CHAPTER XXI 

 TISSUES OF REPRODUCTION, GENERAL OUTLINE 



AMONG multicellular animals and plants the individual exists for a 

 longer or a shorter period and then perishes. Certain cells, however, 

 of these multicellular organisms may separate from the parent individual 

 and, under proper conditions, give rise to new individuals. Through 

 these cells, then, an unbroken chain of living cells is perpetuated. Cells 

 so functioning are known as the reproductive cells. 



Reproductive cells are of two kinds, asexual and sexual. The asexual 

 reproductive cells are known as spores. It is characteristic of them 

 that they may develop directly, without conjugation, into a new indi- 

 vidual. Asexual reproductive cells and their organs are found in plants 

 and certain very low forms of animals. We shall consider them only 

 in the pollen cells of Magnolia. 



The sexual reproductive cells are known as gametes. It is charac- 

 teristic of gametes that without the union of two gametes or their equiva- 

 lent, a new individual will not be developed. Certain apparent excep- 

 tions to this idea can be reconciled with it. 



These gametes are different from any other cell or group of cells that 

 may separate, bud, or divide from the body to form a new individual, 

 in that they go through a peculiar process called maturation, which 

 involves two cell divisions and a reduction of the number of the chro- 

 mosomes by one half. In this case each cell does not develop into a 

 new individual by itself, but joins with another reproductive cell, de- 

 rived usually from another individual of the same species, and by a 

 process analogous to a reversal of the process of amitotic cell division 

 the two unite and become one cell, with the full number of chromosomes, 

 which represents the beginning of a new individual of the species. This 

 union is known as conjugation. 



Every species of animal has its own kind of reproductive cell, but there 

 also are two forms of this cell, a male and & female form. It is always a 

 male and a female cell, usually from the same species, that thus unite to 

 form the new individual, and besides any deeper or more significant 

 difference between the two there is nearly always the comparatively 

 superficial difference that the female cell, called the ovum, is the larger 



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