LABORATORY AND FIELD WORK 401 



d. The gradual contraction and rounding up of the respective cells 



(gametes). What parts of the cells take part in this rounding 

 up ? Do the two gametes begin to contract at the same time ? 



e. The passage of one (the male) gamete through the tube into the 



cavity occupied by the other (the female). 

 /. The union of the two gametes into a single rounded cell (the 



zygote), 

 g. The formation of a thick wall about the zygote. Notice that the 



zygote occupies less space than was filled originally by either of 



the two gametes that have united to form it. 



11. Should the material at hand show the lateral method of 

 conjugation — which is less common than the method above de- 

 scribed — the conjugation tube will be seen to be formed between 

 two adjacent cells of the same plant instead of between cells of two 

 different plants. 



Chapter VI 



1. Place a slice of moist bread (not wet enough to be soggy) in 

 an evaporating dish ; leave the dish uncovered for a half -hour ; 

 then cover it and let it stand in a moderately warm place. If the 

 surface of the bread seems to be drying out at any time, add a little 

 water. In the course of one or two days fluffy spots will appear 

 on the surface of the bread ; in a day or two more, the molds which 

 cause these spots begin to form spores, and the spots take on a 

 more powdery appearance. Among these molds Rhizopus is almost 

 sure to be one ; it can be recognized by the presence of stout upright 

 branches at the top of each of which is a globular spore sac. 



2. If Rhizopus is overgrown by other molds, as sometimes 

 happens, it will be best to prepare a practically pure culture. This 

 can be done by transferring from the first culture, by means of 

 sterile forceps, a small amount of the mold bearing spore sacs 

 to a slice of bread that has been freshly cut with a sterile knife, 

 moistened with boiled water, and placed in a sterilized evaporating 

 dish which is kept covered. 



3. Study the young spots on the bread with a hand lens. Notice 

 that each spot consists of a tangle of branched threads, white or 

 grayish while young. There are two kinds of branches — some 

 that grow horizontally along the surface, and some that grow 

 downward into the bread. 



