THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



243 



being direct derivatives of the venous channels; (2) that the lymph vessels 

 arise in situ through enlargement and coalescence of intercellular tissue 

 spaces, the mesenchymal cells bounding these spaces becoming flattened and 

 rearranged to form the endothelial walls of the vessels, and, as a corollary, 

 that the junction of the lymph vessels with the veins, which occurs at certain 

 definite points, is a secondary matter. 



Here again the scope of the work does not permit presentation in detail 



Ant. cardinal vein I -/~ 



^ 

 Subclavian vein 



/ 



Diaphragm 



Suprarenal gland /C~~ 



Mesonephros 



Kidney-- 



Ant, lymph heart 



Deep lymphatics 



of arm 



Branches to heart 



r 



Branches to lung 

 Aorta 



' Branch to oesophagus 

 'Branches to stomach 

 ^Branch to duodenum 



/ Branches to mesenteric plexus 

 Cisterna chyli 



Post, lymph heart 



Deep lymphatics 

 to leg 



FIG. 213. Diagram showing the arrangement of the lymphatic vessels in a 

 pig embryo of 40 mm. Sdbin. 



of the evidence adduced in favor of either of these views. The advocates 

 of the first view have placed much dependence upon the method of in- 

 jection, which, as in the case of the origin of blood vessels, has been met 

 with the criticism that injection shows only those lymph channels with con- 

 tinuous lumina and leaves undetermined the field beyond the injected area 

 (see page 194). To supplement their studies by the injection method, the 

 investigators who maintain that lymphatic endothelium grows by sprouting 

 of preexisting endothelium have added studies on living tissues in which 

 the sprouting phenomena are claimed to be clearly observable. Those who 



