SEXUALITY 25 



has noticed this in another yeast which he has not named. This was 

 isolated from the mucilaginous secretions on a tree in the Berlin 

 Botanical Garden. 



Copulation of ascospores does not seem to be regarded as a true 

 fecundation but as a phenomenon 1 of parthenogamy a sexual 

 process replacing fecundation. In fact, it may be admitted that the 

 copulation which takes place in Schizosaccharomyces, and Zygosac- 

 charomyces and Debaromyces at the moment when the asc forms 

 represents a normal sexual process of yeasts. The copulation which 

 takes place, then, among the ascospores, is a new process and one 

 which takes the place of normal fecundation. 



The cell which gives rise to the asc ought to be regarded as a 

 gamete developing parthenogenetically. As the formation of ascospores 

 necessitates two successive divisions which are not separated by 

 a period of intercalary nutrition, the nucleus which results is quite 

 devitalized. This may explain why the ascospores felt the need 

 of compensating for the loss of chromatin which the nucleus has 

 experienced in successive divisions. It is probable, however, from 

 what is known with regard to the higher ascomycetes, that the asc 

 of the yeasts is the seat of a reduction in the number of chromo- 

 somes. The copulation of ascospores may intervene to replace the 

 fecundation which should occur at the moment when the ascs are 

 formed and to compensate the loss of chromatin undergone in the 

 course of mitosis of the asc. 



(C) Retrogradation of Copulation Parthenogenesis 



As has been pointed out, in the great majority of yeasts, es- 

 pecially those which are of industrial significance, one does not 

 find any trace of sexuality. As among species which present a copu- 

 lation at the moment when the asc is formed, it has become es- 

 tablished from numerous cases of parthenogenesis that the yeasts 

 which do not have sexuality, represent parthenogenetic forms derived 

 from primitive sex forms. The asc, when it has not resulted from 

 a copulation, has then the import of a gamete having developed by 

 parthenogenesis, that is, a parthenospore. The yeasts may be re- 

 garded as a group of fungi which have gone toward parthenogenesis 

 by evolution or by force of unknown circumstances, and which are 



1 This phenomenon is comparable to that which Brauer has observed in the 

 parthenogenesis of an echinoderm, Artemia salina. In this organism, when fecun- 

 dation has not taken place, there is a fusion of a second polar globule with the 

 egg, and this fusion takes the place of fecundation. Some phenomena which 

 appear to be similar have been found since among various fungi and protozoa 

 and have been grouped under the name of parthenogamy. 



