CH. XXVI.] DEVELOPMENT OF BLOOD-CORPUSCLES 425 



squares is then taken, and this number multiplied by 4000 x 100 gives the number 

 of corpuscles in a cubic millimetre of undiluted blood. 



Dr George Oliver's Haemacytometer is a much easier instrument to use, and 

 the results obtained are accurate ; it does not enable one, however, to ascertain the 

 proportion of red and white corpuscles. A small measured quantity of blood is 

 taken up into a pipette and washed out into a graduated flattened test-tube with 

 Hayem's fluid (sodium chloride 0'5 gramme, sodium sulphate 0*25 gr., corrosive 

 sublimate '25 gr. , distilled water 100 c.c.). The graduations of the tube are so 

 adjusted that with normal blood (i.e., blood containing 5,000,000 red corpuscles per 

 cubic millimetre) the light of a small wax candle placed three yards from the eye in 

 a dark room, is just visible as a thin'bright line when looked at through the tube 

 held edgeways between the fingers, and filled up to the 100 mark with Hayem's 

 fluid. If the number of corpuscles is less than normal, less of the diluting solution 

 is required before the light is transmitted ; if more than normal, more of the solu- 

 tion is necessary. The graduations of the tube correspond to percentages of the 

 normal standard which is taken as 100. 



Development of the Blood-Corpuscles. 



The first formed blood-corpuscles of the human embryo differ 

 much in their general characters from those which belong to the 

 later periods of intra-uterine, and to all periods of extra-uterine, life. 

 Their manner of origin is at first very simple. 



Surrounding the early embryo is a circular area, called the 

 vascular area, in which the first rudiments of the blood-vessels and 

 blood-corpuscles are developed. Here the nucleated embryonic cells 

 of the mesoblast, from which the blood-vessels and corpuscles are to 

 be formed, send out processes in various directions, and these, joining 

 together, form an irregular meshwork. The nuclei increase in number, 

 and collect chiefly in the larger masses of protoplasm, but partly also 

 in the processes. These nuclei gather around them a certain amount 

 of the protoplasm, and, becoming coloured, form the red blood - 

 corpuscles. The protoplasm of the cells and their branched network 

 in which these corpuscles lie then become hollowed out into a system 

 of canals enclosing fluid, in which the red nucleated corpuscles float. 

 The corpuscles at first are from about ^rcr to TTDTT f an i ncn (10/* 

 to 16/x) in diameter, mostly spherical, and with granular contents, 

 and a well-marked nucleus. Their nuclei, which are about nnnr f 

 an inch (5yu) in diameter, are central and circular. 



The corpuscles then strongly resemble the colourless corpuscles 

 of the fully developed blood, but are coloured. They are capable of 

 amoeboid movement and multiply by division. 



When, in the progress of embryonic development, the liver begins 

 to be formed, the multiplication of blood-cells in the whole mass of 

 blood ceases, and new blood-cells are produced ^by this organ, and 

 also by the lymphatic glands, thymus and spleen. These are at first 

 colourless and nucleated, but afterwards acquire the ordinary blood- 

 tinge, and resemble very much those of the first set. They also 

 multiply by division. In whichever way produced, however, whether 



