THE DEVELOPMENT OF THE VASCULAR SYSTEM. 279 



In regard to the origin and development of the mammalian systemic lymph- 

 atics, some of the earlier investigators held that they were outgrowths from 

 preexisting vascular channels. Sabin maintained that the lymph hearts were 

 centers from which the rest of the lymphatics developed by evagination or 

 outgrowth (Figs. 252, 253 and 254). Lewis claimed that the lymphatic vessels 

 grew out not only from the lymph hearts but also from the veins at many 



.Cellular mass 



, Mesenchyme 



:'--.'- "-.- .- "^ Marginal plexus 



FlG. 255. From a section through the axilla of a human embryo of 106 mm. 

 (about 4 months), showing anlagen of lymph glands. Kling. 



points. Subsequent researches of Huntlngton and McClure and of Huntington 

 himself have furnished conclusive evidence, so far as such evidence can be ob- 

 tained from sections of embryos, that the systemic lymphatics, not only the 

 larger trunks but also the smaller ramifications, arise independently of the 

 haemal vascular system by confluence of mesenchymal intercellular spaces and 

 become lined by lymphatic vascular endothelium which represents modified 

 mesenchymal cells lining those spaces. 



The Lymph Glands. 



The lymph glands do not begin to develop for some t'.me after the lymphatic 

 vessels, since there are no indications of them in the human foetus until the latter 

 part of the third month and none in pig embryos until they have reached a 

 length of 30 mm. While it is definitely settled that lymph glands originate 

 in very close relation with the lymphatic vessels, certain points in their latter 

 development need further study. In the axilla and groin, for example, the 

 lymphatic vessels form a dense network in the meshes of which are masses of 

 connective tissue. These masses become more cellular and with the surround- 



