THE ANATOMY OF A 4.2 MM. HUMAN EMBRYO 93 



represented by a single small trunk which branches on the surface of the yolk- 

 sac (Fig. 84). Compared with the arterial circulation of the chick of fifty hours 

 the important differences are (i) the development of the fourth and the 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. 83 and 84). 

 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 joins the umbilical vein 

 of the same side. (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 which empties into the 

 sinus venosus of the heart. As the liver develops it may be seen (Fig. 83) that 

 the vitelline veins break up into blood spaces called by Minot sinusoids. 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 separating, enter the soma- 

 topleure 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 empties 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 can not be described in detail here. 

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

 will be compared with the pig embryos described in Chapter V. Figs. 85 and 86 

 show the human embryos described by His, the age of which was 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 

 called & fetus. The external metamorphosis is due principally: (1) to changes 

 in the flexures of the embryo; (2) to the development of the face; (3) to the 

 development of the external structure of the sense organs (nose, eye and ear); 



