70 MORPHOLOGY AND CULTURE OF MICROORGANISMS 



saccharomyces) the ascus comes from an egg which starts in a similar 

 manner (Fig. 52.) In some species, this egg is formed by a hetero- 

 gamous copulation between an adult cell (macrogamete) and a very 

 young cell which has just separated from the mother cell (micro- 

 gamete) (Fig. 53). On the contrary, in most species, the ascus results 

 from the simple transformation of an ordinary cell without previous 

 copulation. Whatever may be its origin, the ascus shows cytological 

 phenomena quite similar to those which have just been described in 

 Sch. octosporus, with mere differences of detail. Always in Sch f 



FIG. 53. Heterogamous copulation in Zygosaccharomyces chevalieri. 1-3, 

 Gametes sending out a beak in anticipation of copulation. 4-7, Micro- and macro- 

 gametes joined by their channel of copulation. 8, The partition separating the 

 two gametes is absorbed. 9-18, The contents (nucleus and cytoplasm) of the micro- 

 gamete enter the macrogamete and are fused with the contents of the latter. 

 19-21, Ripe asci. 22-23, Freeing of the ascospores by rupture of the membrane of 

 the ascus. 



octosporus are seen only a few metachromatic corpuscles in the ascus. 

 In most of the other yeasts, on the contrary, the ascus contains a very 

 large number of metachromatic corpuscles, and it is easier there to fol- 

 low the evolution of these bodies which present interesting singularities 

 clearly demonstrating their role as reserve substances. 



Let us observe, for example, the cytological phenomena which ap- 

 pear during sporulation in Saccharomyces ludwigii. In this yeast, 

 which shows no sexuality in the origin of the ascus, the cells which are 

 preparing to sporulate assume a finely vacuolar structure (Fig. 54, 8 

 and 9) and produce a large quantity of reserve products: metachromatic 

 corpuscles, glycogen and fat globules. Metachromatic corpuscles spring 

 up in some vacuoles, glycogen in others; as for the fat globules, they 



