BLOOD. 119 



marrow. The formation of erythrocytes in the spleen and 

 liver takes place in embryonic life. In the adult individual it 

 is exclusively in the bone-marrow. 



The colorless blood-corpuscles (leucocytes) are derived, 

 according to some authors, from a different kind of cells which, 

 in contradistinction to erythroblasts, are called leucoblasts. 

 Originally the embryonic blood contains no leucocytes. These 

 come later, after the development of the lymph glands. In 

 post-embryonic life leucocytes increase in connective tissue and 

 in the so-called germinal centres (see below), which are present 

 in adenoid tissue. 



It is believed almost generally now that the polymorphonu- 

 clear leucocytes are derived from the myelocytes of the bone- 

 marrow ; and that lymphocytes arise in the germinal centres 

 of the lymph glands. Uskow and others regard the polymor- 

 phonuclear leucocytes as the terminal stage in the development 

 of leucocytes, and that the other forms constitute a series which 

 leads up to this stage. 



The blood as a whole has two main functions, namely, to 

 carry oxygen to the various parts of the body from the lungs, 

 Miul to carry waste products to the different excretory organs. 

 As mentioned above, the white corpuscles have a protective 

 action in the so-called phagocytosis. According to some 

 authors, they not only are capable of surrounding and digesting 

 bacteria, etc. ; but they also secrete a substance (alexine) which 

 is a powerful poison for bacteria. 



The coagulation of blood, which takes place as soon as it 

 leaves the vessels, or in the vessels after death, is dependent on 

 the formation of fibrin. Details concerning this process should 

 be sought for in works on physiology. Fibrin appears in thin 

 layers, in the form of fine fibrils which form a network. This 

 forms the clot which is characteristic of coagulated blood, and 

 contains within the meshes of the fibrin network all the formed 

 elements of the blood. 



Haemoglobin, the coloring matter of the red corpuscles, sepa- 

 rates in most animals under certain conditions in the form of 

 rhombic crystals. Such crystals are found in old alcoholic 



