Florence R. Sabin 193 



development in the remoter parts, for example, in the feet, is always 

 somewhat retarded. 



While the pig is increasing from 10 to "35 em. in length the two 

 lymphatic plexuses, the deep or primary and the superficial or secondary, 

 become more complicated. Valves begin to develop in the lymphatics and 

 increase the difficulty of obtaining lymphatic injections. By the time 

 the embryo is 16 or 18 cm. long the valves are present and prevent much 

 backward injection. At this stage, and still more clearly in pigs between 

 20 and 25 cm. long, a subcutaneous injection of considerable pressure 

 will usually enter the deep plexus of lymphatics and run centralward in 

 the ducts of the subcutaneous tissue, but not outward into the plexus of 

 the chorium. This is readily demonstrated by injecting into the foot' 

 pads. If the injection is in the hind feet the fluid enters the ducts of 

 the subcutaneous tissue and is carried to the inguinal glands; if in the 

 fore feet, the ducts lead to the glands in the front of the neck. A sub- 

 cutaneous injection then in a pig about 20 cm. long enters the deep 

 lymphatics. The injection mass, however, often enters the chorium, not 

 in the superficial lymphatic plexus, but rather through certain veins 

 that run directly to the surface and spread out in a fine plexus just 

 beneath the epidermis. These vessels are blood capillaries, as can be 

 proved by making a venous injection. Prussian-blue was injected into 

 the umbilical vein of a pig 22 cm. long, under a pressure of 100 mm. of 

 mercury. The skin soon showed fine points of blue, and each point was 

 seen to be a fine plexus of ducts just beneath the skin, the plexus spread- 

 ing out from a small vein which ran to the surface. Thus, since by 

 subcutaneous injections in these stages one usually gets a mixed injec- 

 tion of deep lymphatics and superficial veins, the lymphatics are best 

 studied by complete venous injections. 



Fig. 7 is a section of the skin of the ear of a pig 22 cm. long in which 

 Prussian-blue was injected into the umbilical vein under a pressure of 

 about 100 mm. of mercury. The veins are filled with blue granules, 

 the capillaries with blood corpuscles, while the lymphatics are empty. 

 The epidermis is now several layers deep, the hairs are partly developed, 

 but there are no papillae. There is still some differentiation between the 

 subcutaneous tissue and the chorium, the former being more fibrillar 

 and having wider spaces, the latter being denser and more cellular. The 

 lympliatics are not as large as in earlier stages and they lie in two planes, 

 a primary plexus of ducts in the subcutaneous tissue and a secondary 

 plexus in the chorium. 



To complete the study of the development of the ducts in the skin it 



