198 THE ORGANS 



blood, but occur in large numbers in leukaemia. It is from the 

 myelocytes that some and possibly all leucocytes, which are of bone- 

 marrow origin, are derived. 



(2) Nucleated Red Blood Cells. — These are divisible into primary 

 erythrohlasts and secondary erythrohlasts or normoblasts. The former 

 represents an earlier, the latter a later stage in the evolution of the 

 non-nucleated adult red blood cell. The primary erythroblast, the 

 younger of the two, has a wellformed nucleus with a distinct in- 

 tranuclear network. The protoplasm contains but little haemoglobin. 

 In the secondary erythroblast the intranuclear network has disappeared 

 and the protoplasm has become richer in haemoglobin. The second- 

 ary erythroblast is converted into the adult red blood cell either 

 by extrusion of its nucleus, or by the disintegration of the nucleus 

 within the cell body. The number of erythrohlasts present in a 

 particular marrow is of course proportionate to the activity of red 

 blood-cell formation. 



(3) Giant Cells. — (a) Megakaryocytes. These are from 25-30/4 

 in diameter and are distinctly amoeboid. They have an abundant 

 granular protoplasm which is usually acidophils more rarely baso- 

 phile. Peripherally the protoplasm is comparatively free from 

 granules so that a clear exoplasm and a granular endoplasm may be 

 distinguished. The nucleus varies greatly both in shape and size. 

 It is usually single; may be spheroidal, but is more commonly lobu- 

 lated with the lobules arranged in a circle or like the letter C. There 

 are many nucleoli and centrosomes. The latter may be clumped 

 or distributed through the protoplasm. When the nucleus is circu- 

 lar or C-shaped the centrosomes lie in the centre. Both mitosis and 

 amitosis have been described in these cells. The origin and function 

 of the megakaryocytes is unknown. Wright describes them (p. 114) 

 as giving rise to the blood platelets, Stohr as probably associated 

 with the formation of leucocytes. They sometimes apparently en- 

 tirely lose their protoplasm, thus giving rise to free giant nuclei. 



(b) Polykaryocytes (myeloplaxes — osteoclasts). These are even 

 larger than the megakaryocytes, having sometimes a diameter of 

 ioo/«. They are flat with a thickness of only 6-io,« and are rather 

 scarce in adult bone. In developing bone they are numerous and 

 lie near the bone or cartilage Their protoplasm is granular and 

 frequently contains fat droplets. They are multinuclear (5 to 40) 

 and contain many centrosomes arranged in pairs, the number of pairs 

 apparently corresponding to the number of nuclei. The nuclei mz 



