaps be considered as represent- 

 ing the white of the egg in Birds; and which probably aids in the nutrition 

 of the Embryo, previously to the formation of the Placenta ( 918). This 

 is absorbed during the second month; and the Amnion is then found imme- 

 diately beneath the Chorion. In the Umbilical Cord, when it is completely 

 formed, the following parts may be traced. 1. The tubular sheath afforded 

 by the Amnion. 2. The Umbilical Vesicle, with its pedicle, or Omphalo- 

 Enteric duct. 2. The Vasa Omphalo-Meseraica, or mesenteric vessels of 

 the Embryo, by which the Yolk was absorbed into the body of the Fcetus; 

 these accompany the pedicle. 4. The Urachus, and remains of the Allantois. 

 5. The Vasa Umbilicalia, which, in the later period of gestation, constitute 

 the chief part of the Cord. These last vessels consist in Man of two Arte- 

 ries and one Vein. The Arteries are the main branches of the Hypogastric; 

 and they convey to the Placenta the blood which has to be aerated and other- 

 wise revivified, by being brought into relation with that of the Mother. The 

 Vein returns this to the Foetus, and discharges a part of it into the Vena 

 Porta3, and a part directly through the Ductus Venosus into the Aorta. 



