GENERATION AND DEVELOPMENT. 



283 



right and left lung are very small, and most of the blood which is forced 

 into the pulmonary artery passes through the wide ductus arteriosus into 

 the descending aorta. All these points will become clear on reference to 

 the preceding diagram (Fig. 430). 



As the umbilical vesicle dwindles in size, the portion of the omphalo- 

 mesenteric arteries outside the body gradually disappears, the part inside 

 the body remaining as the mesenteric arteries (Figs. 440, 441). 



Meanwhile with the growth of the allantois two new arteries (umbilical) 

 appear, and rapidly increase in size till they are the largest branches of 



FIG. 440. Diagram of young embryo and its vessels, showing course of circulation in the umbili- 

 cal vesicle; and also that of the allantois (near the caudal extremity), which is iust commencing. 

 (Dalton.) 



FIG. 441. Diagram of embryo and its vessels at a later stage, showing the second circulation. 

 The pharynx, oesophagus, and intestinal canal have become further developed, and the mesenteric 

 arteries have enlarged, while the umbilical vesicle and its vascular branches are very much reduced 

 in size. The large umbilical arteries are seen passing out in the placenta. (Dalton.) 



the aorta: they are given off from the internal iliac arteries, and for a long 

 time are considerably larger than the external iliacs which supply the com- 

 paratively small hind-limbs. 



Veins. The chief veins in the early embryo may be divided into two 

 groups, visceral and parietal: the former includes the omphalo-mesenteric 

 and umbilical, the latter the jugular and cardinal veins. The former may 

 be first considered. 



The earliest veins to appear in the foetus are the omphalo-mesenteric, 

 which return the blood from the yolk-sac to the developing auricle. As 

 soon as the placenta with its umbilical veins is developed, these unite with 

 the omphalo-mesenteric, and thus the blood which reaches the auricle 

 comes partly from the yolk-sac and partly from the placenta. The right 

 omphalo-mesenteric and the right umbilical vein soon disappear, and the 

 united left omphalo-mesenteric and umbilical veins pass through the 

 developing liver on the way to the auricle. Two sets of vessels make 

 their appearance 'in connection with the liver (venae hepaticae advehentes, 



