388 Development of the Lymphatic Nodes in the Pig 



The sinus develops by a proliferation of the cndotlicliura of the ducts, 

 and so, as Kling pointed out, each space in the sinus has its complete ring 

 of endothelium. The fact that the sinus is made of a great number of 

 small lymph ducts packed closely together explains why one cannot get 

 the silver picture of a membrane of endothelium far beyond the periphery 

 of the node. None of the membranes are large enough. In the em- 

 bryonic stages the connective tissue bridges are largely protoplasmic and 

 show connective tissue nuclei. In the adult the bridges are a network 

 of reticulum fibrils so that the endothelial cells appear on the fibers. 



The study of the contents of the sinuses will prove, I think, an 

 important point in the physiology of the lymph nodes. There is a 

 marked difference between the embryonic nodes and the adult in this 

 respect. In the embryonic nodes there are few free, wandering cells 

 in the sinuses as compared with the adult. In the early stages there are 

 almost no free cells in the ducts. After the lymphocytes appear, they 

 occur occasionally in the ducts and sinuses, as well as a few large mono- 

 nuclear forms. As a rule the sinuses are nearly empty of the free cells, 

 so that the bridges stand out with great beauty and clearness. In the 

 adult node, on the other hand, the content of the sinus in free cells is 

 most varied, both in the number and in the kind of cells. Sometimes 

 the sinuses are so packed with cells that the reticulum and endothelium 

 are almost covered up. It often happens that the sinuses of one por- 

 tion of a section are densely packed with cells, while in another portion 

 they are almost empty. These free cells may be any type of white blood 

 cell or may be large phagocytic cells. The sinuses may be filled with 

 phagocytic cells that are crowded either with red blood corpuscles or, in 

 the mesenteric nodes, with fat globules. 



Reticulum. — The complete relation of the connective tissue frame- 

 work or reticulum can only be clear after noting the nature of the sinus. 

 The reticulum fibers first appear in the germ centers where they make 

 a mosaic pattern around the capillary tuft. The trabeculse develop from 

 the capsule in connection first with the sinuses and secondly with the 

 blood-vessels, especially the veins. The reticulum fibers are laid do^vm 

 in a very close protoplasmic syncytium, and this syncytium remains pro- 

 toplasmic long after the surrounding connective tissue has become pre- 

 dominately fibrillar. The reticulum fibers do not show until the embryo 

 is 15 or 16 cm. long. They appear first in the germ centers and in the 

 capsule. In an embryo 23 cm. long they are limited to the capsule and 

 a few trabeculse, while up to the time of birth the connective tissue frame- 

 work is still largely protoplasmic. 



