THE RED BLOOD-CORPUSCLES 941 



fresh haemoglobin and new red corpuscles. The seat of the formation 

 of red corpuscles in the higher vertebrates is the bone-marrow. Here 

 we have a structure protected from pressure where the capillaries and 

 veins are dilated and thin-walled, and allow a slow passage of blood 

 and the entry of newly formed corpuscles through the imperfect walls 

 into the blood-stream (Fig. 361). That the marrow is the tissue involved 

 in the process is shown by the fact that it is the only tissue of the body 

 which undergoes an alteration in appearance when blood formation is 



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FIG. 361. Section of red marrow of mammal. (BoHM and DAVIDOFF.) 



a, e, erythroblasts ; b, reticulum; c, myeloplax ; d, g, marrow cells ; 

 /, a marrow cell dividing ; h, a space which was occupied by fat. 



stimulated by such means as repeated bleeding or destruction of 

 corpuscles by the injection of toxic agents. Under such conditions 

 the red marrow, which in adult mammals is present only in the 

 epiphyses, is found to have increased in extent and in many cases to 

 occupy the greater part of the shaft of the bone, having taken the 

 place of the yellow marrow. It is in the red marrow therefore that 

 we must seek the precursors of the red blood-corpuscles. In the bird 

 the erythroblasts, i.e. the precursors of the red blood-corpuscles, form 

 a sort of inner lining to the dilated capillaries of the marrow (Fig. 362). 

 Here we can see all grades between the colourless nucleated corpuscle 

 which lies nearest the periphery and the fully formed red oval corpuscle 

 containing haemoglobin, lying next the lumen and ready to be carried 

 away in the blood stream. If blood formation has been stimulated 

 by repeated bleeding, this blood-forming tissue is found to occupy the 

 greater part of the lumen of the marrow capillaries. If, however, 

 blood formation has been reduced to its lowest extent by a process of 

 chronic starvation, the erythroblasts form a single layer of cells just 



