18 



Reproauction 



Living organisms have the remarkable power of producing new ones that 

 look and act like themselves, though never exactly so. Many do this by the 

 division of their substance into parts of equal size with nothing remaining 

 to be a parent; all amebas begin life as orphans. Many others divide into 

 parts of very unequal size, a large one, the parent's body and small ones, the 

 cells, two of which, one male and one female must unite to make a new indi- 

 vidual. Whatever the case, parental protoplasm is the first substance of the 

 new individual no matter what its kind, ameba, bird, or man. 



Asexual and Sexual Reproduction 



Either asexual or sexual reproduction increases the population. The main 

 difference is in its variety. By asexual reproduction one cell becomes two cells, 

 and by sexual reproduction two cells become one and this one divides asexually 

 into many (Fig. 18.1). Thus, generations of amebas are produced, and the 

 bodies of multicellular animals increase in size whether small or great, fleas 

 or elephants. Asexual division, the pioneer method of reproduction, has 

 persisted throughout the course of evolution. 



Various invertebrates divide into two or more parts, each of them a new 

 individual. In the marine worm, Autolytus, a second head appears part way 

 down the body. There are soon two fully organized worms attached one be- 

 hind the other. For a time they swim about tandemwise, then separate and 

 each one swims away alone. Sometimes a chain of individuals will form and 

 swim about together. Fresh-water hydras put forth buds that pinch off as 

 independent animals, and thus stop just short of colonial life. The internal 

 buds (gemmules) that form within fresh-water sponges are eventually set 

 free to start new colonies. No higher animals produce buds; cats do not bud 

 off kittens. 



Sexual reproduction differs from asexual in that two individuals furnish 

 different kinds of cells, eggs and sperms. When such cells are fully developed 



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