41 8 How A?ii?nals and Plants Reproduce unit viii 



are called female cells. Others become two unlike gametes is called a fertilized 

 smaller whiplike forms; these are the egg cell or zygote (zeye'goat).* Fertili- 

 male cells. When one of the male cells zation is a much more common form of 

 enters a female cell, the nuclei and cell sexual reproduction than conjugation. In 

 bodies unite. The new cell then goes on higher animals and plants there is always 

 to produce more cells like itself. This a union of unlike gametes; a male cell 

 form of sexual reproduction, where the unites with a female cell. Reproduction 

 two uniting cells or gametes are different is always by fertilization, not by conju- 

 in appearance, is called fertilization. The gation. You should now be able to sum- 

 cell that is produced by the fusion of marize this problem. See Exercise 8. 



* Zygote is the general name for all cells zygospore of Spirogyra is a zygote. It is a spe- 



produced by the fusion of gametes. Thus the cial kind of zygote with a thick spore covering. 



Questions 



1. Explain what is meant bv spontaneous generation. Outline Redi's ex- 

 periment and state his conclusions. 



2. Why \\as the belief in spontaneous generation renewed a century 

 after Redi's experiment? Name two scientists who disproved spon- 

 taneous generation a second time and describe the experiments per- 

 formed bv one of them. 



3. Describe reproduction in a protozoan. What starts the process of 

 division? Define binarv fission. 



4. Explain how budding resembles and differs from binary fission. Give 

 an example. 



5. Explain how spore formation differs from both binarv fission and 

 budding. What is a spore? How is it formed? Into what does it de- 

 velop? Name a common plant that reproduces by spore formation. 



6. Name other organisms that reproduce by spore formation. 



7. In what important respect is reproduction in Spirogyra different from 

 the methods just studied? Explain reproduction in Spirogryra, using 

 the terms gamete and zygospore. How does a zygospore differ from a 

 gamete? What normalh- becomes of the zygospore? 



8. What is the difference between sexual and asexual reproduction? 



9. What is true of the gametes when sexual reproduction is called con- 

 jugation? Name a second simple plant that conjugates. 



10. How does fertilization differ from conjugation? Name a single-celled 

 organism in which fertilization occurs. 



Exercises 



I. Prepare some sterile broth in sterile test tubes by following directions 

 for the preparation of nutrient agar (page 317) but omit the agar. Before 

 sterilizing you must close the tubes w ith wads of cotton to prevent the 



