216 BLOOD 



several types are conceived to arise through process of differentiation along 

 distinct lines. ' The polyphyletic theory holds to a dual origin, from geneti- 

 cally distinct mother-cells, for red and white cells; or to a multiple 

 origin, tracing erythrocytes and the several varieties of leukocytes each 

 along different lines of differentiation to different mother-cells. The later 

 investigations into this problem of blood-cell origin done with an admit- 

 tedly superior technic tend to show that the monophyletic view is correct. 

 This was first well supported by the work of Saxer, who gave the name 



l^f/itf 



pa. 



tf* A^P'g^^iff 

 IT 



FIG. 230. LARGE BLOOD ISLAND FROM (Fio. 210), A 13 MM. HUMAN EMBRYO. 



a 1 and a 2 , mesenchymal cells; a 1 is differentiating into an endothelial cell; a 3 , endo- 

 thelial cells becoming separated from wall to form hemoblasts ('lymphocytes'); 

 6 1 and b 2 , hemoblasts (mesameboids) ; c, megaloblasts; d l and d 2 , normoblasts; R, 

 binucleated giant cell. X 750. 



'primary wandering cell' to the common blood-mother cell; more recently 

 by the work of Maximow, who applies the name 'lymphocyte' (primary 

 lymphocyte) to this cell, on the ground of its resemblance to the large 

 lymphocyte of adult blood. This lymphocyte (hemoblast) is according to 

 Maximow originally a cell of the general mesenchyrna, the earliest differen- 

 tiation stages of which are characterized by increase of basophily of the 

 cytoplasm, separation from the general syncytium, and assumption of 

 ameboid properties. These steps involve blood-island stages. 



We may now trace the steps through which the primitive blood-cell 

 passes in the process of attaining the adult condition of the various types 

 of cells. Of two daughter-cells one differentiates into an erythroblast ; the 

 other may remain as a primitive blood-cell to further function as a mother- 

 cell; at the same time primitive blood-cells may continually differentiate 

 In adult life from the mesenchyme of bone-marrow. An alternative view 



