452 DR THOMAS H. BRYCE ON 
by counting the erythrocytes and white cells in every third of the 112 sections. The 
figures were as follows :— 
Erythrocytes, White Cells. 
Aorta, 1003 ate pp 
Cardinal vein, 2634 wa 140 
The proportion of red cells in the aorta compared with the cardinal vein was thus 
less than 1 to 3, while the proportion of white cells was nearly 1 to 7. 
I have used the term ‘ white cells’ rather than ‘leucocytes’ here because the counts 
include the large mononuclear cells, which are not necessarily all true leucocytes. Those 
in the spaces of the mesenchyme outside the blood sinuses are certainly in great part 
‘leucoblasts,’ in the sense that all the polymorphic and granular cells are derived from 
them by metamorphosis of the nucleus and by the deposition of granules in their 
protoplasm ; but what is the relation of those within the blood stream to the non- 
heemoglobin-containing erythroblasts ? 
As one has, unfortunately, no opportunity of actually seeing one form of cell change 
into another, this question can only be answered in terms of probability. 
A careful scrutiny of these intravascular mononuclear cells in the cardinal vein and 
the spaces communicating with them shows that certain of them have rounder and 
larger nuclei than others, though identical in general characters. 
Several considerations point to the probability that these cells are progenitors of 
the erythroblasts. 
1st. The polymorphism of the erythrocyte series is in favour of the view that the 
blood is at this stage receiving new formed elements. There seems no reason why 
certain corpuscles in the general blood stream should retain, under the same conditions, 
their primitive generalised characters if all the corpuscles are survivors from earlier 
stages. 
2nd. If new elements are being added, the fact that erythroblasts are never found 
outside the blood stream indicates that they must be derived from less specialised cells 
in the blood stream, which in turn should have the characters of cells outside the blood 
stream. The large mononuclear cells fulfil the conditions and complete a logical chain. 
But the question arises as to the relations of the cells without and within the blood 
stream. Numerous cases prove that cells are passing from the spaces in the mesenchyme 
into the blood stream, or vice versa, but it is not possible absolutely to say in which 
direction they are moving. 
The fact that these mononuclear cells occur in disproportionately large numbers in 
a vessel,* the current in which would carry them away from this locality, indicates 
that in all probability they do arise in this tract of mesenchyme, and are there set free 
* The sections in which the above count was made were behind the liver, a long distance posterior to the point 
where the hepatic vein joins the cardinal. 
