92 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS 



to the gut corresponds to the open mid-gut of the chick embryo. The hind-gut and 

 tail-fold of this embryo are greatly elongated as compared with the chick embryo 

 of fifty hours. The hind-gut terminates blindly in the tail. Near its caudal 

 end it is dilated to form the cloaca. Into the ventral side of the cloaca opens the 

 stalk of the allantois. Dorso-laterally the primary excretory (Wolffian) ducts 

 which we saw developed in the fifty-hour chick have connected with and open 

 into the cloaca. Caudal to the cloaca on the ventral side is the cloacal mem- 

 brane, which later divides and breaks through to form the genital aperture and 

 anus. That part of the hind-gut between the cloaca and the yolk-stalk forms 

 the rectum, colon, caecum, and appendix, with a portion of the small intestine 

 (ileum) . 



Urogenital Organs. We have seen that the primary excretory (Wolffian) 

 ducts open into the cloaca. These are the ducts of the mid-kidney or meso- 

 nephros. At this stage the nephrotomes, which in the chick embryos formed the 

 anlages of these ducts, are also forming the kidney tubules of the mesonephros 

 which open into the ducts (Fig. 82). The mid-kidneys project into the peri- 

 toneal cavity as ridges on each side. A thickening of the mesothelium along the 

 median halves of the mesonephroi forms the anlage of the genital glands or 

 gonads (Fig. 213). 



Circulatory System. The heart is an S-shaped double tube as in the fifty- 

 hour chick. The outer myocardium is confined to the heart, while the inner 

 endothelial layer is continuous, at one end with the veins, at the other end with 

 the arteries. The disposition of the heart tube is well seen in a ventral view of a 

 younger embryo (Fig. 83). The veins enter the sinus venosus just cranial to 

 the yolk-sac. Next in front is the atrium with the convexity of its flexure di- 

 rected cephalad. The ventricular portion of the heart is U-shaped and is flexed 

 to the right of the embryo. To the left is the ventricular limb, to the right is 

 the bulbus. The arteries begin with the ventral aorta which bends back to the 

 midline and divides into five branches on each side of the pharynx (Figs. 83 and 

 84). These are the aortic arches and they unite dorsally to form two trunks, the 

 descending aortce. The aortic arches pass around the pharynx between the gill 

 clefts in the branchial arches. The arrangement is like that of the adult fish 

 which has its gill slits, branchial arches and aortic arches to supply the gills. 

 The descending aortae run caudad and opposite the lung buds unite to form a 

 single median dorsal aorta. This in the region of the posterior limb buds 

 divides into the two umbilical 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 



