1).\ THE DEVELOIMIENT OF THE LYMPHATICS OF THE LUNGS IN THE 



EMBRYO ?I(i. 



Bv R. S. Cunningham. 



From an analysis of the literature on the development of the lymphatic sys- 

 tem, it is clear that there is a general agreement among recent workers that the 

 mammalian lymph-sacs precede the lymj)h-vessels in the time of their appearance, 

 and hence constitute what may be called a primary lymjihatic system. This 

 system consists, in mammals, of 8 sacs: 3 paired, the jugular, the subclavian, and 

 the posterior iliac lymph-sacs; and 2 unpaired, the retroperitoneal sac and the cys- 

 terna chyli. 



The further development of the lymj^hatic system — that is, the formation of 

 the thoracic ducts and the peripheral vessels — has been discussed at length by 

 numerous workers during the past decade. These workers have been grouped 

 into two general schools : the one holding that the lymi)hatics grow by a centrifugal 

 sprouting of pre-existing endothelium, the other believing that these vessels are 

 formed by a coalescence of numerous isolated spaces develoj^ing in the mesenchyme. 



According to the centrifugal theory, briefly stated, the sacs arise from the 

 veins and are joined together by vessels that sprout out from their endothelial walls. 

 Thus the thoracic duct arises from both the retroperitoneal sac and the left jugular 

 sac, and the two elements unite somewhere between the two points of origin. 

 Supporters of the centrifugal theory claim that the secondary lymphatic system 

 (the capillary bed) arises by the sjjrouting of the endothelial walls of the sacs and of 

 the right and left thoracic ducts. These sprouts invade the organs and, becoming 

 progressively more complex, assume the adult form of the lymphatic system. The 

 supporters of the multiple-anlagen theories (whether they believe in coalescing tissue- 

 spaces, multiple venous origins, or degenerating veno-lymi)hatics) agree in claiming 

 that lymphatics do not grow by the centrifugal s]irouting of the i)re-existing endo- 

 thelial walls. 



It is not my intention to review here all the various theories that have been 

 advanced, but only to call attention to the two general views, in order to correlate 

 my findings with them. A very thorough discussion of these two views, as well as 

 a comprehensive review of the literature, may be found in the Ergebnisse der 

 Anatomie und Entwickelungsgeschichte, 1913. (Dr. F. R. Sabin, Der Ursprung und 

 die Entwickelung des Lymphgefiissystems.) 



Though primarily concerned with the problems of origin and the method of 

 growth of the lymphatic vessels, the supporters of both theories have aided in 

 establishing the morphology of the primary system and have laid the foundation for 

 the further study of the development of the system as a whole. If the centrifugal 



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