190 The Development of the Lymphatics in the Skin 



the scapula; second, at the angle of the Jaw, for the ducts of the face; 

 third, in the front of the neck, for the ducts of the lower jaw, chest and 

 fore legs. Tlie ducts of all these systems anastomose freely in the skin. 

 In the' posterior part of the hody there are two centers for the radiation 

 of the ducts, first over the crest of the ilium for the ducts of the pos- 

 terior part of the back and of the hip, and secondly in the inguinal region 

 for the ducts that grow into the abdominal wall and down the leg. The 

 ducts of the anterior and posterior systems anastomose freely over the 

 body. 



Having traced the spreading of the superficial lymphatics in the skin 

 from the time the ducts first come to the surface in the neck and over 

 the crest of the ilium to the time when they have reached the remotest 

 parts of the body, namely, the top of the head and the tips of the toes, 

 it remains to trace the development of these ducts in the different layers 

 of the skin. 



In pig embryos 13 mm. in length the epidermis is from two to four 

 cells deep and is separated from the connective tissue beneath by a dis- 

 tinct basement membrane. The connective tissue beneath is loose or com- 

 pact in different parts of the body, and is not divided into layers, so that 

 there is no differentiation between the chorium and the underlying sub- 

 cutaneous tissue. The blood capillaries in this connective tissue are 

 large, having a width of from two to four or five times the diameter of 

 the red blood corpuscles, which at this stage are large nucleated cells. 



By the time the embryo is 15 mm. long, a stage just about the time 

 that the lymphatics are budding oft" from the veins but before they have 

 reached the skin, there are certain areas of the body, for example, over 

 the arm bud, where the connective tissue beneath the epidermis is divided 

 into two distinct layers, a denser layer next the epidermis and a looser 

 meshed layer just within. The outer, denser layer is to become the 

 chorium, and the inner, looser layer the subcutaneous tissue. The blood 

 capillaries lie at this stage on tlie inner border of the chorium between 

 it and the subcutaneous tissue. There are numerous vessels in the sub- 

 cutaneous tissue but none in the true skin. 



Soon the blood capillaries begin to grow outward into the chorium 

 and give up their position along its inner border. The vessels in the 

 subcutaneous tissue remain and become larger, iis the blood capillaries 

 advance into the true skin the lymphatic capillaries grow in just behind 

 them, taking the position along the inner border of the chorium. In an 

 embryo 3 cm. long the lymphatic capillaries lie in this border just inter- 

 nal to the blood capillaries. Fig. 6 shows the lymphatics in the skin 



