DEVELOPMENT OF THE LYMPHATICS 



707 



and not from blood-vessels or mesenchyme cells. This view is supported especially by the 

 work of Sabin, MacCallum, Hoyer and his pupils and E. R. Clark. 



On the other hand, F. T. Lewis has suggested that the spreading of lymphatics occurs 

 by the transformation of blood-vessels into lymphatics; while Huntington and McClure and 

 their pupils maintain that it occurs by the continued transformation of mesenchyme cells. 



The Ij^mphatics gi-owing from the various primary centres meet and anastomose with 

 one another, and gradually lose all connections with the veins save those at the base of the neck 

 Sylvester has found, however, that in South American monkeys the connections with the veins 

 in the region of the renal veins are maintained in the adult. Valves do not appear in the lym- 

 phatic vessels until quite late, in human embryos about 5 or 6 cm. long. (Sabin.) 



The lytnphatic nodes do not make their appearance until the system of vessels is well 

 established. They are at first represented by masses of lymphoid tissue in the meshes of a 

 lymphatic network. Later the lymphoid mass breaks up into smaller portions, into which 

 the blood-vessels and branches from the surrounding network penetrate; and each mass, together 



Fig. 553. — The Sprouting of Lymphatic Capillaries in the Pig. (After IMacCallum.) 

 The lymphatics are injected and the sprouts are both single cells and clumps of cells. 



'^' 





with the portions of the network surrounding it, becomes enclosed in a connective-tissue capsule. 

 The original lymphoid tissue becomes transformed into the medullary cords and cortical nodules 

 of the node, while the enclosing lymphatic capillaries form its peripheral lymph-sinus. 



The earUest nodes appear in the places occupied by the primary lymphatic plexuses or 

 sacs (Miss Sabin, F. T. Lewis, Jolly), and have been termed the "primary nodes" (Miss Sabin). 

 Secondary and tertiary sets of nodes develop later at places of confluence of many Ivmphatics 

 (cf. A. H. Clark.) " ^% 



Regeneration and new growth of lymphatic vessels and glands. — While blood-vessels 

 are known to possess throughout life the capacity for regeneration and new growth, this process 

 in lymph-vessels has been very httle studied. Yet enough has been learned from the work of 

 Coffin and Evans to justify the statement that lymphatic vessels also possess the capacity?'for 

 new growth. Evans made the interesting observation that lymphatic vessels grow into a tumor 

 of connective-tissue origin (a round-celled sarcoma), while they fail to grow into a tumor of 

 epitheUal origin (an experimentally-produced peritoneal carcinoma in mice). 



