i86 



HISTOLOGY 



growths, such as extend from the lymphatic vessels into the peripheral 

 tissues as described by Clark. The mesenteric sac thus becomes connected 

 with the left jugular sac (symmetrical connections with both jugular sacs 

 occur in some animals) and the connecting vessels constitute the thoracic 

 duct. The cisterna chyli is a secondary enlargement dorsal to the aorta. 

 In the adult the sacs are replaced by plexuses of smaller vessels. 



The origin of the detached or apparently detached lymphatic spaces in embryos, 

 which precede the formation of the well-defined vessels, has been studied with great 

 diligence by Huntington (The Anatomy and Development of the Lymphatic System, 

 Mem. Wistar Institute, 1911) and McClure (Anat. Rec., 1912, vol. 6, pp. 233-248), 

 to whose many contributions references will be found in the papers cited. They con- 

 sider that the lymphatic spaces arise in large part as mesenchymal spaces, but the 

 possibility suggested by Bremer's recent work on the blood vessels, that uninjectable 

 endothelial strands of great delicacy may pass to these cavities, has not been set aside, 

 and further work upon this subject is being conducted under Professor McClure's 

 direction. The reasons which led the writer to consider the origin of the lymphatic 

 vessels from mesenchymal spaces as improbable, were stated as follows (Amer. Journ. 

 Anat, 1905, vol. 5, pp. 95-120). 



a bed 



FIG. 177- BLOOD VESSELS AND LYMPHATIC VESSELS BETWEEN THE CIRCULAR AND LONGITUDINAL LAYERS 



OF SMOOTH MUSCLE FIBERS IN THE SMALL INTESTINE OF A CAT. X 775. 



a, d., Lymphatic vessels; b, vein; c, artery. 



" i. The lymphatic spaces do not resemble mesenchyma even when it is oedematous, 

 but on the contrary, are scarcely distinguishable from blood vessels (Langer)." 



" 2. After being formed, the lymphatics increase like blood vessels, by means of 

 blind endothelial sprouts, and not by connecting with intercellular spaces (Langer, 

 Ranvier, MacCallum, Sabin)." The subsequent work of Clark is here conclusive. 



"3. In early embryos detached blood vessels may be seen without proving that 

 blood vessels are mesenchymal spaces. These detached vessels are not far from the 

 main trunks, from which they may have arisen by slender endothelial strands, yet 



