ROLLESTON, ON NUCLEATION OF BLOOD-CELLS. 129 
small as to resemble very closely the normal mammalian 
blood, may be found very constantly in the blood of certain 
ovipara.* Here, as in so many other cases, the morphological 
value of a structural arrangement depends, not upon an in- 
variable presence or an invariable absence, but upon the 
constancy of its quantitative preponderance. And upon this 
principle, whatever other affinities to the sauroids the sloth 
may be supposed to possess, the microscopy of its blood can- 
not be held to point in that direction. That the red blood- 
cell—the carrier of oxygen, and, probably enough, the distri- 
butor of heat + generated in the body—should present such 
different structural characters in the two classes, Aves and 
Mammalia, which are both alike warm-blooded, is a fact of 
the greater morphological importance for that it is physiolo- 
gically so hard to understand. From the purely anatomical 
point of view it may be allowable to suggest that the enor- 
mous relative preponderance of the lymphatic and lacteal 
gland system in the mammalia may account for the almost 
exclusive presence in their blood of the small non-nucleated 
red blood-cell. 
Since writing the above I have, through the kindness of 
T. J. Moore Esq., of the Liverpool Museum, had the oppor- 
tunity of examining the blood of an elephant, Elephas Indicus, 
which had died a week previously in Mr.: Manders’ Mena- 
gerie. 
In this blood very many nucleated red blood-cells were 
visible; but in all observed, with perhaps one exception, 
the coloured factor was internally placed, whilst the colourless 
formed the envelope. It is, of course, impossible to explain 
this arrangement as being a retention in a mammal of the 
condition usually met with in ovipara; for in these latter 
creatures it is the nucleus which is colourless, whilst the 
parts exteriorly to it are coloured. When the elephant’s 
blood-cells turned over in the slide, they presented much the 
appearance which a figure of a blastodermic vesicle does 
when its area pellucida is dumb-bell shaped, the envelope 
holding, in many cases, almost as favorable a relation in point 
of size to the nucleus, if so it may be called, as the blasto- 
dermic vesicle does to its area pellucida. This appearance I 
have noted also in the blood of the horse, of the rabbit, and 
of the human subject. 
* Funke, ‘ Lehrbuch der Physiologie,’ 4th ed., i, p. 213. 
+ Beale, in odd and Bowman’s ‘ Physiological Anatomy,’ p. 137. 
VOL. VII.—NEW SER. I 
