THE HISTOGENESIS OF DENSE LYMPHATIC TISSUE 171 



One factor apparently common to the formation of lymphatic 

 tissue anywhere is that of an extensive supply of lymph vessels 

 or lymph plexuses. It seems probable, therefore, that there is 

 some influence exerted by the lymph or lymph vessels on the 

 surrounding mesenchymal or loose connective tissue to induce 

 lymphopoiesis. These lymph plexuses are always found in places 

 where the blood supply is also very extensive. This very rich 

 blood supply affords excellent nutritive conditions for the growth 

 and multiplication of lymphocytes after the lymphocytopoietic 

 reaction has been initiated by the lymph plasma or the lymph 

 vessels. 



Coincident with the differentiation of mesenchymal cells of 

 the mucosa into free lymphocytes, there occurs a formation by 

 other cells of the mesenchymal reticulum of connective tissue. 

 This is at first a richly cellular, embryonic connective tissue. As 

 the formation and proliferation of lymphocytes proceeds, the 

 free cells separate the fibers of the connective tissue so that a 

 branching, reticular network of fibers, reticular tissue, is formed, 

 a skeletal framework, in the meshes of which the lymphocytes 

 proliferate. 



The first masses of .cells formed in the region of the future 

 Peyer's patch and of the nodular tissue of the appendix, com- 

 posed mainly of small lymphocytes and transitional stages be- 

 tween them and mesenchymal cells or those of the embryonic 

 connective tissue, bear little resemblance to adult lymphatic 

 nodules (fig. 1). Lymphoid hemoblasts appear first to any extent 

 at an age of eight days or later. Until this time the formation of 

 most of the free cells has been by differentiation from mesen- 

 chymal cells. Many transitional stages between mesenchymal 

 cells and free lymphocytes were seen in all preparations of 

 tissues studied up to this time in development. At about the 

 ninth or tenth day after birth mitotic figures begin to appear 

 with some frequency, especially in the small lymphocytes, and 

 the masses of cells at the same time begin to assume a more defi- 

 nite nodular shape. Lymphoid hemoblasts constantly increase 

 in number, both by differentiation from mesenchymal cells and 

 by growth from small lymphocytes. These lymphoid hemoblasts 



