700 SUMMARY OF CURRENT RESEARCHES RELATING TO 
Red Blood-corpuscles.* — Prof. W. H. Howell has made an extended 
study of the formed elements of the blood of Mammals. He finds that in 
the very young embryo there are two forms of red corpuscles, one large, 
oval, and always nucleated, which resembles the corpuscles of the lower 
vertebrates ; and one small, biconcave, circular in outline, and nucleated 
or not. The latter are the true mammalian corpuscles, and the former 
possibly represent ancestral corpuscles, The true mammalian cor- 
puscles lose their nuclei by extrusion. In the first half of embryonic 
life new red corpuscles are produced in the liver from groups of 
mesoblastic cells outlining the position of future blood-vessels. The 
central cells of these cords become red corpuscles, while the peripheral 
form the walls of the veins. It is probable that new red corpuscles are 
formed in all parts of the body when blood-vessels are being developed. 
In the second half of embryonic life red corpuscles are formed in the 
liver, the spleen, and the marrow of the bones. In the Cat the two former 
lose this function three or four weeks after birth. 
Leucocytes and blood-plates do not occur in the circulating blood of 
young embryos, but make their appearance in later embryonic life. The 
red corpuscles produced in the red marrow first occur as nucleated cells ; 
these differ in structure with age. Two extreme types may be recognized 
— one mature and ready to be converted into a non-nucleated corpuscle, 
and one immature, as shown by the character of the nucleus and the 
amount of hmmoglobin. This latter multiplies by karyokinesis and the 
daughter-cells form mature nucleated red corpuscles, which lose their 
nuclei by extrusion. The liberated nuclei go into solution in the blood- 
plasma. The immature red cells are derived from spherical colourless 
cells, erythroblasts, which have a definite histological structure and are 
found in the marrow ; they multiply actively by karyokinesis. These 
erythroblasts are derived from larger embryonic cells, which are usually 
described in the adult as ordinary marrow-cells, which also multiply 
by karyokinesis. The leucocytes of the blood are derived from lympho- 
cytes; these enter the circulation as small corpuscles with vesicular 
nuclei and scanty protoplasm, and they are not amoeboid. They develope 
into larger cells, with finely granular protoplasm, which possess the power 
of amoeboid movement. These have at first an oval vesicular nucleus 
which afterwards becomes elongated ; from this last form the multi- 
nucleated cells are derived by fragmentation of the nucleus. 
Giant-Cells of Marrow.j — Prof. W. H. Howell divides the giant- 
cells of the marrow into two classes. Some are polykaryocytes or 
multinucleated giant-cells found in developing bone, in pathological 
formations, or porous bodies kept in lymph-cavities. Others are 
megakaryocytes or large nucleated giant-cells found in the red marrow 
of the adult and in the blood-forming organs of the embryo. The former 
have no special formation, are not related to the latter, and are formed by 
the fusion of smaller cells in consequence of too rapid growth. The 
megakaryocytes form a peculiar class of cells; they arise from the 
growth of small lymphoid cells, and afterwards reproduce by direct 
division. During their life they form a secreted material which can be 
seen for a time by the Microscope, but which finally dissolves in the 
Journal of Morphology, iv. (1890) pp. 57-116 (1 pi.). t T. c., pp. 117-29. 
