82 DEVELOPMENT OF BLOOD. 



process, blood is formed, and the originally solid heart and 

 bloodvessels are hollowed out. 



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

 to y-g-ftfl of an inch in diameter, mostly spherical, pellucid, and 

 colorless, with granular contents, and a well-marked nucleus. 

 Gradually, they acquire a red color, at the same time that the 

 nucleus becomes more defined, and the granular matter clears 

 away. Mr. Paget describes them as, at this period, circular, 

 thickly disk-shaped, full-colored, and, on an average, about 

 2~5"0 f an i ncn m diameter ; their nuclei, which are about 

 WOTT f an i ncn m diameter, are central, circular, very little 

 prominent on the surfaces of the cell, and apparently slightly 

 granular or tuberculated. 



Before the occurrence, however, of this change from the 

 colorless to the colored 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, 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. 



D E ^^^ F 



Development of the first setof blood-corpuscles in the mammalian embryo. A. A 

 dotted, nucleated embryo-cell in process of conversion into a blood-corpuscle: the 

 nucleus provided with a nucleolus. B. A similar cell with a dividing nucleus ; at c, 

 the division of the nucleus is complete ; at D, the cell also is dividing. E. A blood- 

 corpuscle almost complete, but still containing a few granules. F. Perfect blood- 

 corpuscle. 



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, according to Kolliker, and new 

 blood-cells are produced by this organ. Like those just de- 

 scribed, they are at first colorless and nucleated, but afterwards 

 acquire the ordinary blood tinge, and resemble very much 

 those of the first set. Like them they may also multiply by 



