240 
DK. E. KLEIN ON THE SMALLPOX OE SHEEP. 
(the distension of the interfascicular lymph-system) spreads from the deeper strata 
gradually towards the surface of the corium, and that the nearer the stage of the 
formation of the pustules is reached the more the epithelial cells of the middle layer 
of the rete Malpighii become dropsical and the proliferation of the deeper cells increases 
in activity, it appears evident that the excessive irrigation of the corium gradually 
extends towards the surface. I am inclined to think that this is probably attributable 
to stagnation in the large subcutaneous lymphatics and in the veins ; for I have found 
the lymphatics leading from infiltrated parts dilated and plugged up by dense myce- 
lium and lymph-corpuscles. I have also observed veins densely packed with lymph- 
corpuscles and mycelium. 
It can therefore be easily understood that inasmuch as the lymphatics and veins of 
the deeper parts of the pocks become prevented from carrying away the morbid products, 
and inasmuch as there is a constant addition of them, as shown by the increase of the 
infiltration, it must naturally lead to a stagnation in the passages leading to the efferent 
vessels, which stagnation may be the cause of the morbid material of these passages 
being gradually carried in another direction, i. e. towards the surface. This view, 
as we shall see presently, is further supported by the fact that the lymph-corpuscles 
of the papillary tissue not only find their way in great numbers into the rete Malpighii, 
but also into the epithelium of the sebaceous glands, the hair-follicles, and the sweat- 
glands, which structures become surrounded by more and more numerous layers of these 
bodies. That the spontaneous movement of the lymph-corpuscles is to a certain extent 
of importance in determining their migration, cannot of course be denied ; but it is 
improbable that this is the only factor, for it is very difficult to see how that could be 
the case, considering how densely they are packed in some places. 
The formation of the vesicular cavities invariably depends on the transformation of 
individual epithelial cells of the middle layer of the rete (sometimes nearer, sometimes 
further from the surface) into spherical or elliptical vesicular structures, which possess 
a thick membrane and clear contents. The process of transformation is usually as 
follows : — An individual cell expanded by dropsical swelling presses on the surrounding 
cells so much that they gradually become flattened and so compressed that they almost 
coalesce. In this way a vesicle is formed, the membrane of which is composed of 
concentrically arranged scales. These scales when seen in profile appear to be spindle- 
shaped. Of the epithelial cell from which the vesicular structure originated, all that 
remains is the nucleus surrounded by a thin zone of granular protoplasm (the original 
substance of the cell). This remains attached to one side of the vesicle, and finally 
disappears. 
But there is also another way by which individual epithelial cells become transformed 
into vesicles, i. e. by vacuolation. A cell first shows a small vacuole ; by the enlarge- 
ment of the vacuole the nucleus becomes pressed at the periphery, while the original 
cell-substance expands into a vesicular membrane enclosing the vacuole ; with this 
membrane the nucleus, which becomes more flattened the larger the vacuole grows, is 
