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



Caudally the two limbs terminate by uniting below the yolk-stalk. Slightly 

 above this level the pleuro-peritoneal cavity joins the exocoelomic cavity on either 

 side and in front where the anterior body-wall is deficient; that is, following the 

 line of reflection of the amnion (plate 6, fig. 3). That portion of the exoccelomic 

 cavity which lies in front bridges across the space between the two above-described 

 limbs. The bridge lies just above the yolk-stalk, while lying between it and the 

 pericardial cavity are the amnion, the amnionic cavity, and the anterior peri- 

 cardia! wall. 



The lower portion of the pleuro-peritoneal cavity lies in relation to the hind- 

 gut dorsally and the posterior surface of the yolk-sac ventrally. On either side it 

 is in wide communication with the exoccelomic cavity. As seen in plate 6, figure 

 3, its anterior surface is irregularly pitted, the cast of the irregular surface of the 

 yolk-sac. Viewed from behind (plate 6, fig. 4), is a deep groove which marks the 

 position of the hind-gut and dorsal mesentery. At the dorsal bend in the back 

 of the embryo the ccelomic cavity continues caudally by means of two prolonga- 

 tions (plate 1, fig. 2, and plate 6, fig. 4). These lie laterally to the hind-gut and 

 cloaca. They extend about as far caudally as the cloacal membrane. It is to be 

 noted that these are not united together ventrally; consequently there exists in 

 this portion of the embryo a ventral mesentery as well as a dorsal one. 



MESOTHELIUM. 



The body-cavity is everywhere lined with a mesothelium which varies in 

 character and thickness in different regions. As a rule, however, it may be said 

 that the visceral layer is thicker than the parietal. This is not entirely true 

 regarding that portion of the parietal pericardium which lies next to the fore-gut, 

 for this is thicker, being composed of two to three layers of cubical cells. Since, 

 however, this layer lies next to the fore-gut, it is in reality visceral, so that its 

 being thick is not actually at variance with the general rule. The parietal peri- 

 cardium elsewhere, i. e., lining the body-wall and the pericardial surface of the 

 septum transversum, is thin, being composed of a single layer of cubical cells. 

 The visceral pericardium, the previously described epicardium, is formed by a 

 single layer of cubical or rounded cells. 



The parietal peritoneum, with the exception of that covering the peritoneal 

 surface of the septum transversum, is thin and formed by a single layer of flattened 

 cells. Where this becomes continuous with the visceral peritoneum (that is, on 

 the dorsal mesentery) it gradually becomes thicker. The inferior surface of the 

 septum transversum possesses a comparatively thick epithelium, being composed 

 of three to four layers of cells. The visceral peritoneum is thick over the fore-gut 

 and yolk-stalk, somewhat thinner over the hind-gut, and thinnest over the yolk- 

 sac, where in most places it becomes lost as a distinct layer. 



The lining of the extra-embryonic cavity, i. e., the inner lining of the chorionic 

 vesicle, is not in the form of a distinct cell layer, but appears to be made up of 

 uncovered mesenchyma. Mitotic figures are found throughout all parts of the 

 visceral and parietal epithelium, except in the epicardium. 



