194 



The Development of the Lymphatics in the Skin 



was necessary to find out wliether the lymphatic capillaries enter the 

 papillae or not. The papilla are present in the skin of the new-born 

 pig, but the hairs make the skin so diflficnlt to study that the papillae 

 are best seen in the tongue. By making a complete arterial injection 

 and forcing the injection mass over into the veins of a pig a week old, 

 it was easy to demonstrate the lymphatics in the subpapillary layer and 



Fig. 7. Skin of the ear of a pig 22 cm. long. Tlie veins are injected with Prussian 

 blue represented as black. About x 05. a, arteries ; be, blood capillaries ; c, chorium ; 

 Vl, primary lymphatics ; si, secondary lymphatics ; si, subcutaneous tissue, v, veins. 



in the center of the larger papilla. The smallest papillae contain just 

 a tuft of blood capillaries in the center, while the larger ones at the side 

 of the tongue have a central artery which is bordered by a central 

 lymphatic duct lined with epithelium. This makes the papillse in the 

 tongue analogous with the villi of the intestine as far as the central 

 lymjihatic duct is concerned. 



Thus the course of the development of the lymphatics has been fol- 

 lowed in the skin. The ducts are defined as channels with an endothe- 



