THE BLOOD. 151 
are globular masses of protoplasm, containing one or more 
nuclei, and with the general character of amoeboid organisms. 
Tye HisToRY OF THE BLOOD-CELLS. 
We have already seen that the blood and the vessels in 
which it flows have a common origin in the mesoblastic cells of 
the embryo chick; the same applies to mammals and lower 
groups. The main facts may be grouped under two head- 
ings: 1. Development of the blood-corpuscles during embry- 
onic life. 2. Development of the corpuscles in post-embryonic 
life. 
In the bird and the mammal, cells of the mesoblast in the 
area opaca give off processes which unite; later they become 
hollowed out (vacuolated), 
and thus form capillaries. 
At the same time the nuclei 
of these cells multiply (pro-— 
liferate), gather small por- 
tions of the protoplasm of 
the main cells about them, 
become colored, and thus 
form the nucleated corpus- 
cles of: the embryo. This, 
or asimilar process, is known 
to occur in some animals 
(rat) after birth; but in the 
human foetus there is a grad- 
ual decline in the number of 
nucleated cells found free in 3 
the blood, and at birth they Fie. 147,—Surface view from below of a small por- 
tion of posterior end of pellucid area of a chick 
i I is of thirty-six hours, 1 x 400 (Foster and Bal- 
are very rare, which a prob four). 6. c, blood-corpuscles ; a, nuclei, which 
ably the case with most subsequently become nuclei of cells forming 
walls of blood-vessels ; p. pr. protoplasmic 
mammals. Le apei containing nuclei with large nu- 
While the origin of the 7 
red cells, as above described, may be regarded as the earliest 
and most general, it is not their exclusive source. 
When the liver has been formed this organ seems to carry 
on a development begun in the spleen, for the nucleated but as 
yet colorless cells formed in the spleen seem to become pig- 
mented in the liver. 
There is also evidence that colored corpuscles may arise by 
endogenous formation in the lymphatic glands. 
