THE LYMPHATIC SYSTEM 



281 



Rapidly accumulating evidence seems to favor the latter view. The method 

 of injection, upon which Miss Sabin and her followers have largely relied, has its 

 Umitations, for it obviously can furnish no information regarding discrete spaces 

 prior to their linkage into continuous channels. Concise summaries of these views 

 are given by Sabin (Anat. Rec, vol. 5, 1911) and by McClure (Anat. Rec, vols. 5, 

 1912; 9, 1915). 



Lymph Glands. — Paired lymph glands appear during the third month, first 

 in the axillary, iliac, and maxillary regions. Those from the lymph sacs develop 

 later. Plexuses of lymphatics first form either as ordinary networks of peripheral 

 vessels or as secondary networks produced by a connective tissue invasion of the 

 primitive lymph sacs. In either case a capillary plexus, with simple connective 





Mesolhelium-^^^^^ 



Mesothdium 



'^■■Mesoderm 



—Spleen anlage 

 A "" 



Fig. 288. — Two stages in the early development of the human spleen (dorsal is below). A, from an em- 

 bryo of 10.5 mm. (Kolhnann); B, from a, 20 mm. embryo (Tonkofi). 



tissue septa, marks the first stage of development. Next (Fig. 289 A), lympho- 

 cytes collect in the connective tissue, forming follicles which become associated 

 with blood capillaries. Finally, the lymphoid tissue is channeled by sinuses 

 formed from l)miphatic capillaries. The peripheral sinus develops afferent and 

 efferent vessels. The central sinuses cut the lymphoid tissue into cords. In the 

 larger lymph glands (Fig. 289 B) the connective tissue forms a definite capsule 

 from which trabeculae dip into the gland. 



Hemolymph glands, according to Schumacher (Arch. f. mik. Anat., Bd. 81, 

 1912), begin their development like lymph glands, but soon after the formation 

 of the peripheral sinus the lymphatic connections degenerate and blood escapes 

 from the blood capillaries into what are henceforth hlood sinuses. 



Spleen.— This appears in embryos of about 10 mm. as a swelHng on the left 



