Florence K. Sahiii 387 



This point is hrou.iilit out hv coni})aring Figs. 5 and G, from the primary 

 node at 3.(j and 4.!) em. long. The sections are cut in the same plane 

 and can be related by the sac. It a^II be seen that the increase in size of 

 the second is due to the development of the lymph plexus. This invasion 

 of ducts can take place as long as there is no definite capsule. The cap- 

 sule is a late development, as shown in the last diagram, where it is still 

 incomplete. After the capsule is formed the nodes increase in size by 

 the fusion of many small nodes in the neighborhood. The capsules of 

 the smaller nodes become the trabecule of the larger ones. The marked 

 tendency of the fusion of nodes in the pig has been noted by other ob- 

 servers. In the pig not only do the ordinary lymph nodes fuse, but a 

 haemolymph node may fuse with a ])ure lymph node and the two remain 

 distinct but enclosed in a common capsule. The primary lymph node 

 from the lymjih sac, and the node over the crest of the ileum show but 

 few of the small nodes in the neighborhood and hence few evidences of 

 fusion. On the other hand, the inguinal node which represents a group 

 of nodes in higher forms, appears like a conglomerate of small nodes in 

 the new-born pig. 



The lymph nodes give inuch evidence of a repeated tearing down and 

 relniilding in the process of growth. For example, in passing from the 

 stage of the first to the second diagram, there must be a destruction of 

 many lymph duets in the formation of the primordial follicle. It will 

 be noted that this destruction is not complete in Fig. 8, for there are a 

 few lymph duets scattered through the follicle. In following through the 

 diagrams it will be noted that there is a constant change in the propor- 

 tions of the lymphoid or vascular portion and the lymphatic portion of 

 the node. The small nodes of Fig. 16 make this point clearer. The 

 youngest ones have a predominance of the lymphoid portion, the very 

 youngest ones having no lymph ducts at all. Others have merely a peri- 

 pheral sinus, while others slightly larger are made almost entirely of 

 a plexus of lymph ducts. As these nodes develop farther the lymphoid 

 tissue will increase until the balance of lymphoid tissue, sinus, and 

 lymph plexus characteristic of the adult node is reached. This point 

 illustrates the extreme variation met with in the development of the 

 lymphatic apparatus. 



Sinus. — The lymph sinus develops out of the lymph ducts by a mul- 

 tiplication of the ducts along certain lines. The areas in which the 

 lymph ducts multiply until the connective tissue bridges are reduced to 

 the thickness of fibers, are determined by the blood-vessels. That is, the 

 einuses grow in between the arteries thereby bounding the lymph cords. 



