76 THE BLOOD 



rare in man, but occurs in greater numbers in lower vertebrates. 

 It has less motility than either of the other types of granulocytes 

 and little is known concerning its function. The invasion of foreign 

 materials may produce an increase in their number. They closely 

 resemble the mast cells found in loose fibroelastic connective tissues. 



Platelets.— In addition to the red and white cells of the blood 

 there are found in mammals certain cytoplasmic elements called 

 platelets. These are much smaller than the red cells and in smears 

 take the blue basic dye and appear in clumps or masses. They are 

 thought to be associated with blood clotting, though not found in 

 clots. Their origin is traced to large giant cells in the bone-marrow, 

 known as megakaryocytes, which are thought to extend pseudopodia 

 into the marrow sinuses where they are cut off as platelets. 



Thrombocytes or Spindle Cells.— In the blood of vertebrates below 

 mammals platelets are not usually found, but a cellular element, 

 the spindle cell or thrombocyte, is thought to have an analogous 

 role in blood clotting. These cells are about the size of the red cells 

 or lymphocytes and have a heavily staining spherical or oval nucleus 

 in which a nucleolus is usually not present. The cytoplasm is clear 

 and varies in its reaction from a more or less neutral to a lightly 

 basophilic condition. The cells appear oval or spindle-shaped in 

 outline and several cells may fuse and give the appearance of a cyto- 

 plasmic mass with several nuclei. The origin of these cells is traced 

 to differentiation of the endothelial cells and of cells indistinguishable 

 from lymphocytes. 



Blood Cell Formation. Hemapoiesis is the name given to the 

 process of blood cell formation, and a study of it in different groups 

 of vertebrates shows some variation in its location. Embryologically 

 blood cells as well as the blood vessels originate in mesenchyme, 

 occurring first as isolated masses or cords, called l)lood islands, in 

 the wall of the yolk sac. The peripheral cells of these masses be- 

 come flattened and form the endothelium of the vessels, while the 

 central cells become surrounded by plasma and form the first blood 

 cells. Other similar centers arise from the mesenchyme of the 

 embryo proper and with subsequent development a vascular net- 

 work is formed that gradually gives rise to a system of blood vessels 

 and developing blood cells. During the early stages blood cells are 

 formed by division of the primitive blood cells in the vessels and to a 

 lesser extent from the surrounding endothelial cells. After this early 

 embryonic stage, blood cells no longer proliferate in the Aessels, but 



