OYTOLOGICAL PHENOMENA OF SPORULATION 49 



and forms ascospores. From recent studies 1 it has been shown that 

 in the formation of the asc, especially in the Exoascees, which constitute 

 a family of the Ascomycetes very close to the yeasts, this fusion is 

 wanting. 



In many yeasts which do not represent sexuality, and among 

 others in Saccharomyces cerevisiae, Janssens and Leblanc 2 considered 

 that they observed a nuclear fusion in the cells which were about to 

 form ascs and considered this as a sexual act. According to these 

 authors the nucleus of these cells undergoes a division since both 

 nuclei, which result after being separated for some time, blend together 

 into a single nucleus which by successive divisions furnishes the nuclei 

 for the ascospores. But Guilliermond 3 has shown that this obser- 

 vation is inaccurate and that one is not able to verify any nuclear fusion 

 in the yeast cells which are destined to sporulate without preliminary 

 copulation. Thus karyogamy preceding sporulation is lacking in the 

 yeasts, a fact which seems definitely established today. 



We shall stop now to observe the processes which take place in 

 the cell when it transforms into ascs the division of the nucleus 

 and the formation of ascospores. 



Let us take S. Ludwigii as the example. In this yeast, sexuality 

 before the formation of the asc has not been observed. As we have 

 said, the cells pass directly into ascs without preliminary copulation 

 and this phenomenon is replaced by a fusion (parthenogamy) of the 

 ascospores at the moment of germination. 



The cells which are preparing to sporulate assume a very complex 

 structure. They possess an alveolar cytoplasm in which the network 

 which surrounds the alveoli shows inclusions of fat and numerous 

 basophile granules. Two sorts of alveoli may be distinguished: 

 some are filled with a considerable quantity of metachromatic cor- 

 puscles; others with glycogen. The nucleus is placed at the side "of 

 the cell. It surrounds itself with a thin layer of very dense zone of 

 protoplasm (plasma sporogenic) at the expense of which the asco- 

 spores are built up. In this sporogenic plasma, the greater portion 

 of the basophile grains accumulate. (Figs. 47, a, and 51.) All the 

 rest of the cytoplasm, which possesses an alveolar structure, will not 

 be used in the formation of ascospores, but will form the epiplasm, 

 the plasma which will be absorbed by the ascospores during their 



1 Dangeard, who observed this nuclear fusion first, regarded it as a true 

 fecundation. The explanation of this phenomenon is not completely elucidated and 

 remains obscure. 



2 Janssens, F., and LeBlanc, A. Recherches cytologiques sur la cellule de 

 levure. La cellule, 14, 1898. A propos du noyau de la levure. La cellule, 20, 1903. 



3 Guilliermond, A. Le noyau de la levure. Annales mycologici, 2, 1904. 



