86 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS 



Ulical arteries, which, curving cephalad and ventrad, enter the body stalk on 

 each side of 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 

 (1) the development of the fourth and the fifth pairs of aortic arches, and (2) 

 the presence of the chorionic circulation by way of the umbiUcal arteries in addi- 

 tion to the vitelUne 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: (1) The blood from the body of the embryo is 

 drained, from the head end by the anterior cardinal veins; from the tail end of the 

 body 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 

 umbiKcal 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 vitelMne arteries. The trunks of these veins pass back into the body on 

 each side of the yolk stalk and Uver, and with the paired umbihcal veins form a 

 trunk which empties into the sinus venosus of the heart. As the hver develops 

 it may be seen that the vitelline veins break up into blood spaces called by Minot 

 sinusoids (Fig. 279). When the liver becomes large and the yolk sac rudimentary 

 the vitelline veins receive blood chiefly from the hver 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 separating, 

 enter the somatopleure on each side. They run cephalad to the septum trans- 

 versum where they unite with the vitelhne veins to form a common vitello-umbil- 

 ical trunk which joins the common cardinal and empties into the sinus venosus. 



The veins of this embryo are thus hke 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, hke 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. Two embryos 

 will be compared with the pig embryos described in Chapter V. Figs. 90 and 91 

 show the human embryos described by His , the ages of which were estimated by 

 him at from two weeks to two months. The figures show as well as could any 

 description the changes which lead to the adult form when the embryo may be 



