386 Development of the Lvmpliatic Nodes in the Pig 



plexus not transformed into the sinus. Thus the hsvmolymph node is a 

 later and less developed organ occurring along the l)loo(l-vessels. 



The luvmolymph node found in the neck of the pig is from the l)egin- 

 ning a distinct organ, different in type from the lymphatic node. In the 

 adult pig these IiaMnolymj)li nodes in the neck are sometimes fused into 

 the same capsule witli an ordinary lymph node, hut the two remain as 

 distinct structures with trabecuhv between. 



Summary and General Discussion. 



Follicle.— From the study of the development of lymph nodes we find 

 that there are in general three types of follicles. The simplest follicle 

 consists of a collection of lymphocytes in a reticulum around an artery 

 and its capillaries. This type occurs in the embryo and probably in the 

 adult. The second type consists of the follicle of the first type sur- 

 rounded by the lymphatic sinus. This type occurs either singly or in 

 groups in the ordinary lymphatic gland. In the third type the lymphoid 

 follicle is surrounded by a blood sinus or a less developed plexus of blood- 

 vessels. This occurs in the hai-molymph node and spleen. 



Lymphocytes. — In connection with the nature of the lymphoid tissue 

 it has already been brought out that the lymphocyte occurs in a fine 

 reticulum around the artery and its capillaries. For the ultimate origin 

 of the lymphocyte we have as yet no proof, but the lymphocytes divide, 

 as Flemming showed, in the germ centers. The germ centers are definite 

 organs around capillary tufts or glomeruli. In the embryo the division 

 of lymphocytes is so constant that the germ centers are always filled 

 with them. The lymphocyte develops independently from the lymph 

 ducts. 



General structure and rjroirtli. — In regard to the general structure 

 of the node, it has been shown to consist of two elements — a lymphoid 

 or vascular and a sinus element, which is either venous or lymphatic. 

 Through the study of the development of the nodes, it becomes clear 

 that the sinus comes from a plexus of ducts or vessels by increasing the 

 number of ducts and reducing the size of the connective tissue bridges. 

 In the pig the plexus element of the node is not completely transformed 

 into the sinus in the adult, so that the node has three structures, the 

 lymphoid element, the plexus, and the sinus. There are wide variations 

 in the proportions of these elements. 



The question of growth and increase in size of the lymph node is an 

 interesting one, closely liound up with the process of absorption in devel- 

 opment. All of the diagrams show that the nodes in the early stages 

 increase in size by the invasion of surrounding tissue by lymph ducts. 



