390 Dr. E. Klein on the Smallpox of Sheep. [June 18, 



contents can be distinguished from the enclosing membrane by their yel- 

 lowish-green colour. 



6. At the same time that these appearances present themselves in the 

 corium, those changes are beginning in the now much thickened rete Mal- 

 pighii which are preparatory to the formation of the vesicular cavities 

 already mentioned. By a process which the author designates horny 

 transformation, having its seat in the epithelial cells of the middle layer of 

 the rete Malpighii, a horny expansion, or stratum, appears, lying in a plane 

 parallel to the surface, by which the rete Malpighii is divided into two 

 parts, of which one is more superficial, the other deeper than the horny 

 layer. Simultaneously with the formation of the horny layer the cells of 

 the rete nearest the surface of the corium undergo very active germina- 

 tion, in consequence of which the interpapillary processes not only 

 enlarge, but intrude in an irregular manner into the subjacent corium. 

 At the same time, the cells immediately below the horny stratum begin to 

 take part in the formation of the vesicular cavities, some of them en- 

 larging into vesicles, while others become flattened and scaly, so as to 

 form the septa by which the vesicular cavities are separated from each 

 other. 



7. The vesicles, once formed, increase in form and number. Originally 

 separate, and containing only clear liquid, they coalesce, as they get 

 larger, into irregular sinuses, and are then seen to contain masses of vege- 

 tation similar to those which have been already described in the lymphatic 

 system of the corium with this difference, that the filaments of which 

 the masses are composed are of such extreme tenuity, and the conidia 

 are so small and numerous, that the whole possesses the characters of 

 zooglaea rather than of mycelium. However, the author has no doubt 

 that these aggregations are produced in the same way as the others, viz. 

 by the detachment of conidia from the ends of filaments.. In the earlier 

 stages of the process the cavities contain scarcely any young cells. 

 Sooner or later, however, so much of the rete Malpighii as lies between 

 the horny stratum and the papillae becomes infiltrated with migratory 

 lymph-corpuscles. The process can be plainly traced in the sections. At 

 the period of vesiculation, i.e. at a time corresponding to the commence- 

 ment of the development of the vesicles in the rete Malpighii, the cutis 

 (particularly towards the periphery of the pock) is infiltrated with these 

 bodies. No sooner has the coalescence of the vesicles made such progress 

 as to give rise to the formation of a system of intercommunicating 

 sinuses, than it is seen that the whole of the deep layers of the rete Mal- 

 pighii become inundated (so to speak) with migratory cells, which soon 

 find their way towards the cavities, and convert them into microscopical 

 collections of pus-corpuscles, the formation of which is proved to be due 

 to migration from the corium, not only by the actual observation of 

 numerous amoeboid cells in transitu, but by the fact that the corium itself, 



