THE VASCULAR SYSTEM 363 



to connect these with the lymphatic nodes, the extreme theory 

 being that here is the center of origin for all the latter, 

 and that they migrate from these patches during development, 

 first invading the mesentery and forming the mesenteric 

 glands; thence passing from these along the lymphatic chan- 

 nels to all parts of the body. As the nodules of Peyer's 

 patches are endodermic in origin, it would follow that, with 

 such an origin, all of the lymphatic nodes, wherever found, 

 must be also endodermic, and it was thus held that we had here 

 an example of elements, originally endodermic, wandering 

 over the entire body and invading practically all the tissues. 

 The more- recent exposition of the development of the lym- 

 phatic system, as given above, renders such theories no longer 

 tenable and shows the lymphatic system, at least in mammals, 

 to be a definite system, budding out from that of the circula- 

 tion, and, like it, mesenchymatous in origin. Whether this 

 is the case in the lower Classes of vertebrates and whether 

 the various spaces utilized by the lymph can be thus derived 

 cannot be ascertained until the development of the lymphatics 

 in these forms is as well known as it is in mammals ; but with 

 our present knowledge it seems probable that certain definite 

 channels that possess walls of their own, like the sub-verte- 

 bral space, are produced as outgrowths from the blood-vessels, 

 and that these enter into secondary communication with nu- 

 merous irregular spaces which can well be utilized as adjuncts 

 of the lymphatic system until their function can be supplied 

 by definite lymphatic vessels. 



Aside from the solitary and aggregated nodules, both of 

 which appear to be centers of origin of leucocytes, there are 

 numerous other places in which the cellular constituents of 

 the blood are developed. Many of these, as in the case of the 

 aggregated nodules of the intestine, are developed within the 

 wall of the alimentary canal and are therefore endodermic in 

 origin. These include the tonsils, the thymus, and thyreoid 

 glands, the associated epithelial bodies, and perhaps, the spleen. 

 The marrow of the bones is especially important in this re- 

 spect, and develops large quantities of the blood-cells, espe- 



