PROBLEM I. Why OjJ Spring Resemble Their Parents 463 



sion every' gamete gets the / number of chromosomes. The full num- 

 ber is restored in the process of K. Now make a short statement 

 explaining why offspring resemble their parents. 



Exercises 



1. Draw a dissected animal, such as a frog or fish, with mature sex or- 

 gans. Using different colored crayons, shade in all the somatoplasm in one 

 color; the germ plasm in another color. 



2. Illustrate mitosis by drawing a series of clear diagrams. Assume that 

 the cell has four chromosomes when cell division begins. 



■},. Demonstration. Mitosis in onion root tip (or similar material). Ex- 

 amine stained sections of onion root tips under the microscope. Can you 

 find nuclei that are dividing? Can you find chromosomes? Draw or de- 

 scribe the chromatin material in cells that are at the beginning of division, 

 in cells that are half through division, and in cells that are near the end of 

 division. What structures do vou find that have not been described? 



4. Study the table of chromosome numbers. How can you explain the 

 fact that the numbers are all even numbers? 



V The mosquito has six chromosomes. Draw a series of diagrams to 

 show the three steps in the maturation of its sperm. 



6. Why is it that the primary sex cell has chromosomes exactly like 

 those of the fertilized tg^} Explain and draw. Compare the number and 

 kind of chromosomes of a skin cell wdth those of the fertilized tgg and 

 those of the primar\^ sex cell. In what process is the number of chromo- 

 somes reduced to one half? Explain and draw. In what process is it re- 

 stored to the full number? 



Further Activities in Biology 



1 . Mitosis and maturation can be shown by means of movable chromo- 

 somes. You might make small wooden chromosomes of different shapes 

 to be hung on a board. Draw the outlines of cells on the board and put in 

 screw hooks on which to hang the chromosomes. This demonstration can 

 be made in less permanent form by using paper chromosomes with thumb 

 tacks. You could use small, painted bar magnets on a sheet of tin or iron. 



2. There are many devices for making permanent models to illustrate 

 the whole process of mitosis: the frame of a box could represent the cell 

 wall; wires, the spindle; and so on. 



3. If your school has a good slide showing mitosis in a root tip, photo- 

 graph parts of it under low power. If your pictures are very good, enlarge 

 them for use in the class. 



