DEVELOPMENT OP THE EMBRYO. 995 



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 sub- 

 stance, which probably aids in the nutrition of the embryo previously to the 

 formation of the placenta. 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 following parts may be traced : 1. 

 The tubular sheath afforded by the Amnion. 2. The Umbilical Vesicle (Fig. 

 266, 0, with its pedicle, or vitelline duct. 3. The Vasa Omphalo-Meseraica 

 (q_j r), or mesenteric vessels of the embryo, by which the yelk was absorbed 

 into its body ; these accompany the pedicle. 4. The Urachus, and remains of 

 the Allantois. 5. The Vasa Umbilicalia (nn, o), 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 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 Venosus into the Vena Cava. 



1003. A change in the type of the Circulating system of the foetus, from 

 that at first presented by it ( 1000), 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 in the 3d 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 aorticus" 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 Sub- 

 clavian and Carotid arteries ; whilst of the middle pair, the arch on the right 

 side is obliterated, and the other becomes the "arch of the aorta." The lowest 

 pair arises from the Pulmonary trunk, and forms the right and left Pulmonary 

 arteries; that on the left side, however, goes on to join the descending aorta as 

 before, and thus constitutes the Ductus Arteriosus. A knowledge of these 

 different stages in the development of the Heart and Arterial system enables us 

 to explain many of the malformations which they occasionally present in Man; 

 these being for the most part due to arrest of development, whereby the circu- 

 lating apparatus is permanently fixed in conditions that are properly characteristic 

 of cold-blooded animals. And it is interesting to remark, too, that the varieties 

 which not unfrequently present themselves in the arrangement of the principal 

 trunks, given off from the Aorta, find their analogues in the arrangements that 

 are normally characteristic of some or other of the Mammalia. 1 The Venous 

 system undergoes changes which are even more remarkable than those of the 

 arterial trunks. In its earliest condition, it has been ascertained by Rathke 3 to 

 present essentially the same type in the embryos of all Vertebrated animals, 

 the peculiarities of each group being acquired by a process of subsequent trans- 

 formation. There is at first a pair of anterior venous trunks (Figs. 266, 267, 



1 See "Princ. of Phys., Gen. and Comp.," Am. Ed., $ 491, 492. 



2 " Ueber den Bau und die Entwickelung des Venensystems der Wirbelthiere," 1838. 



