THE LATER DEVELOPMENT OF THE FROG 191 



of the tail, where it divides into dorsal and ventral branches. 

 The dorsal and ventral branches of each side then unite forming 

 two large vessels which extend through the tail, lying above 

 and below the myotomes (Fig. 69). 



The large subcutaneous lymph sacs, so characteristic of 

 both the tadpole and the adult frog, are formed very early 

 from the network growing out from these vessels. The small 

 lymphatics in the subcutaneous connective tissue branch 

 abundantly and anastomose freely, forming a rich network; 

 their walls then disappear and the wide lymph sacs are left, 

 still connected with the lymph hearts by way of the lateral 

 trunks described. 



The thoracic ducts also appear to arise from the anterior 

 lymph hearts, as a pair of outgrowths which extend poste- 

 riorly, between the dorsal aorta and the posterior cardinal 

 veins. When the hind-legs appear, from one to three pairs of 

 posterior lymph hearts develop in connection with the inter- 

 segmental veins of the region, in much the same way that the 

 anterior hearts developed. They open for a time into the pos- 

 terior cardinal veins, and later, therefore, into the renal-portal 

 veins, whether by the intersegmental veins or by the ischiadic 

 branch is not clear. 



The spleen is first indicated in larvae of about 10 mm. by a 

 collection of mesenchymal lymphoid cells in the mesentery, 

 around the mesenteric artery, just dorsal and posterior to the 

 stomach (Radford). These cells multiply and in a 15 mm. 

 larva form a definite projection from the mesentery, covered 

 therefore, by a coelomic or peritoneal epithelium. During 

 this period of enlargement, the spleen appears to receive some 

 cells which wander out from the intestinal epithelium. Later 

 this organ becomes very vascular and in the 25-27 mm. 

 larva it forms a definite ovoid body, in the position where it is 

 found in the adult. 



6. The Formation of the Septum Transversum 



We have seen that the pericardial cavity is formed as a 

 median ventral section of the ccelom. This remains completely 



