DEVELOPMENT OF THE EMBRYO. 617 



Cord. This state of things is represented in the preceding diagram. The 

 Allantois has a correspondence 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 suspen- 

 sory ligament of the Bladder, by which it is connected with the Umbilicus. 

 Before this takes place, however, the Allantois is the receptacle for the secre- 

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

 765. 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 foetus ; 

 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 short 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 dilute 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 ^latinous- 

 looking substance; which may perhaps be considered as representing the 

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

 Embryo, previously to the formation of the Placenta ( 747). This is absorbed 

 during the second month ; and the Amnion is then found immediately beneath 

 the Chorion. In the Umbilical Cord, when it is completely formed, the fol- 

 lowing parts may be traced. 1. The tubular sheath afforded by the Amnion. 

 2. The Umbilical Vesicle, with its pedicle, or Omphalo-Enteric duct. 3. The 

 Vasa Omphalo-Meseraica, or mesenteric vessels of the Embryo, by which the 

 Yolk was absorbed into the body of the Foetus ; 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 Arteries and one Vein. The Arte- 

 ries are the main branches of the Hypogastric ; and they convey to the Pla- 

 centa 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 Portae, and a part directly 

 through the Ductus Venosus into the Aorta. 



766. A change in the type of the Circulating system of the Foetus, from 

 that at first presented by it ( 763), takes place at a very early period. At 

 about the 4th week, in the Human Embryo, a septum begins to be formed in 

 the Ventricle ; and by the end of the 8th week it is complete. The Septum 

 Auriculorum is formed at a somewhat later period, and it remains incomplete 

 during the whole of foetal life ; it is partly closed by the valvular fold covering 

 the Foramen Ovale, which fold is developed during the third month. During 

 the same period, a transformation takes place in the arrangement of the large 

 vessels proceeding from the Heart; which ends in their assumption of the 

 form they present until the end of Foetal life ; and this undergoes but a slight 

 alteration, when the plan of the circulation is changed at the moment of the 

 first inspiration. The number of Aortic arches on each side, which was five 

 at first, soon becomes reduced in the Mammalia to three, by the obliteration of 

 the two highest pairs, the Bulbus Arteriosus is subdivided, by the adhesion 

 of its walls at opposite points, into two tubes, of which one becomes the Aorta 

 and the other the Pulmonary artery ; and of the three pairs of (branchial) 

 arches, the highest, being connected with the Aortic trunk, contributes to the 

 formation of the subclavian and Carotid arteries; whilst of the middle pair, 

 the arch on the right side is obliterated, the other becoming the arch of the 



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