938 OF GENERATION EMBRYONIC DEVELOPiMENT. 



they ramify in its substance, and send filaments into its villi ; and in pro- 

 portion as these villi form that connection with the uterine structure which 

 has been already described ( 752 755), do the vessels increase in size. 

 They then pass directly from the foetus 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 this time withdrawn ; and from henceforth the foetus is completely depend- 

 ent 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 Figs. 344, 345, n n', o o' . The Allantois is 

 commonly said to give origin to the Urinary Bladder ; but this organ is 

 really formed by an enlargement of the upper part of the uro-geuital sinus 

 ( 787), with which the allautois communicates by a duct which gradually 

 shrivels, only a vestige of it remaining permanent, to form the Urachus or 

 suspensory ligament of the bladder, by which this is connected with the 

 umbilicus. Before this takes place, however, the Allautois is the receptacle 

 for the secretion of the Corpora Wolffiana, and also for that of the true 

 Kidneys, when they are formed. 



780. It will be seen from the succeeding diagram, that the Amnion forms 

 a kind of tubular sheath around the umbilical cord; it is continuous at the 

 umbilicus with the integument of the foetus; and at the point where the cord 

 enters the placenta, it is reflected over its internal or foetal surface. It thus 

 forms a shut sac, resembling that of the pleura, arachnoid, etc.; and it con- 

 tains a fluid known as the liquor amnii, which consists of water holding in 

 solution a small quantity of casein, creatin, lactic acid, grape sugar, 1 and 

 saline matter, and resembles, therefore, very diluted serum. During the 

 first two months of gestation, the amnion and the inner lining of the choriou 

 (which is really the reflected layer of the amuion, Fig. 343, h, just as the 

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

 by a gelatinous-looking substance; which probably aids in the nutrition of 

 the embryo, previously to the formation of the placenta. This is absorbed 

 during the second mouth; and the amnion is then found immediately be- 

 neath 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 (Fig. 344, t), with its pedicle, or vitel- 

 line duct. 3. The Vasa Omphalo-Mesaraica (q, r), or mesenteric vessels of 

 the embryo, by which the yolk was absorbed into its body; these accompany 

 the pedicle. 4. The Urachus, and remains of the Allantois. 5. The Vasa 

 Umbilicalia (n n, o), which in the latter period of gestation, constitute the 

 chief part of the Cord. These last vessels consist in Man of two arteries 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 otherwise 

 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 

 Portse, and a part directly through the Ductus Veuosus into the Vena 

 Cava. 



781. A change in the type of the Circulating system of the foetus, from 

 that at first presented by it ( 777), takes place at a very early period. Be- 

 tween the fourth and eighth week the venous portion of the heart becomes 

 much enlarged, and a septum begins to be forned which gradually divides 

 the single ventricular cavity into two, the separation being completed at the 

 seventh week. The septum of the auricles commences in the eighth week, 



1 Hoppe-Seyler, Hundbuch der Chem. Anal., 1867, p. 502. 



