8 LIVING CELLS (2) 



Exercise 



developed in the same culture, are heritably 

 different from each other. We speak of such 

 different strains as different mating types. At 

 least 28 mating types are now known. These 

 occur as 14 complementary pairs. Fertile con- 

 jugations occur only between individuals of 

 complementary mating types. 



We shall examine conjugation in two comple- 

 mentary mating types of Paramecium aurelia. 

 Types XIII and XIV. Place 3 drops of Type 

 XIII culture in the left depression and 3 in the 

 center depression of a 3-depression glass slide. 

 Place 3 drops of Type XIV culture in the right 

 and center depressions. The right and left 

 depressions will serve as controls for what hap- 

 pens in the center depression, where both types 

 have been mixed. 



When two mating types are compatible, as 

 are these, the individuals first clump, their cilia 

 sticking together. At this stage a narrow space 

 can still be seen between them. After a short 

 time, the Paramecia pair off, the mating indi- 

 viduals uniting side by side, and the pellicles 

 fusing. It is at this stage that exchange of 

 genetic material (haploid gametic micronuclei) 

 takes place. Conjugation goes on for several 

 hours. Then the mating individuals separate, 

 and each resumes reproduction by fission. 



After you have observed the original clump- 

 ing, set the slide aside and go on with the rest 

 of the exercise. After about an hour, a few 

 pairs should have separated. Find such pairs 

 and make a quick sketch of what you can see. 



Euglena, a green flagellate 



Place a drop of Euglena culture on a clean 

 slide. Add a drop of 4*^ methyl cellulose. 

 Cover with a cover slip, and observe, first under 

 low, then under high power. Note the whiplike 

 flagellum (often better seen by dimming the 

 light), the chloroplasts, and the eye-spot, which 

 seems to be a genuine light-receptor, guiding 

 the motion of the organism toward or away 

 from the light. Sketch one cell and its parts. 



The green flagellates are hard to classify. 

 Zoologists include them among the one-celled 



animals, the protozoa; botanists among the 

 algae; or both avoid the issue by calling them 

 "protists." What are their plant, and what 

 their animal characteristics? One characteristic 

 of typical plants is that they can incorporate 

 inorganic nitrogen (nitrates, ammonium salts), 

 whereas animals require their nitrogen in or- 

 ganic form (e.g., amino acids). On the basis 

 of this criterion, how would you set up an 

 experiment to classify Euglena ? 



Ameba, a Rhizopod (Sarcodine) 



This is the famous protozoan whose name 

 has become a household word for the simple 

 and listless among living organisms. Put a drop 

 of ameba culture on a clean slide, and observe 

 with the naked eye against a dark background. 

 The amebas can be seen as whitish dots about 

 as large as a pinpoint. Look at them under low 

 power, without a cover slip. See how they move 

 by means of outward bulgings of the cytoplasm 

 (pseudopodia = "false feet"). Watch carefully 

 the formation of a pseudopod, and the asso- 

 ciated flow of cytoplasm. Distinguish in the 

 cytoplasm a clear outer layer, an inner, granular 

 mass, and the single, ovoid nucleus. Make a 

 series of 6 outline sketches of a progressing 

 ameba, recording the time of each sketch. 



For study under high power, it is preferable 

 to use a cover slip. So that this will not crush 

 the ameba, place four bits of broken cover slip 

 around the drop of culture, and set the cover 

 slip on these. Under high power observe the 

 granules, food vacuoles, nucleus, contractile 

 vacuoles, and pseudopodia. 



It must be plain to you now that the ameba 

 has been maligned. It is not as simple as it 

 looks. It packs more into a small space than 

 anything yet designed by man. It can move, 

 react to stimuli, reproduce, adapt to the environ- 

 ment (how?), ingest solid food, excrete waste, 

 and regulate its water content. How do you 

 manage to do more? With all that churning up 

 of its contents, how does the ameba keep its 

 functions sorted out and balanced ? 



