THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



249 



which transport blood cells and which have been defined as haemophoric 

 lymphatics, or veno-lymphatics, fulfil their haemophoric function they are 

 retained as permanent lymph channels in the general lymphatic organiza- 

 tion. In a broader interpretation, the haemophoric function of certain 

 lymph vessels during ontogeny is particularly significant in that it indicates 

 essential and fundamental similarity of lymphatic vascular development to 

 haemal vascular development. 



The Lymph Glands. 



The lymph glands do not begin to develop for some time after the lym- 

 phatic 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 thev 



Efferent lymph, ves. 



Blood vessel 



'. '' V ' * " "" *'*-' : &" V >' V''' V 



iH K|r Marginalsi 



mmm-A 



i*.'.:,V*:&v' .;; Capsule 



S&W&fi? ''i'> 



P^S :'V{ 



^^,x-^a 



rtv ,; v* 



' '. / 'I \ **~ ," ' fV 



Afferent ^.,1. r ..;/'"'<"- ,r ?'^^ 

 lymph, ves. i s , j ,*; ; j ,\vW'l 



U . j >. , , 



FIG. 218. From a section through the axilla of a human embryo of 125 mm. (4-5 months), 

 showing an early stage of a lymph gland. Kling. 



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 later 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 surrounding vessels constitute the anlagen of lymph 

 glands (Fig. 218). The new cells which appear in the masses are lympho- 

 cytes which may pass through the walls of the neighboring blood vessels and 

 lodge here or may be derived directly from connective tissue (mesenchymal) 

 cells in situ. Whatever the origin of the lymphocytes may be, they have the 

 opportunity here to divide freely. The mass becomes still more cellular and 

 enlarges at the expense of the lymphatic vessels which then come to form a 



