110 DR W. B. DRUMMOND. 
like the polymorphonuclear cells. Such marrow cells Tf 
shall have to speak of again when describing the origin of 
the red corpuscles. 
The Red Corpuscles of the Blood—We rust now turn 
our attention to the coloured corpuscles. It is to these 
corpuscles that the blood owes its red colour, but although 
the corpuscles are ved when seen in mass, they are yellow 
when seen singly under the microscope by transmitted 
light. Their colour is due to a pigment termed hemoglobin. 
In all animals except mammals the red corpuscles are oval 
cells, each containing a nucleus. The protoplasm is non- 
granular and contains the hemoglobin. In mammals, on 
the other hand, the red corpuscles are non-nucleated. They 
are oval in form in the camels, but in all other mammals 
they are circular biconcave discs. 
Red corpuscles vary greatly in size. The largest are 
found in the Amphibia, the largest of all being those of 
Amphiuma, which measure 35 inch in their long diameter. 
The smallest corpuscles are found in the mammals. Those 
of Man are only 3399 inch in diameter. What they lack in 
size they make up in numbers. Every cubic millimetre of 
blood contains 5,000,000 of them. 
When a drop of blood is examined under the microscope, 
the concave character of the discs is indicated by the fact 
that each disc looks paler in the centre, where it is thinnest, 
than at the periphery. This character can also be readily 
made out when a corpuscle is found standing on its edge. 
The corpuscles have a strong tendency to stick together 
by their concave surfaces, so as to form masses or rouleaux. 
The Origin of the Red Corpuscles—With this brief de- 
scription of the appearances presented by the red corpuscles, 
we may pass on to consider the different stages of their 
life-history. First of all, let us ask where they come from. 
Where are they born ? 
Red corpuscles are recognisable in the embryo at 
quite an early stage of development. They are described 
as originating in the substance of nucleated masses of 
protoplasm, which are situated in the middle of the three 
layers of cells of which the body of the embryo is composed. 
The nuclei of these masses subdivide, and the multinucleated 
