2 16 



VASCULAE SYSTEM 



one another. Between these primary thickenings there are two smaller ridges, so 

 that the cavity is divided into two triangular passages (fig. 269). The main folds 

 run in a spiral direction from a point on the ventral aorta between the last two 

 aortic arches to the base of the truncus, and as they lie dorso-ventral in front, and 

 right and left behind, it follows that when the vessels are separated from one 

 another they are placed dorso-ventral at their proximal, and right and left at their 

 distal ends. The actual cleavage of the truncus arteriosus is brought about by two 

 folds of the connective-tissue wall which correspond to the primary fused endocardial 

 ridges. These endocardial ridges are prolonged into the part of the right ventricle 



uJu ^ 



VLV. 



cat.. 



FIG. 270. PROFILE VIEW OF A HUMAN EMBRYO OF ABOUT FIFTEEN DAYS, WITH THE 

 ALIMENTARY CANAL IN LONGITUDINAL SECTION. (His.) 



Two arterial arches are formed at this stage. 



FIG. 271. SIMILAR VIEW OF A SOMEWHAT OLDER EMBRYO, ^SHOWING FIVE ARTERIAL ARCHES. 



1, 2, 3, 4, 5, are opposite the respective secondary cerebral vesicles; from the side of the fore-brain 

 the primary optic vesicle is seen projecting ; ot, otic vesicle, still open in fig. 270 ; p.v., septum between 

 mouth and pharynx (primitive velum). This has disappeared in fig. 271 ; I, commencing liver in septum 

 transversum ; v, vitelline stalk; all, allantois enclosed within stalk ; j.v., jugular vein; c.v., cardinal 

 vein; s.r., sinus venosus within septum transversum; u.a., left umbilical (allantoic) artery ; l.u.v., left 

 umbilical vein ; end, endothelial tube of heart. The sharp curve of the trunk of the embryo towards 

 the yolk-sac is normal at this period of development. 



formed from the ascending limb of the ventricular loop (proximal chamber of aortic 

 bulb), and are continuous with the endocardial cushions proper to it. As these 

 also unite with one another, the aortic septum becomes extended into the right 

 ventricle to divide its distal portion (aortic bulb) into two passages, pulmonary 

 and aortic. These become the conus of the right, and the aortic vestibule of 

 the left ventricle respectively (fig. 263). The semilunar valves are formed by a 

 hollowing of the endocardial cushion from the distal side at the mouths of the aorta 

 and pulmonary artery (fig. 269). 



The inter ventricular foramen, however, still persists. It is now closed by 

 a somewhat complicated process. We left the interventricular septum at a 



