516 H. M. WAHL 



outer side of the anlage of the body wall as this extends forward 

 at the expense of the membrana reuniens to enclose the thoraco- 

 abdominal viscera. This plexus at first drains forward through 

 the membrana reuniens into the umbilical vein, but later the 

 thoraco-epigastric vein becomes differentiated so as to drain 

 the plexus into the main vein of the arm. From a similar plexus 

 on the inner surface of the body wall, which at first drains into 

 the cardinal veins, later also into the subclavian and femoral 

 veins, the intercostal, internal mammary and deep epigastric 

 vessels are differentiated. Between these and the superficial 

 vessels numerous anastomoses exist. At a later period the terri- 

 tory of the thoraco-epigastric vein comes to drain into the femoral 

 vein as well as into the axillary vein and the arteries correspond- 

 ing to the superficial veins are formed.^ 



Although the mammary line arises comparatively early it 

 appears to exert no influence on the formation of the chief ven- 

 tral cutaneous vessels. The latter are fonned in response to the 

 needs for more direct vascular connections in the ventral region 

 of the abdominal wall as the umbilical veins change their courses 

 and the distance from the dorsal axis increases. 



The first noticeable influence of the gland region on the cutane- 

 ous vessels appears at the period when the individual gland 

 anlages begin to differentiate in the mammary line. At this 

 time in the deep layer of the skin the branches of the thoraco- 

 epigastric vein spread out widely and anastomose. Superficial 

 to these larger branches there is a network of smaller capillaries. 

 This network becomes especially well developed about the lentic- 

 ular thickenings of the epidermis which mark the anlages of the 

 glands (fig. 1). As the epithelial buds of the ducts project 

 downwards and the connective tissue begins to condense about 

 their bulb-like extremities a special capillary plexus develops 

 in this connective tissue sheath. 



By the time the ducts extend laterally from the primitive 

 gland anlage veins and arteries are differentiated in the capillary 



^ Helen W. Smith, The development of the superficial veins of the body-wall 

 in the pig. Am. Jour. Anat., vol. 9, p. 439, 1909; F. T. Lewis, The development 

 of the lymphatic system in rabbits. Am. Jour. Anat., vol. 10, p. 113, 1905. 



