DEVELOPMENT OF BLOOD. 



blood-cells ; some fluid plasma being at the same time 

 secreted. Thus, by the same process, blood is formed, and 

 the originally solid heart and blood-vessels are hollowed out. 



The blood-cells produced in this way, are from about 

 ^ . 1 OU to T3Vir f an i ncn i n diameter, mostly spherical, 

 pellucid, and colourless, with granular contents, and of 

 well-marked nucleus. Gradually, they acquire a red 

 colour, at the same time that the nucleus becomes more 

 denned, and the granular matter clears away. Mr. Paget 

 describes them, as, at this period, circular, thickly disc- 

 shaped, full- coloured, and, on an average, about -^-^ of 

 an inch in diameter ; their nuclei, w r hich are about 3-^ Vo f 

 an inch in diameter, are central, circular, very little pro- 

 minent on the surfaces of the cell, and apparently slightly 

 granular or tuberculated. 



Before the occurrence, however, of this change from 

 the colourless to the coloured state in many instances, 

 probably, during it, and in many afterwards, a process of 

 multiplication takes place by division of the nucleus and 

 subsequently of the cell, into two, and much more rarely, 



Fig. 30 * 





D F 



three or four new cells, which gradually acquire the 

 characters of the original cell from which they sprang 

 Fig. 30 (B, c, D, E). 



* Fig. 30. Development of the first set of blood-corpuscles in the 



