‘in Physiology. 291 
form of a careful paper by Dr. W. H. Howell,‘ of Michigan Univer- 
sity. As a result of work extending over a period of two years, most 
of which was confined to the cat, the author concludes that the cor- 
puscles originate not, as is usually assumed, in different ways, but in 
accordance with one scheme of reproduction, which is essentially the 
same in health as in disease, in the embryo as in the adult. 
As regards the red corpuscles, these arise in the very young embryo 
from cords of mesoblastic cells, which outline the position of future 
veins; the central cells of the cord form corpuscles, while the periph- 
eral ones form the walls of the veins. Such developing blood vessels 
were found in the liver and in the muscular tissue of the posterior 
limb, and it seems probable that corpuscles are thus formed wherever 
there are developing blood vessels. In the second half of embryonic 
life red corpuscles are formed in the liver, the spleen, and the red 
marrow. At first this function is most active in the liver, next in the 
spleen, and lastly in the red marrow. A few weeks after birth, in the 
cat, the liver and spleen cease to take part in their formation, and in the 
adult healthy animal they are produced in the red marrow alone. In 
case of extreme anzemia, resulting from bleeding or whatever cause, the 
spleen may resume its embryonic function. Wherever and whenever 
red corpuscles are produced, nucleated forms precede the mature non- 
nucleated forms, the latter being derived from the former by the 
extrusion or migration of the nucleus,—a process which the author was 
able to follow in part in the living cell. The life-history of the cor- 
puscle was studied most fully in preparations from the marrow, and is 
given in brief below. In the very young embryo two forms of red 
corpuscles-occur. One is very large, oval, and always nucleated, which 
the author regards as possibly an ancestral form. These disappear in 
early embryonic life. The other, the true mammalian corpuscle, is 
much smaller, circular in outline, and is found both nucleated and 
non-nucleated. These apparently arise from colorless, spherical cells 
—erythroblasts—found in the marrow and elsewhere. The marrow 
erythroblasts are derived from large embryonic cells, known in the 
adult simply as marrow-cells,—the unchanged descendants apparently 
of the original mesoblastic cells from which the marrow is formed. 
ese embryonic marrow-cells multiply by karyokinesis, the daughter 
cells sooner or later acquiring the structure of the erythrob’ The 
erythroblasts multiply rapidly by karyokinesis, giving rise ultimately 
to cells from which the nucleated red blood corpuscles are derived 
by the development of hemoglobin within the cell substance. These 
* Journal of Morphology, Vol. IV., p. 57, 1899. 
