CHAPTER XIII 



DISTRIBUTION OF SEXUAL AND ASEXUAL METHODS. 

 ALTERNATION OF GENERATIONS 



SUMMARY OF THE METHODS OF REPRODUCTION 



REPRODUCTION in all animals and plants is the result of divi- 

 sion, but according to whether the division takes place with 

 or without cell union, we have the two following types: 



1. Asexual reproduction. Asexual reproduction is division 

 without cell union. Under this head there are at least four 

 different methods. 



A. Division by fission. 



B. Division by budding. 



C. Division by spore formation. 



Each of these three types of reproduction is found among 

 unicellular as well as multicellular organisms. 



D. Parthenogenesis, or reproduction by eggs without fer- 

 tilization. 



2. Sexual reproduction. Sexual reproduction is division 

 preceded or accompanied by a union of cells, the uniting cells 

 being called gametes. According to whether the uniting cells 

 are alike or unlike, we find tw9 types. 



A. Conjugation. When the uniting cells are microscopically 

 identical with each other, the process is conjugation. In these 

 cases there are neither eggs nor sperms, and the cell resulting 

 from their union is a zygospore. 



Conjugation has apparently for its purpose the reinvigora- 

 tion of the process of cell division, since, after two individuals 

 have united, cell division begins to take place more rapidly. 

 After many generations of simple cell division the process 

 tends to become slower, and conjugation then may occur to 



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