The Origin and Development of the Lymphatic System. 71 



both of whom state that vessels may function now as veins and now as 

 phatics, but beginning with the amphibia, it is certain that lym- 

 phatics bud off from the veins in certain definite areas and form 

 plexuses which are transformed into sacs. The essential structure of 

 these sacs is the endothelial lining. A primary lymph sac is there- 

 fore one which buds off from the endothelium of a vein. 



In the amphibia, probably in the reptilia, and in the case of the 

 posterior lymph sacs in birds, the primary sacs bud off from segmental 

 veins and rest on the myotomes. They derive striated muscle from 

 the myotomes, which has been shown at least in the case of the 

 anterior lymph hearts in amphibia (Knower 74), and the posterior 

 lymph hearts in reptiles (Stromsen 145) and pulsate rhythmically; in 

 a word, they become lymph hearts. 



The lymphatic sacs for the anterior part of the body in birds and all 

 of the lymphatics in mammals do not come from the vertebral and 

 coccygeal veins, but from the anterior cardinal veins in the neck, and 

 the renal veins in the abdomen. These sacs do not lie on the myotomes, 

 they do not receive striated muscle, but rather are transformed into 

 lymph glands. Thus the bird represents a transition stage having a 

 posterior lymph heart and a jugular lymph gland. I shall keep, 

 therefore, the term anterior and posterior lymph hearts, and use the 

 terms jugular and renal lymph sacs. The embryological classification 

 is: 



"1. Lymph hearts. 



a. Anterior, amphibia. 



b. Posterior, amphibia, reptilia and birds. 



2. Lymph sacs (glands). 



a. Jugular — birds and mammals. , 



(Retroperitoneal. 

 Iliac. 

 Cisterna chyli. 



The subcutaneous and deep lymph sacs of the amphibia are not 

 primary, but secondary. They are transformed lymphatic ducts. 

 Thus the amphibia have primary lymph sacs which become lymph 

 hearts, lymph ducts which become secondary sacs, and lymphatic capil- 

 laries. Mammals have primary lymph sacs which become primary 

 lymph glands, lymphatic ducts which develop valves and along which 

 secondary glands are formed, and thirdly, lymphatic capillaries without 

 valves. 



Primary 



Lymph 



Sacs. 



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