CLASSIFICATION OF THE MAIN SUBDIVISIONS 471 



stratified epithelium of the epidermis. In this situation they run 

 through two cycles the one cytoplasmic, the other intranuclear. 

 The first is the vaccine-cycle, and is the only part of the develop- 

 ment of which the harmless vaccine-organism is capable ; the 

 variola-organism, however, after passing through a vaccine-cycle, 

 proceeds to the extremely pathogenic intranuclear cycle. 



The vaccine-cycle, according to Calkins, begins with the appear- 

 ance of " gemmules " in the cytoplasm of the cells affected. Each 

 gemmule is a minute grain of chromatin without cytoplasm of its 

 own at first, but as it grows a cytoplasmic body is formed. When 

 full-grown, the parasite sporulates by fragmentation of its nucleus 

 into a great number of grains, which, as gemmules, pass into other 

 cells and repeat the development already described. Several 

 generations of this type may succeed each other before giving rise 

 to the next type. 



The intranuclear variola-cycle begins in the same way with 

 gemmules, which, however, penetrate into the nucleus, and develop 

 a cytoplasmic body. According to Calkins, they become sexually 

 differentiated, and produce gametes which conjugate. The final 

 result is the production of numerous spores, which are probably the 

 means of spreading the infection. 



Calkins referred Cytoryctes to the Microsporidia. Now, however, 

 he inclines to the opinion that the genus should be placed amongst 

 the E/hizopods (4). 



Negri (910) also describes a developmental cycle for Neuroryctes 

 hydrophobice, which he regards as a true Protozoon, and which 

 Calkins refers also to the Rhizopoda. Siegel (914) describes under 

 the name Cytorhyctes organisms of a type perfectly different from 

 those described by Calkins. He distinguishes four species Cyto- 

 rhyctes vaccinice of vaccine and smallpox, C. luis of syphilis, G. scarla- 

 tinas of scarlet fever, and C. aphtharum of foot-and-mouth disease. 



3. The parasitic life-cycles described by Calkins and others have 

 been criticized by a number of investigators, who have maintained 

 that the bodies in question are not Protozoa, nor even independent 

 living organisms at all, but merely degeneration-products of the cell 

 itself, provoked by a virus yet to be found. Thus, with regard to 

 Guarnieri's bodies (Cytoryctes) of vaccine, it is maintained by Foa, 

 Prowazek, and others, that they consist of nucleolar substance 

 (plastin) extruded from the nucleus ; that they have no definite 

 developmental cycle ; and that infection can be produced by lymph 

 in which Guarnieri's bodies have been destroyed, or by tissue in 

 which they are not present. With regard to the Negri bodies, 

 Acton and Harvey (906) come to the same conclusions, and state 

 that similar nucleolar extrusions can be brought about also by 

 other stimuli than the rabies- virus. 



