140 A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 



extending from the region of the sinus venosus to the yolk-stalk, and divisible 

 into cranial and caudal portions. The upper of these he states goes into the for- 

 mation of the liver proper and the hepatic duct; the lower into the formation of 

 the gall-bladder and the cystic duct. The two portions are designated by Maurer 35 

 the "pars hepatica" and "pars cystica" respectively. The twin embryos which 

 Watt described are apparently too young to show these divisions of the liver 

 in fact, the liver forms merely a slight swelling on the ventral wall of the foregut 

 where the latter joins the yolk-stalk. It is called by Watt the "liver bay." 

 Thompson, however, recognizes the hepatic and cystic portions of the liver 

 diverticulum in the Robert Meyer embryo No 300. Somewhat similar divisions 

 are described by Ingalls in his embryo of 4.9 mm., but his specimen is considerably 

 older than the above-mentioned ones. Evidence of a division into two portions 

 is apparently altogether lacking in the Bremer embryo of 4 mm., the liver divertic- 

 ulum of which has been modeled by Bremer and more recently by Lewis. 



YOLK STALK AND SAC. 



Just below the hepatic diverticulum the gut becomes narrow, but a little 

 more caudally it again gradually broadens. This broadening leads out into the 

 cavity of the yolk stalk and sac. The yolk-stalk is short, being in fact merely 

 the constriction between the yolk-sac and the gut. It is flattened antero-poster- 

 iorly, but is broad transversely (plate 1, fig. 2). It measures roughly 0.5 mm. 

 from side to side and 0.16 mm. antero-posteriorly. 



The yolk-sac is a flattened vesicle, rather irregular in form, and with a num- 

 ber of folds of various shapes and sizes on its surface. It fills up practically the 

 entire space between the embryo and the wall of the chorion, and extends into 

 the artificially made fold of the chorionic wall as described above. Its dimensions 

 are roughly as follows: length, measured parallel to long axis of embryo, 3.3 mm.; 

 width, measured parallel to dorso-ventral axis of embryo, 2.7 mm.; thickness, 

 measured transverse to embryo, 1.1 mm. Its histological structure will be con- 

 sidered later. 



HIND-GUT AND CLOACA. 



Caudal to the place at which the yolk-stalk passes out is the beginning of the 

 hind-gut. It has a funnel-shaped opening which tapers as it passes toward the 

 tail into a small rounded tubule. It is surrounded by loose mesenchyma, the 

 whole being attached to the dorsal body-wall by a short, thick mesentery. The 

 hind-gut occupies a position slightly to the left of the median plane of the embryo. 

 It bends backward with the body of the embryo at the ventral bend in the back. 

 It passes without sharp demarcation into the cloaca (text-figs. 8 and 9). The 

 cloaca is the cephalic portion of the spindle-shaped termination of the hind-gut. 

 Its cephalic limit is not definitely indicated, but its caudal extent is marked by 

 the cloacal membrane. 



