192 INFECTIONS CAUSED BY MOLDS 



room temperature revert to a hyphal type of growth and appear as 

 white or brownish cottony molds. On the hyphae of such a culture 

 two types of spores are commonly formed. There are small spherical 

 conidia 1 to 3 microns in diameter borne on lateral conidiophores of 

 varying length but usually not exceeding 4 or 5 microns. These 

 conidia may be smooth, in which case they are practically indistin- 

 guishable from the conidia of Blasto7nyces dermatitidis. Other 

 conidia, and particularly the larger ones, have a rough or spiny outer 

 wall. In many cultures and at early stages of sporulation there are 

 conidia intermediate between these rough-walled conidia and a larger 

 type of spore to be described. 



It is the second type of spore which characterizes H. capsulatum. 

 It is large, varying in size and shape from a spherical cell 10 to 15 

 microns in diameter to a clavate cell reaching a size of 12 by 20 

 microns. The outer wall is adorned with warty, finger-like, or occa- 

 sionally spiny excrescences which give it a distinctive appearance. 

 The variation in size and shape of these excrescences and their homo- 

 geneous structure suggest that they are produced by the proliferation 

 of the cell wall itself. They are not asci, as suggested by some in- 

 vestigators. The spores bearing these external structures are usu- 

 ally called chlamydospores, but one can find intermediate forms 

 which seem to relate them closely to the smaller conidia in a single 

 series. No ascomycetous form is known. When these conidia are 

 injected into an experimental animal they give rise to buds which 

 reproduce the budding life cycle characteristic of the parasitic phase 

 of growth. 



Taxonomy. The fungus was described and named in the mistaken 

 belief that it was a protozoan. However, since the genus was erected 

 for this organism and has never actually included protozoa the name 

 is valid. There are some differences between the various strains of 

 the fungus so far isolated but these are insignificant and do not 

 merit specific separation. A single species, Histoplasma capsulatum, 

 is recognized. 



Geographical Distribution. The distribution appears to be cir- 

 cumglobal. Reports have come from the United States, Central and 

 South America, Europe, Russia, Java, and Africa. 



Habitat. The natural habitat of the fungus is unknown. It has 

 been reported in several cases from dogs, but there is not yet suffi- 

 cient evidence to incriminate dogs as a reservoir. 



