380 Dovelopnieiit of the Lymphatic Nodes in the Pig 



the pig at this stage have the center made of definite cords and sinuses 

 and the entire periphery made of a plexus of lymph ducts; and this is 

 the condition of the primary lymph node in a pig 2 weeks old. That is 

 to say, there are three kinds of tissue in the nodes : first, lymph cords 

 and germ centers ; second, lymph sinuses ; and thirdly, plexuses of lymph 

 ducts not yet transformed into sinuses. It is clear that the lymph 

 plexus is a stage in the development of tlie lymph sinus, and hence is a 

 less highly developed structure. 



These points are clearly shown in Fig. 15, which is taken from the 

 inguinal node in a pig 2-1.5 cm. long. A portion of the capsule is shown 

 which is not as yet a limiting membrane. The outer part of the node 

 consists of a plexus of h'mph ducts with connective tissue bridges. The 

 nuclei in these bridges are large and oval. The inner portion of the 

 node, away from the capsule, consists of lymph cords and sinuses. 

 Within the cords the predominating cell is the lymphocyte, with which 

 the germ centers are closely packed. The sinuses are groups of lyiuph 

 ducts and transitions between the lymph plexus of the edge of the node 

 and of the developed sinus are to be seen. 



In the pig the lymph plexus persists up to adult life. In specimens 

 where the lymph ducts are all collapsed, these areas look like masses of 

 connective tissue, hence Delamere^ pictures them and calls them homo- 

 geneous areas. Eanvier " found the same tissue in the ]uesenteric nodes 

 of the pig, but since his specimens had the lympli ducts distended he 

 called the areas cavernous or erectile tissue. As has been said, in study- 

 ing large numbers of lymph nodes in fresh specimens one often finds the 

 lymph ducts or sinuses so distended Avith fluid that all the spaces are 

 rounded. The presence of this lymph plexus not developed into a sinus 

 in the nodes in the pig is, I believe, an important point not only in the 

 study of development but in the understanding of the structure of 

 allied organs. In all of the hsemolymph nodes I have seen, there have 

 been three types of tissue, the lymphoid areas, true sinuses, and zones 

 filled with blood but not definitely sinuses. These zones appear like 

 veins not crowded enough to be sinuses. 



We have thus followed the development of the primary lymph node 

 which comes from the lymph sac. The stages are, in brief : first, a 

 preliminary stage in which the entire node consists of a plexus of 

 lymphatic capillaries with connective tissue bridges containing blood 



" Poirer, Cuneo, and Delamere: The Lymphatics, p. 100, translated from 

 Poirier's Anatomie. 



^"Ranvier: C. R. Acad. Sciences, 1S95, p. 800, or C. R. Soc. Biol., 1895, p. 774. 



