280 THE DEVELOPMENT OF THE VASCULAR SYSTEM 



sprouts, arising from the endothelium of veins, form the single and paired 

 sacs already described. From these five sacs the thoracic duct and 

 peripheral lymphatics develop chiefly as endothelial outgrowths. Thus, 

 lymphatic vessels grow to the head, neck, and arm from the jugular sacs ; 

 to the hip, back, and leg from the posterior sacs; and to the mesentery 

 from the retroperitoneal sac. 



Other investigators (Huntington, 1911; 1914; McClure, 1915) hold 

 that the lymph sacs are formed in situ by the fusion of discrete mesen- 

 chymal spaces which become lined with an endothelium of transformed, 

 bordering mesenchymal cells. Venous connections are purely secondary. 

 The thoracic duct and the peripheral vessels develop similarly by the 

 progressive fusion of separate clefts; hence, endothelium can differentiate 

 continually from young mesenchyma. The further growth of endothelium 

 already formed, is not denied. This general doctrine of the local origin of 

 endothelium is unquestionably correct (cf. p. 247). 



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 pro- 

 duced by a connective-tissue invasion of the primitive lymph sacs. In 

 either case, a capillary plexus, with simple connective-tissue septa, marks 

 the first stage of development. Next (Fig. 288 A), lymphocytes collect 

 in the connective tissue, forming cortical nodules which become associated 

 with blood capillaries and later acquire germinal centers. Finally, the 

 lymphoid tissue is channeled by sinuses formed from lymphatic capillaries. 

 The peripheral sinus develops afferent and efferent vessels. The central 

 sinuses cut the lymphoid tissue into medullary cords. In the larger 

 lymph glands (Fig. 288 B) the connective tissue forms a definite capsule 

 from which trabeculoe dip into the gland. 



Haemal (Haemolymph) Glands. Their origin is traced by Meyer 

 (1917) to condensations of mesenchyme which' develop in relation to 

 blood vessels, not lymphatics. The peripheral sinus arises independently; 

 its vascular connections are secondary. 



The Spleen. This appears in embryos of about 10 mm. as a swelling 

 on the left side of the dorsal mesogastrium near the dorsal pancreas (Fig. 

 289 A). The thickening is due to a temporary proliferation and invasion 

 of mesothelial cells into the underlying mesenchyme, which, meanwhile, 

 has also undergone local enlargement and vascularization. It is said that 

 these cells from the peritoneal epithelium give rise to a large part, at least, 

 of the future spleen. The splenic anlage becomes pinched off from the 

 mesogastrium (Fig. 289 B) with which it is ultimately joined by a narrow 

 band only. 



