TTTF. BT.nOD 1 13 



known as elementary granules and are not present in normal human 

 blood. 



Development of the Blood 



At an early stage of embryonic development certain mesodermic cells of the 

 area vasculosa, which surrounds the embryo, become arranged in groups known 

 as blood islands. It is from these "islands" that both blood and blood-vessels 

 develop. The peripheral cells arrange themselves as the primitive vessel walls, 

 within which the central cells soon become free as the first blood corpuscles. In 

 this way vascular channels are formed, inside of which are developing blood 

 cells. This division of the mesoblastic cells of the blood islands into (i) endothe- 

 lial cells of the vessel walls and (2) progenitors of the blood cells or primitive 

 blood cells, is quite generally accepted. From this point, however, opinions 

 diverge, the two main theories of blood-cell formation being known as the poly- 

 phyletic and the monophyletic theories. 



According to the polyphyletic theory, after the original division of the meso- 

 blast cells of the blood islands into vessel wall cells (endothelium) and blood 

 cells (primary blood cells), the latter go on to the development of erythroblasts 

 and these to the development of erythrocytes. Leucocytes develop later in con- 

 nective tissue, in the liver, spleen and bone marrow, while lymphocytes have 

 their origin in the germ centers of the lymphoid organs. There are thus three 

 separate sources of blood cells: (i) Red blood cells originally from erythroblasts 

 of the blood islands; in adult life from the erythroblasts of the bone marrow and 

 possibly of spleen; (2) leucocytes, first in connective tissue, liver, spleen and mar- 

 row; in adult Hfe in the marrow; (3) lymphocytes, in the lymphoid organs. Ac- 

 cording to the polyphyletic theory these three types are genetically independent 

 and remain so throughout life, the cells of each type undergoing mitotic division 

 to produce new cells of the same type only. 



According to the monophyletic theory the blood-cell-forming elements of the 

 blood islands fall into two groups as regards their future development: (i) cells 

 which give rise to primary erythroblasts, these to secondary erythroblasts and 

 these again to erythrocytes, thus completing the line of red blood cells, and (2) 

 cells which remain undifferentiated, not only in the embryo but throughout life, 

 and retain the capabiUty of differentiating into erythroblasts or into white blood 

 cells, either leucocytes or lymphocytes. According to Weidenreich^ and other 

 supporters of this theory, red and white blood cells stand in very close genetic 

 relation. He claims that an undifferentiated mother cell exists which is capable 

 of differentiating either in the direction of the red cell or of the white cell; that 

 this mother cell exists not only in the embryo but throughout life and is con- 

 stantly giving birth to cells which are to develop into red blood cells or white 

 blood cells to replace those being constantly used up and cast oft". Morpho- 

 logically the mother cell is apparently identical with the lymphocyte. In post- 

 embryonic life, this mother ceU is found in marrow in the germ centers of lymphoid 

 organs and possibly in connective tissue. On the other hand a once differentiated 

 cell remains a differentiated cell, and while able to divide mitotically and thus 



^ Ant. Rec, vol. iv., Sept., 1910, 

 s 



