604 THE EMBRYOLOGY OF THE FROG 



Their first openings are into the posterior cardinal vein, which means 

 that later they empty into the renal portal veins. 



The Spleen. This ductless gland is first seen in the developing tad- 

 pole at about ten millimeters. It appears as a mass of mesenchymal 

 lymphoid cells in the mesentery, immediately dorsal and posterior to the 

 stomach and around the mesenteric artery. 



These cells then multiply and project from the mesentery so as to 

 have a peritoneal covering, as do all the organs in the body cavity. 

 Later, as the spleen enlarges, there seem to be various wandering cells 

 from the intestinal epithelium added to it. The spleen is complete by the 

 time the tadpole is twenty-five to twenty-seven millimeters in length 

 and is extremely vascular. 



THE SEPTUM TRANSVERSUM (Fig. 348) 



The pericardial cavity, already discussed, has remained open pos- 

 teriorly into the abdominal cavity, with the exception of the region 

 covered by the liver. Now, as the ducts of Cuvier form and pass from 

 the body-wall to the sinus venosus, they pass through this open region 

 and carry with them incomplete peritoneal folds from the body-wall. 

 These folds are called the lateral mesocardia. They remain incomplete 

 dorsally for a long time, but gradually extend ventrally so as to form a 

 complete separation between pericardial and peritoneal cavities This 

 transverse partition is called the pericardio-peritoneal septum or the 

 septum transversum. To this septum transversum is added a medial 

 portion of peritoneum from the anterior face of the liver, while on the 

 right side the septum becomes continuous with the posterior suspensory 

 fold of the liver commonly called the mesohepaticum. 



After metamorphosis, the septum unites dorsally with the dorsal 

 mesentery and completes the separation between pericardial and peri- 

 toneal cavities. 



