1896.] MICTtOHCOPlCAL JOURNAL 275 



colonies consisting of varying numbers of ye<as"t cells; 

 take care not to confuse single cells merely in mechanical 

 contact with cells that are really in vital relation. Study 

 different colonies and note the exact size and position of 

 its different members. Do the colonies furnish any evi- 

 dence by which to decide on the mode of reproduction of 

 thecell? This mode differs how from fission } It is called 

 gemmation or budding. Do you find any symmetry in 

 yeast? Do tiie new cells tend to arise at definite jK)ints 

 on their progenitors? Note that both gemmation and 

 fission take place without the intervention of other cells. 

 It is called the asexual mode of reproduction. What 

 other mode of asexual reproduction have you noticed ? 

 How do they differ from conjugation. Examine for com- 

 parison yeast which has been standing an equal time 

 in pure water; do you find any indication of growth? 



Stain a colony with iodine, and study the cells with the 

 strongest magnifying power at your command. Examine 

 the oldest cell of a colony and locate in it a clear space — 

 the vacuole surrounded by protoplasm. Examine cells of 

 different ages, and determine whether a vacuole is found 

 in all. Why does the vacuole change from light to 

 dark in different focal levels? In some cells you will find 

 minute droplets of fat. Do you find any chlorophyll ? 

 Can you find a nucleus ? How do you know that the 

 vacuole is not a nucleus ? Is the vacuole exactly com- 

 parable with anything found in previous studies ? Can 

 you recognize a cell-wall ? Is it thick or thin, and is it 

 rigid or flexible ? Mount and examine some dead yeast, 

 the cell contents have disappeared, leaving an empty 

 cell, the wall can now be seen. Sometimes you can burst 

 yeast cells by pressure and get views of the fracttired 

 wall and escaping protoplasm. This can be facilitated by 

 staining. 



23. Penicillium. — Examine a series of yessels con^in- 



