92 DEVELOPMENT OF BLOOD. 



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 described, they are at first colourless 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 division. In whichever 

 way produced, however, whether from the original for- 

 mative cells of the embryo, or by the liver, these coloured 

 nucleated cells begin very early in fcetal life to be mingled 

 with coloured non-nucleated corpuscles resembling those of 

 the adult, and about the fourth or fifth month of embryonic 

 existence are completely replaced by them. 



The manner of origin of these perfect non-nucleated 

 corpuscles must be now considered. 



I. Concerning the cells from which they arise. 



a. Before Birth. It is uncertain whether they are 

 derived only from the cells of the lymph, which, at about 

 the period of their appearance, begins to be poured into 

 the blood; or whether they are derived also from the 

 nucleated red cells, which they replace, or also from similar 

 nucleated cells, which Kolliker thinks are produced by the 

 liver during the whole time of fcetal existence. 



b. After Birth. It is generally agreed that after birth the 

 red corpuscles are derived from the smaller of the nucleated 

 lymph or chyle-corpuscles, the white corpuscles of the blood. 



II. Concerning the Planner of their Development. 

 There is not perfect agreement among physiologists 



mammalian embryo. A. A dotted, nucleated embryo-cell in process of 

 conversion into a blood-corpuscle ; the nucleus provided with a nucle- 

 olus. 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. Per- 

 fect blood-corpuscle. 



