THE ANATOMY OF A 4.2 MM. HUMAN EMBRYO 85 



becomes enormously elongated as compared with the rest of the digestive 

 tube. The yolk stalk is still expansive. The region of its attachment 

 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 the cloaca and open into it. 

 Caudal to the cloaca, on the ventral side, is the cloacal membrane, 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, together with a portion of the 

 small intestine (ileum). 



Urogenital Organs. The opening of the primary excretory (Wolffian) 

 ducts into the cloaca has been noted. These are the ducts of the mid- 

 kidney, or mesonephros. At this stage, the nephrotomes, which in the 

 chick embryos were seen to form the anlages of these ducts, are also form- 

 ing the kidney tubules of the mesonephros which open into the ducts 

 (Fig. 87). The mid-kidneys project into the peritoneal cavity as ridges 

 on each side. A thickening of the mesothelium along the median halves 

 of the mesonephroi forms the anlages of the genital glands, or gonads 

 (Fig. 220). 



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. 88). The veins enter the 

 sinus venosus just cranial to the yolk sac. Next in front is the atrium, 

 with the convexity of its flexure directed cephalad. The ventricular 

 portion of the heart is U-shaped and is flexed to the right of the embryo. 

 The left limb is the ventricle, the right the bulbus. 



The arteries begin with the ventral aorta which bends back to the mid- 

 plane and divides into five branches on each side of the pharynx (Figs. 88 and 

 89) . 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 pharyngeal pouches in the branchial arches. The arrangement is like 

 that of the adult fish which has gill slits, branchial arches, and aortic 

 arches to supply the gills. The descending aortae run caudal, 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 



