916 



EMBRYOLOGY. 



in the auricles ; at a point corresponding to it, a septum is developed in 

 their interior which remains incomplete during the whole of foetal life, being 



perforated by the foramen of 



Fig. 443. Botal. With regard to the 



aortic bulb, it contracts and 

 divides into two vessels, the 

 aorta and pulmonary artery. 



The arteries are developed 

 partly at the expense of the 

 vessels of the primary circula- 

 tion, and partly in the vascular 

 lamina of the embryo. The 

 heart, when it was only a simple 

 cylindrical tube, presented at 

 its anterior extremity two aortic 

 arches, which curved backwards 

 and united to form the single 

 aorta, then the vertebral or 

 common aortee. The aortic 

 arches are situated at the inner 

 face of the two first pharyngeal 

 arches; afterwards more are 

 developed, which are placed 

 within the other pairs of arches, 

 until the number is increased to 

 five, though they never all exist 

 at the same time. Some atrophy, 

 while others are being developed : 

 the two first entirely disappear ; 

 the third form the carotids : the 

 fourth the axillary arteries and 

 the arch of the aorta ; the fifth 

 atrophies on the right, and on 

 the left originates the pulmonary artery, the ductus arteriosus, and the aorta. 

 The latter is continued along the spine by the fusion of the two primitive 

 aortas; they present, at their posterior extremity, the pelvic vessels, which 

 are very small, and the umbilical arteries, which are, on the contrary, 

 remarkable for their volume. 



The peripheral arteries arise, independently of the central vessels, on the 

 interior of the vascular lamina. They appear in the form of solid cellular 

 branches, which are hollowed in the centre by a cavity in which the cells 

 become free. In proportion as these new vessels are developed, the omphalo- 

 mesenteric vessels disappear, until at last there only remain one or two ducts 

 that pass to the umbilical vesicle. 



The umbilical veins are developed immediately after the formation of the 

 omphalo-mesenteric veins ; they enter the common trunk of the latter, and 

 when its ramifications diminish in volume, the umbilical veins increase 

 rapidly ; when the liver is formed around them, they throw into it branches, 

 which are the rudiments of the hepatic network. Between the hepatic and 

 subhepatic veins, the umbilical vein communicates with the vena cava by 

 the ductus venosus of Aranzi, which, according to M. Colin, does not exist in 

 the foetus of Solipeds in the last moiety of uterine life. 



The veins of the embryo form four principal trunks at first : two anterior, 



HEART OF AN EQUINE F(ETUS. THE RIGHT AURICLE 

 AND POSTERIOR VENA CAVA HAVE BEEN OPENED 

 TO SHOW THE FORAMEN OF BOTAL. 



men of Botal. 



