THE DEVELOPMENT OF THE VASCULAR SYSTEM 



277 



body wall (Fig. 252) . Outgrowths from the original axial drainage line invade 

 the various visceral organs (Fig. 251). Thus the lymphatic drainage of the 

 body is effected through outgrowths from a few primary centers which 

 represent derivatives of the venous channels. The lymph glands are second- 

 ary foci of development along the lymph vessels (see page 280), 



The view that lymphatics arise as enlarged isolated intercellular spaces 

 in the mesenchyinal tissue does not include any dispute as to the general 



FIG. 254. From a photograph (X 600) of a cross-section through the caudal end of a chick 



embryo of 15 mm., showing enlarged mesenchymal intercellular spaces as rudiments 



of the posterior lymph sac. West. 

 3, Coccygeal vein; 6, caudal muscle plate; 8, isolated enlarged intercellular spaces, the bounding 



cells becoming flattened; 9, lateral branch of coccygeal vein; 10, lymphatics containing 



collections of developing blood cells. 



disposition of the lymph channels in the body, but comprises a funda- 

 mentally different concept of the origin of these vessels. Upon a long and 

 exhaustive series of observations on closely graded series of embryos of 

 Fishes, Amphibia, Reptiles, Birds, and Mammals is based the conclusion 

 that not only the lymph sacs but the peripheral lymphatics as well originate 

 independently of the veins; and that the opening of the main lymphatic 

 drainage lines into the jugular or subclavian veins near their junction, 



