SPORE FORMATION IN THE COLONY 4_2 



Fig. 4-1 shows a section of the yeast colony. The outer layer 

 of autolyzed cells stains very lightly, and the inner central mass of 

 vegetative cells with their dense protoplasts is much darker. A 

 pseudo-mycelium of yeast cells penetrates the agar and is thickest 

 and deepest at the edges of the colony, apparently where oxygen is 

 most abundant. The thin peripheral film of cells at the edge of the 

 colony spreads over the surface of the agar. Fig. 4-2 shows details 

 at the ends of the section, with cells penetrating the agar. At the 

 points of origin of penetration the cells are very abundant. This may 

 be due to the channelling of the substrate nutrient into these regions 

 along the cracks made by the pseudo -mycelium. 



Figs. 4-3 and 4-4 illustrate the autolyzed layer containing the 

 asci. Autolysis apparently occurs early in the history of the colony, 

 at least before competition reduces cell size. The autolyzed layer 

 contains the only asci found, suggesting that autolysis supplies es- 

 sential nutrients on which sporulation depends. 



Fig. 4-5 shows the central vegetative cells, many of which are 

 extremely small, indicating that cell division continued after the 

 nutrients became less readily available and competition resulted 

 in a decrease in cell size. Fig. 4-6 shows the pseudo -myceluim 

 produced in the agar in higher magnification. 



In some of the contact smears the autolyzed cells were not so 

 shrunken as those obtained by the paraffin method. The walls seemed 

 relatively intact, but there were no stainable cell contents. The 

 * ghost" cells were larger than the densely stained cells in the vege- 

 tative section of the colony. These autolyzed cells, which appar- 

 ently served as sources of nutrients for the sporogenous cells, have 

 a parallel in the paraphyses found in Pyrenomycetes and Discomy- 

 cetes which also act as nurse cells. In some regions of the auto- 

 lyzed layer small clusters of round, apparently haploid cells were 

 foimd, suggesting that some spores may germinate in the layer. New 

 genetic rearrangements could result from such spore germination 

 and subsequent copulation. These experiments suggested that yeast 

 autolysate is an essential nutrient for sporulation and led to the ad- 

 dition of 2 per cent dried brewers' yeast to our pre -sporulation me- 

 dium, with excellent results. 



The striking parallelism between the structure of yeast colonies 

 and those of bacteria, as shown by Legroux and Magrou, suggests 

 that the life cycles may be similarly parallel. Yeasts possess a nu- 

 clear mechanism, and the vegetative cells undergo meiosis and 

 sporulation in the outer layer of the colony. In bacterial colonies 

 a similar division of the cell contents into two or four bodies oc- 

 curs in the corresponding layer. The analogy suggests strongly 

 that the granulation of the bacterial cell is the result of a reduc- 

 tion division similar to that known to occur in yeasts. Badian (1933) 

 and Lindegren (1941) have shown that bacteria undergo a series of 



