CH. XXIX.] OKIGIN OF RED CORPUSCLES 459 



and Welcker, is 5,000,000 in adult men, and 4,500,000 in women ; this corre- 

 sponds to an average of 11-2 and 12 '5 corpuscles respectively per square of the 

 instrument. 



FIG. 324. 



The enumeration of the colourless corpuscles depends on the same principle, 

 but the counting has to be carried out over larger areas than the small squares, 

 and the differentiation of the varieties of colourless corpuscles (which is most 

 important from the standpoint of disease) can be accomplished after appropriate 

 staining. 



Development of the Blood-Corpuscles. 



Origin of the Red Corpuscles. Surrounding the early embryo 

 is a circular area, called the vascular area, in which the first rudi- 



FIQ. 325. Part of the network of developing blood-vessels in the vascular area of a guinea-pig. U, 

 Blood-corpuscles becoming free in an enlarged and hollowed-out part of the network ; a, process of 

 protoplasm. (E. A. Schiifer.) 



ments 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 mesh work. 

 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 (fig. 325). The protoplasm 

 of the cells and the 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 -arVir to T-^TT of an inch (10 /UL to 16 /*) in diameter, 



