88 HUMAN EMBRYOS AND FETAL MEMBRANES 



the allantois and eventually ramify in the villi of the chorion. The 

 vitelline arteries, large and paired in the chick, are represented by a single, 

 small trunk which branches on the surface of the yolk sac (Fig. 271). 

 Compared with the arterial circulation of the chick of fifty hours the 

 important differences are: (i) the development of the fourth and fifth 

 pairs of aortic arches, and (2) the presence of the chorionic circulation, 

 by way of the umbilical arteries, in addition to the vitelline circulation 

 found in the fifty-hour chick. 



The veins are all paired and symmetrically arranged (Figs. 88 and 279). 

 There are three sets of them : (i) The blood from the body of the embryo 

 is drained, from the head end by the anterior cardinal veins, from the tail 

 end by the posterior cardinal veins. These veins on each side unite dorsal 

 to the heart and form a single common cardinal vein which receives the 

 vitelline and umbilical veins of the same side before joining the heart. 

 (2) Paired vitelline veins in the early stages of the embryo drain from the 

 yolk sac the blood carried to it by the vitelline arteries. The trunks of 

 these veins pass back into the body on each side of the yolk stalk and liver, 

 and, with the paired umbilical veins, form a trunk that empties into the sinus 

 venosus of the heart. As the liver develops, it may be seen that the vitel- 

 line veins break up into blood spaces, called sinusoids (Fig. 279). When 

 the liver becomes large and the yolk sac rudimentary, the vitelline veins 

 receive blood chiefly from the liver and intestine. (3) A pair of large 

 umbilical veins which drain the blood from the villi of the chorion and are 

 the first veins to appear. These unite in the body stalk, and, again sepa- 

 rating, enter the somatopleure on each side. They run cephalad to the 

 septum transversum where they unite with the vitelline veins to form a 

 common vitello-umbilical trunk which joins the common cardinal and emp- 

 ties into the sinus venosus. 



The veins of this embryo are thus like those of the fifty-hour chick 

 save that the umbilical vessels are now present and take the place of the 

 allantoic veins of later chick embryos. The veins, like the heart and arteries, 

 are primitively paired and symmetrically arranged. As development 

 proceeds, their symmetry is largely lost and the asymmetrical venous 

 system of the adult results. 



The later stages of the human embryo cannot be described in detail 

 here. The student is referred to the texts of Minot, and Keibel and Mall. 

 Figs. 90 and 91 show a series of human embryos described by His, the 

 ages of which lie between four and eight weeks. The figures show as 

 well as could any description the changes which lead toward the adult 

 form when the embryo may be called a fetus (stage a/). The external 

 metamorphosis is due principally: (i) to changes in the flexures of the 



