288 



THE DEVELOPMENT OP THE VASCULAR SYSTEM 



region from the jugular sacs. Connective-tissue septa occur between the vessels 

 of the lymphatic network. Next lymphocytes collect in the connective tissue and 

 a capillary network is formed connected with an artery and vein. The lymphocytes 

 multiply and form lymphoid tissue (Fig. 280, A), and a peripheral lymph sinus is 

 developed with afferent and efferent lymphatics. The blood-vessels enter the 

 lymph gland at one point, the hilus. Soon the lymphatic vessels invade the 

 lymphoid tissue and form anastomosing channels, so that the lymph enters at the 

 periphery and is drained through the hilus. In the larger glands (Fig. 280, B) 

 the connective tissue forms a definite capsule and extends through the gland as 

 cords or trabeculce, in which course the larger blood-vessels. At the periphery of 

 the gland the lymphocytes divide actively and form dense lymph nodules with 



Lig. hepato-gastric 



' Ccelom epithel. 

 ~Gk - -Mesoderm 



Stomach 



T '""'.- Mesog. post. 

 Ccelom epithel. 



FIG. 281. Two stages in the early development of the spleen: A, from an embryo of 10.5 mm. 

 (Kollmann); B, from a 20 mm. embryo (Tonkoff). 



germinal centers wherein the lymphocytes are actively dividing. The peripheral 

 nodules constitute the cortex of the gland. At the center of the gland, and near 

 the hilus, the network of lymph sinuses divides the lymphoid tissue into anasto- 

 mosing cords and this region of the gland becomes the medulla. 



Haemolymph glands, according to Schumacher, begin their development 

 like lymph glands, but soon after the formation of the peripheral sinus the lym- 

 phatic connections degenerate and the blood escapes from the blood capillaries 

 into the sinuses. 



The Spleen. Little is known of the early development of the spleen in hu- 

 man embryos beyond the fact that it originates as a thickening of the dorsal 

 mesogastrium due to the division of the cells of the peritoneal epithelium (Fig. 

 281). The spleen is a lymphoid organ in which blood sinuses, instead of lymph 



