356 Developiiiont of tlie Lyniphatie Xoilos in the Pig 



ing heart in the aniphil)ia becomes transformed into a node in the higlier 

 forms. The lym])li heart is the point from which ducts radiate in de- 

 velopment to drain a large area, generally speaking a quarter of the 

 surface of the body, and here the primary lymph node develops. Subse- 

 quently there are secondary points, from which ducts growing from the 

 Ivmph heart radiate to drain lesser areas, and here other nodes are 

 formed. These are subcenters through which lymph passes before reach- 

 ing the primary node. 



As to the nodes themselves, the study of their development Ijrings out 

 that they are made of two fundamental structural elements. First, a 

 lymphoid or adenoid tissue, consisting of lymphocytes in a reticulum 

 around the blood-vessels making the lymph cords and germ centers; and 

 secondly a lymphatic tissue, or sinus, made of large numbers of lymph 

 ducts closely packed together. In brief, an ordinary lymph node is a 

 blood vascular organ, made in part of structures derived from the blood- 

 vessels and in part of structures derived from the lymphatics. The vas- 

 cular unit of the node is the terminal artery and its capillary plexus, the 

 artery being bordered by the cord and the capillary tuft surrounded by 

 the germ center. These two elements, the vascular and the lymphatic, 

 are found in varying proportions in the ordinary node. However, both 

 in the embryo and in the adult, either element may be found alone. In 

 the emliryo and probably in the adult pig, there are small lymph follicles 

 consisting of a tuft of blood papillaries surrounded by lymphocytes and 

 entirely without a sinus. In the hfemolymph node and in the spleen the 

 same type of lymphoid tissue is found, but here the sinuses are made, 

 not of lymph ducts, but of veins. Thus in the blood vascular organs 

 the lymphoid element consisting of lymphocytes in the adventitia of an 

 artery is constant, while the sinus element varies, being absent, or made 

 of veins, or of the modified veins wdiich are called lymphatics. 



Throughout the paper certain terms have been adopted. Lymph node 

 is used as a general name to cover all lymphatic glands; the term follicle 

 is used to represent a simple node consisting of the structures that go 

 with a single artery. The simplest node consists of one follicle; other 

 nodes are groups of follicles. The follicle is the anatomical or structural, 

 unit; it is also the vascular unit. The follicle may be without a sinus, 

 or surrounded by a lymphatic sinus or surrounded by a venous sinus. 



The term lymph heart has been retained notwithstanding that in the 

 pig there is no muscle in its wall at any stage. It is, however, a sac 

 from which all the ducts for the skin radiate and in that sense is 

 homologous with the lymph heart of the amphibia. 



