NORMAL HISTOLOGY. 



maturer stages. The formation of the lymphatic vessels takes 

 place in a manner very similar to that by which the blood-channels 

 are produced. The protoplasmic net-works established by the united 

 processes of the connective-tissue corpuscles are at first solid ; sub- 

 sequently they acquire a lumen and become converted into a series 

 of protoplasmic tubes, the nuclei of whose endothelial plates are de- 

 rived from the proliferated nucleus of the original elements. The 

 earliest lymph-corpuscles are, probably, migrated mesoblastic 

 cells which have entered the young vessels. The additional coats 

 of the larger lymphatic trunks are derived from the condensation and 

 differentiation of the surrounding young connective tissue. 



The development of the lymphoid tissue occurs at a rela- 

 tively late period. The position of the future lymph-gland is indi- 

 cated by a cleft or fissure which appears within the mesoderm and 

 completely isolates the gland-area on all sides except that destined 

 to become the future hilum, where the tissue devoted to the produc- 

 tion of the gland and the surrounding mesoderm are continuous. 

 The development of the lymphoid tissue is marked by increased 

 numbers and greater compactness of the mesodermic elements ; the 

 supporting reticulum, the capsule, and other details of the adenoid 

 tissue appear later. 



The development of the spleen begins about the commence- 

 ment of the third month, some time after the pancreas has become 

 defined; a condensation of the mesodermic cells, lymphoid in 

 character, within the posterior mesentery, or the mesogastrium, in 

 the near vicinity of the pancreas, is the earliest indication of the 

 future organ. The lymphoid aggregation first established is sup- 

 plemented by the elements lying beneath the peritoneum, which 

 differentiate into elongated spindle-cells especially devoted to the 

 formation of the trabeculse and connective-tissue framework. 

 Numerous blood-vessels soon grow into the splenic tissue, the sub- 

 sequent accumulations of lymphoid cells within the tissue around 

 some branches of the arteries giving rise to the Malpighian cor- 

 puscles. 



The history of the development of the thymus body demon- 

 strates an origin markedly at variance with the character of the fully- 

 formed organ, since, notwithstanding the pronounced lymphatic type 

 of the tissue constituting almost the entire body when most complete, 

 its structure in the earliest stages corresponds entirely to embryonal 

 epithelium which is derived as the direct outgrowth of the ento- 

 derm. The first trace of the thymus body appears as a cylindrical 

 bud of entodermic tissue springing on either side from the third 

 pharyngeal pouch, or inner visceral furrow. The epithelial nature 

 of the early thymus is for some time very evident, the original cell- 



