1890.) Morphology of the Blood Corpuscles. 1023 
The above review shows that the vertebrate blood corpuscles 
are of three kinds: 1, red cells; 2, white cells; 3, plastids. 
The red and white cells occur in all(?) vertebrates; the plastids 
are confined to the mammals, The red cells present three chief 
modifications ; whether the primitive form occurs in any living 
adult vertebrate I do not know; the second form is persistent in 
the Ichthyopsida, the third form in the Sauropsida. According 
to this we must distinguish: 
A.—ONE-CELLED BLOOD, 2. e. first stage in all vertebrates ; 
the blood contains only red cells with little protoplasm. 
B.—Two-ceLLep Broop, having red and white cells; the red 
cells have either a large, coarsely granular nucleus (Ichthy- 
opsida) or a smaller, darkly staining nucleus (Sauropsida, 
mammalian embryos). 
C.—Prasrip Broop, without red cells, but with white cells 
and red plastids; occurs only in adult mammals. 
Mammalian blood in its development passes through these 
stages, as well as through the two phases of stage B, all in their 
natural sequence; the ontogenetic order follows the phylogenetic. 
I pass by the numerous authors whose views conflict with 
mine, partly because the présent is not a suitable occasion for a 
detailed discussion, partly because those authors who have 
asserted the origin of one kind of blood corpuscle by meta- 
morphosis from another have failed to find just the intermediate 
forms; it seems to me, therefore, that most at least of the oppos- 
ing views collapse of themselves. 
