DK. E. KLEIN ON THE SMALLPOX OF SHEEP. 
239 
appearance as the process advances, becoming at the same time much thinner. In 
advanced pocks, i. e. shortly before and during the formation of the pustules, they 
become homogeneous or finely granular ; this is to be found in the papillse as well as 
in the deep stratum of the corium and in the subcutaneous tissue (Plate 30. fig. 5). 
In all primary pocks which have depressed centres and peripheral thickening the 
infiltration with lymph-corpuscles presents the characters described in the foregoing 
pages. In the absence of peripheral thickening the peripheral infiltration was not 
much greater than the central. 
D. Pustular Stage of the Development of the Primary Pustules. 
Pustulation commences by the formation of isolated vesicles in the rete Malpighii, 
which, as they gradually increase in number and size, become eventually fused together 
so as to form larger cavities and canals. This occurs generally at a time when the rete 
Malpighii has increased so much in thickness that it sometimes exceeds 2 millims. and 
more in vertical diameter ; the papillae of the corium have reached an extreme length 
by the extensive growth of the interpapillary process of the rete Malpighii, and at the 
same time the papillary tissue contains more or less numerous lymph-corpuscles. We 
have mentioned previously that the infiltration of the papillary tissue is most intense in 
the peripheral parts of the pock, and that the epithelial cells of the rete Malpighii in 
the middle layers are very transparent and large, the peripheral substance (membrane) 
of each cell looking as if it were much thickened. To this it is to be added that the 
papillary tissue, and in general the superficial stratum of the corium, contains, in its 
dilated interfascicular lymph-spaces and lymphatics, either a distinct mycelium with the 
spores as products of its fructification, or those zooglcea-like masses of spherical Micro- 
cocci or their necklace-like chains. In Those primary pocks which show a central 
depression the formation of the vesicles invariably commences at the periphery and 
soon extends towards the centre in a horizontal direction, so that the formation of the 
vesicles in the centre of the pock takes place later than that in the periphery. 
Notwithstanding this, however, numerous vesicles are seen to make their appearance 
in the centre, which stand in no direct connexion with those of the periphery. But 
in pocks of which the central portion, instead of being depressed, is elevated (as in 
many pocks on the mammary glands), the formation of the vesicles commences centrally 
and gradually spreads towards the periphery. Now the question arises, In what way 
do the vesicles form 1 That the vesicles in their first stage are filled with a trans- 
parent fluid lymph and that they afterwards gradually become filled with lymph or 
pus-corpuscles are well-known facts ; but the question of their mode of origin has, I 
think, not been investigated with sufficient detail. The assertion of Luginbuhl that 
the formation of the pustule is to a great extent due to the appearance of giant cells, 
must be at once abandoned so far as relates to variola of sheep, for in no instance have 
I been able to see any indication of such giant cells in the rete Malpighii. 
Considering that the infiltration of the corium with lymph-cells, fungi, and serum 
2 k 2 
