422 



THE BLOOD. 



[CH. xxvi. 



have in the mixture the proper dilution. The blood and the saline solution 

 are well mixed by shaking the pipette, in the bulb of which is contained a 

 small glass bead for the purpose of aiding the mixing. The other part of 

 the instrument consists of a glass slide (fig. 353) upon which is mounted 



Fig. 353- 



a covered disc, ?, accurately ruled so as to present one square millimetre 

 divided into 400 squares of one-twentieth of a millimetre each. The micro- 

 meter thus made is surrounded by another annular cell, c, which has such a 

 height as to make the cell project exactly one-tenth 

 . millimetre beyond m. If a drop of the diluted blood 



is placed upon m, and c is covered with a perfectly flat 

 cover-glass, the volume of the diluted blood above each 

 of the squares of the micrometer, i.e., above each Jg, 

 will be jig of a cubic millimetre. An average of ten 

 or more squares are then taken, and this number multi- 

 plied by 4000 x too gives the number of corpuscles in 

 a cubic millimetre of undiluted blood. 



Dr. George Oliver's Hasmacytometer is a much easier 

 instrument to use, and the results obtained are accurate ; 

 it does not enable one, however, to ascertain the pro- 

 portion 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 o'5 gramme, sodium sulphate 

 o - 2S gr., corrosive sublimate 0^25 gr., distilled water 

 loo 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 trans- 

 mitted ; if more than normal, more of the solution is 

 necessary. The graduations of the tube correspond to 

 percentages of the normal standard which is taken as 100. 



Development of the Blood- Corpuscles. 



Figs. 353 and 354. 



Thoma-Zeiss 

 Heemacy tometer . 



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 



