CHAP, i.] BLOOD. 37 



the protoplasm and which we may regard as food for itself, into 

 one or other or both of these bodies. 



When this differentiation has taken place or while it is still 

 going on, the material in which the nuclei are imbedded divides 

 into separate cell bodies for the several nuclei ; and thus the nest 

 of nuclei is transformed into a group of nucleated red corpuscles, 

 each corpuscle consisting of a nucleus imbedded in a haemoglobin- 

 holding stroma to which is still attached more or less of the original 

 undifferentiated protoplasm. 



Still later on in the life of the embryo the nucleated red cor- 

 puscles are replaced by ordinary red corpuscles, by non-nucleated 

 discs composed almost exclusively of haemoglobin-holding stroma. 

 How the transformation takes place, and especially how the nucleus 

 comes to be absent is at present a matter of considerable dispute ; 

 there is much however to be said for the view that the normal 

 red corpuscle is a portion only of a cell, that it is a fragment of 

 cell substance which has been budded off and so has left the 

 nucleus behind. 



In the adult as in the embryo the red corpuscles appear to be 

 formed out of preceding coloured nucleated cells. 



In the interior of bones is a peculiar tissue called marrow, 

 which in most parts, being very full of blood vessels, is called red 

 marrow. In this red marrow the capillaries and minute veins 

 form an intricate labyrinth of relatively wide passages with very 

 thin walls, and through this labyrinth the flow of blood is compara- 

 tively slow. In the passages of this labyrinth are found coloured 

 nucleated cells, that is to say, cells the cell-substance of which has 

 undergone more or less differentiation into haemoglobin and stroma. 

 And there seems to be going on in red marrow a multiplication of Ap 

 such coloured nucleated cells, some of which transformed, in some 

 way or other, into red non-nucleated discs, that is into ordinary 

 red corpuscles, pass away into the general blood current. In other 

 words, a formation of red corpuscles, not wholly unlike that which 

 takes place in the embryo, is in the adult continually going on in 

 the red marrow of the bones. 



According to some observers the coloured nucleated cells arise 

 by division, in the marrow, from colourless cells, not unlike but 

 probably distinct in kind from ordinary white corpuscles, the 

 formation of haemoglobin taking place subsequent to cell division. 

 Other observers, apparently with reason, urge that, whatever their 

 primal origin, these coloured nucleated cells arise, during post- 

 embryonic life, by the division of previous similar coloured cells, 

 which thus form, in the marrow, a distinct class of cells continually 

 undergoing division and thus giving rise to cells, some of which 

 become red corpuscles and pass into the blood stream, while others 

 remain in the marrow to undergo further division and so to keep 

 up the supply. Such repeatedly dividing cells may fitly be called 

 hcematoblasts. 



