214 BLOOD 



When extravasations of blood occur within the tissues of the body, 

 as, for example, in the corpus hemorrhagicum of the ovary, the hematin 

 is frequently deposited as hematoidin., an iron-free derivative of hemo- 

 globin which forms stellate groups of yellowish needle-like crystals.- He- 

 matoidin is apparently identical with bilirubin, a bile pigment. 



COAGULATION 



According to Howell's theory of coagulation, prothrombin, probably 

 produced at least in part by disintegrating platelets, in the presence of 

 calcium is converted into thrombin. The latter precipitates from the 

 fibrinogen of the blood plasma the fibrin needles which mass to produce 

 a mesh in which the corpuscles become aggregated to form the thrombus 

 or clot. Howell has recently studied with the ultramicroscope the process 

 of coagulation in oxalated blood plasma and solutions of fibrinogen to 

 which a solution of thrombin was added. He describes clotting as pro- 

 ceeding after the manner of crystal formation; the crystals begin as 

 granules which adhere to form threads. Clotting is f an aggregation of 

 the invisible particles (amicrons) to visible particles and then the further 

 consolidation of these particles into rigid looking needles' (Amer. Jour. 

 Physiol., 35, 1, 1914). 



HEMOPOIESIS 



A consideration of blood development involves the questions of the 

 sources of primary origin and the genetic relationship of the different 

 varieties of cells. The blood-forming or hemopoietic organs comprise (1) 

 the yolk-sac; (2) body mesenchyme and endothelium of primitive blood- 

 vessels of the embryo; (3) liver and spleen; assisted by lymph organs in 

 the production of lymphocytes, and finally (4) marrow of fetal and adult 

 bone. These several sources function in development in the order enumer- 

 ated, though the successive stages overlap to some extent. With the 

 assumption of function on the part of bone-marrow the earliest foci being 

 the scapular, pelvic, vertebral and costal elements the other blood-forming 

 organs cease activity. However, in certain diseases the spleen may possibly 

 be stimulated to renewed function; and lymph organs function throughout 

 life in the production of lymphocytes. From the standpoint of the germ 

 layers hemapoiesis is more probably limited to the mesoderm (mesenchyme). 

 This is certainly true largely at least, probably completely, of sources 

 outside of the yolk-sac. 



