236 
DR. E. KLEIN ON THE SMALLPOX OE SHEEP. 
shorter dimensions in the section. This transformation of the epithelial elements is at 
first limited to the central parts of the pock and the middle strata of the rete Malpighii ; 
but as the pock increases in size, the horny streaks, at first isolated, increase in number 
and length until they become confluent, so as to form one more or less continuous horny 
stratum, thus dividing the rete Malpighii of the central part of the pock into a deep 
layer below the horny stratum, and a superficial layer between the newly formed horny 
stratum and the original stratum corneum. This change goes hand in hand with the 
growth of the pock in breadth and with the appearance of the central pale depression 
mentioned on several occasions (see Plate 29. fig. 4 and Plate 30. figs. 5 & 6). 
Simultaneously with the formation of the horny stratum, the superficial as well as 
the deep part of the rete Malpighii (fig. 6, B & C) undergoes remarkable changes. The 
cells of the former become more transparent and flatter ; commencing from the centre 
of the pock, they gradually assume the characters of horny scales, the nuclei of which 
gradually disappear ; in this way the horny stratum (D in fig. 6) increases rapidly in 
thickness towards the surface. The cells of the superficial layers of the deep 
stratum of the rete Malpighii (viz. the layers directly beneath the horny stratum) 
exhibit rows of highly refractive granules, which are seen to be the more deeply stained 
by hsematoxylin the further the cells containing them are from the corium, while the 
cells themselves are larger the nearer they are to the horny stratum. Many of those 
nearest to the horny stratum look as if they possessed a thick membrane enclosing clear 
contents, in which there are here and there a few granules besides the nucleus. These 
cells are no doubt swollen dropsical epithelial cells. 
The deepest stratum of the rete Malpighii is in a state of very active proliferation. 
This is evinced by the fact that the interpapillary processes grow to extremely large 
dimensions, and that cylindrical processes composed of young epithelial cells penetrate, 
?. e. grow, into the papillae of the corium from them, the papillae themselves being 
longer and thicker than natural. These epithelial processes penetrate into the papillae 
in different depths and in different directions ; so that in sections many of them present 
themselves as isolated patches, surrounded by papillary substance, in the neighbourhood 
of the Malpighian layer. These, as well as the processes which in the section are not 
severed from their natural connexion with the rete Malpighii, are composed of the 
same granular substance as the deepest layer of the rete Malpighii, and contain spherical 
nuclei at more or less regular intervals, which exactly resemble those of the epithelial 
cells of the rete Malpighii. 
In most cases these processes show in section a distinct division into “ territories,” 
each surrounding an individual nucleus. Sometimes, however, this separation is indi- 
stinctly seen, and then the areas in the section which correspond to the tips of processes 
cut through resemble, to a certain extent, large multinuclear giant cells. Considering 
what has been stated on this subject by certain writers, it is necessary to repeat that the 
conical, cylindrical, thin or thick, short or long processes which penetrate into the 
papillae are direct outgrowths of the rete Malpighii. And it must be added that the 
