THE ANATOMY OF A 4.2 MM. HUMAN EMBRYO 83 



into the ventral mesentery cranial to the wall of the yolk stalk. It is much larger 

 than in the fifty-hour chick, where its paired anlage was seen cranial to the 

 fovea cardiaca, and is separated from the heart by the septum transversum. The 

 small intestine between the Hver and yolk stalk is short and broad. In later 

 stages it becomes enormously elongated as compared with the rest of the diges- 

 tive 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 bhndly 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 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, csecum, and appendix, with a portion of the small intestine 

 (ileum). 



Urogenital Organs. — The opem'ng 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 

 formed the anlages of these ducts, are also forming the kidney tubules of the meso- 

 nephros 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 anlage of the genital glands or 

 gonads (P'ig. 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 

 endothehal 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 di- 

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

 to the right of the embryo. The left Kmb is the ventricle, the right the bulbus. 

 The arteries begin with the ventral aorta which bends back to the midhne 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 phar3Tix between the gill clefts in the 



