CHAPTER XII 



REPRODUCTION: SEXUAL AND ASEXUAL METHODS 

 GENERAL TYPES OF REPRODUCTION 



THE process by which reproduction is brought about is always 

 fundamentally the same. In spite of all of the numerous 

 modifications of the method in different animals and plants, 

 they are all reducible to some form of division; the original 

 animal or plant divides itself into parts, each of which is ca- 

 pable of growing into an individual like the one from which it 

 came. 



The numerous varieties of reproduction may be grouped 

 together under two general types. In one of these the original 

 organism divides itself directly into two or more parts by 

 simple division. In the other the division is always compli- 

 cated by the union of two parts with each other. In the latter 

 case certain cells of the original organisms unite with each 

 other, and the union is followed by a rapid division of the cells. 

 The two types of reproduction are, therefore, (1) Division un- 

 accompanied by cell union and (2) Division accompanied by 

 cell union. The type of division in which cell union is found 

 is often spoken of as sexual reproduction, and the uniting cells 

 are the sex cells; the type in which the division is not accom- 

 panied by cell union is called asexual reproduction. 



REPRODUCTION IN UNICELLULAR ORGANISMS 

 Simple Division. All of the single-celled animals multiply 

 by the process of simple division; Figs. 19, 23. A careful 

 study of the internal changes that are going on in the cells 

 during this reproduction shows that they are essentially 

 identical with those described on pages 85-89. In other words, 

 there is a division of the chromatin material in the nucleus, 

 followed by the formation of two nuclei, which again is fol-* 

 lowed by the division of the cell into two parts. After having 



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