BLOOD AND LYMPH 217 



the body, occurring either in patches of varying shape and size, 

 or as regular organs, the lymphatic glands (fig. 104). These 

 are placed on the course of lymphatic vessels, and consist of 

 a sponge-work of fibrous tissue, in the interstices of which 

 are set the patches of lymph tissue or germ centres, each 

 surrounded by a more open network, the sinus, through which 

 the lymph flows, carrying away the lymphocytes, which are 

 the characteristic elements produced, from the germ centres. 

 Round some of the lymphatic glands of certain animals large 

 blood spaces or sinuses are seen, and these glands are called 

 hsemolymph glands (fig. 106). They are intermediate between 

 lymphatic glands and the spleen. When erythrocytes are 



FIG. 104. Section of a Lymph Gland, a, capsule ; b, germ centres of cortex ; 

 c, sinuses ; d, trabecula ; e, germ centres of medulla. 



destroyed by hsemolytic agents the pigment and the iron 

 derived from the haemoglobin are often found abundantly in 

 the cells in the sinuses of lymph and hsemolymph glands. 



2. Bone Marrow. The structure of bone marrow is con- 

 sidered below, but it may be stated here that young leuco- 

 cytes or leucoblasts, in the condition of mitosis, are abundant, 

 and that they pass away in the blood stream. They are of 

 all varieties. In certain pathological conditions the formation 

 of these cells is increased and a leucocytosis results (fig. 105). 



II. Erythroeytes. In the embryo these cells seem to be 

 formed by a process of budding from the mesoblast cells, 

 which become vacuolated to form the primitive blood vessels. 

 Later they develop in the liver and spleen, but after birth 



