‘ 
THE BLOOD. 153 
nuclei of these cells remain or not; but as all grades of transi- 
tion forms have been found in the bone-marrow; since anemia 
occurs in disease of bones; since the bone-marrow has been 
found in an unusually active condition after hemorrhage and 
under other circumstances demanding a rapid replacement of 
lost cells—there seems to be little room for doubt that in the 
adult the red marrow of the bones is the chief site of the devel- 
opment of red corpuscles. It is not, however, the only one, for 
under peculiar stress of need even the lymphatic glands pro- 
duce red cells, and the latter have been seen to be budded off 
from the spleen in a young animal (kid). 
The colorless cells of the blood first arise as migrated undif- 
ferentiated remnants of the early embryonic cell colonies, That 
they remain such is seen by their physiological behavior, to be 
considered a little later. Afterward they are chiefly produced 
from a peculiar form of connective tissue known as leucocy- 
tenic, and which is gathered into organs, the chief function of 
which (lymphatic glands) is to produce these cells, though this 
tissue is rather widely distributed in the mammalian body in 
other forms than these. 
Summary.—The student may, with considerable cerjainty, 
consider the colorless corpuscle of the blood as the most primi- 
tive; the red, derived either from the white or some form of 
more specialized cell; the nucleated, as the earlier and more 
youthful form of the colored corpuscle, which may in some 
groups of vertebrates be replaced by a more specialized (or de- 
graded ?) non-nucleated form mostly derived directly from the 
former; that in the first instance the blood-vessels and blood 
arise simultaneously in the mesoblastic embryonic tissue; that 
such an origin may exist after birth, either normally in some 
mammals or under unusual functional need; that the red 
marrow is the chief birthplace of colored cells in adult life; 
that the spleen, liver, lymphatic glands, and other tissues of 
similar structure contribute in a less degree to the develop- 
ment of the red corpuscles; and that the last mentioned organs 
are the chief producers of the colorless amceboid blood-cells. 
Finally, it is well to remember that Nature’s resources in 
this, as in many other cases, are numerous, and that her mode 
of procedure is not invariable; and that, if one road to an end is 
blocked, another is taken. , 
The Decline and Death of the Blood-Cells.—The blood-corpuscles, 
like other cells, have a limited duration, with the usual chapters 
in a biological history of rise, maturity, and decay. There is 
