246 THE MESODERMAL DERIVATIVES 



with the sinus venosus and establishes a new connection with the 

 hepatic portal vein, originally the right vitelline vein, which enters the 

 liver lobes. 



The Lymphatic System. The lymphatic system of the frog is 

 made up of very large lymph spaces in various parts of the body, with- 

 out well-defined connecting lymph vessels such as occur in mammals. 

 However, all of the four lymph hearts are protected by valves so that 

 the lymph always passes from a lymph heart to a lymph or blood ves- 

 sel, and never in the reverse direction. There are no muscular coats to 

 the lymph spaces, but they are lined with flat endothelial cells and 

 connective tissue. The connective tissue forms septa which divide the 

 lymph spaces and attach them to the underlying muscle. 



Between the third and fourth somites between the peritoneum and 

 the integument, at the time of hatching (6 mm. body length), there 

 are developed crude anterior lymph hearts, surrounded by muscle 

 fibers. These are connected by an ill-defined pair of vessels just be- 

 neath the skin, the subcutaneous lymph spaces, one of which extends 

 anteriorly and the other posteriorly, each to merge with the venous 

 plexuses. The anterior paired lymph vessels send extensions ventrally 

 to the branchial region considerably after the time of hatching. Paired 

 thoracic ducts develop from the anterior lymph hearts (at the 26 mm. 

 stage) just between the posterior cardinal vein and the dorsal aorta. 

 The posterior vessels give rise to both dorsal and ventral vessels in the 

 larval tail. After metamorphosis posterior lymph hearts appear in 

 association with the intersegmental veins near the hindlimbs. As pre- 

 viously described, there is flow of lymph from the peritoneal cavity 

 into the venous sinuses of the kidney by way of the ciliated peritoneal 

 funnels of that organ. 



The Spleen. 



This hematopoietic organ arises in the vicinity of the stomach at 

 about the 10 mm. body length stage as a cluster of cells within the 

 dorsal mesentery. By the 15 mm. stage the spleen is seen as a definite 

 projection from the mesentery, covered by coelomic epithelium. Ap- 

 parently some wandering cells from the intestinal epithelium invade 

 the spleen at this time. By the 30 mm. stage these cells become reticu- 

 lar and are vascularized and can be recognized as splenic cells, and 

 the organ as the ovoid spleen of the adult. 



