MORPHOLOGICAL, CYTOLOGICAL INVESTIGATIONS 137 



nal canal without harm. The intestinal canals of certain diptera 

 seem to be the normal habitat for certain yeasts; in fact, he has ob- 

 served S. apiculatus and ellipsoideus. Such conclusions are in accord 

 with the work of Neumayer, who has demonstrated that yeasts are 

 very resistant to digestive juices. It is well to point out that this 

 means of dissemination is not mentioned by other authors, Hansen, 

 for instance. 



(C) Morphological and Cytological Investigations on Yeasts 



It has just been stated that, under no circumstances, are we able 

 to transform yeasts into molds, or a mold into a true yeast; this has 

 not been observed in nature. Hansen did not hold this view and re- 

 garded the yeasts as an autonomous group of fungi, Ascomycetes. 



Such an hypothesis was not a new one. Before this, Reess and 

 de Bary had suggested this idea and noticed the superficial similarity 

 between the asc of the yeasts and the sporangium of the molds. The 

 asc is a single character which distinguishes between the true yeasts and 

 yeast-like structures of other fungi. So little was known, then, about 

 the cytological characteristics of the asc that it was difficult to make 

 any definite statements. 



For a long time this morphological problem remained untouched. 

 Were the sporangia of yeasts similar to the ascs of the Ascomycetes 

 as was maintained by Hansen? Or, should we regard them as approach- 

 ing more closely the sporangia of the Mucors, as was thought by 

 Brefeld? Do the yeasts represent a bona-fide group of fungi or are 

 they developmental forms of the molds? These questions remained 

 unanswered. One may always suppose that the yeasts resulted from the 

 molds by some process, hitherto unobserved, and that they have lost 

 the possibility of returning to the state of a mycelium. We have 

 negative proofs in favor of the autonomy of the yeasts. 



But in these later times, new facts have been discovered. It has 

 been shown in the preceding chapter that the cytological studies 

 on the asc and the discovery of sexuality in yeasts have furnished 

 definite proof of the ascogenous nature of the sporangia of yeast, and 

 have proven the relationship of the fungi to the Ascomycetes. 1 



1 It might be well to point out that what distinguishes the group of Ascomy- 

 cetes is their possession of an aso enclosing from 4 to 8 ascospores. The asco- 

 spores are differentiated on the interior of the asc only at the expense of part of 

 the protoplasm. The rest, or epiplasm, is absorbed by the ascospores when they 

 develop. Among the lower ascomycetes (Endomycetes) the ascs form only at 

 the expense of terminal cells on the filaments. Among the higher Ascomycetes, 

 they are united in great numbers in organs called perethecia. 



A sexual process, rudimentary in certain types, always intervenes in the origin 

 of the yeasts. Among the Exoascee there is a simple nuclear fusion. Among the 



