FROM SINGLE CELLS TO MANY CELLS 



83 



Fig. 4-13. A single-celled animal can reproduce by simply dividing into two individuals. Higher forms (some of the 

 Protozoa likev/ise) provide special cells, eggs and sperms, which unite and give rise to a new individual. 



the job of reproduction to special cells, 

 eggs and sperms, which, with a few excep- 

 tions, must unite to form new individuals. 

 This device has the advantage of bringing 

 two lines of protoplasm together which 

 results in variation in the offspring, for each 

 offspring possesses a combination of the 

 characteristics of its parents and is there- 

 fore different from either. Variation seems 

 to have some advantage in survival of the 

 species and probably has been important 

 in the gradual evolution of complex forms. 

 Specialized reproductive cells are pro- 

 duced by special organs, the gonads. To 

 insure the union of eggs and sperms spe- 

 cial tubes are necessary to conduct these 

 cells out of the body, and ultimate union is 

 still more effectively assured in higher ani- 

 mals by the development of copulatory 

 organs. To insure greater survival, the off- 

 spring of many of the higher animals are 

 either retained within large egg shells or 



the body of the mother for various periods 

 of their early development. All of this intri- 

 cate machinery came into being because 

 cells "insisted" on aggregating into large 

 masses. 



The penalty of organization. The organi- 

 zation of cells into masses and the subse- 

 quent specialization of different kinds have 

 resulted in organisms that are able to 

 penetrate a greater variety of environments 

 because of their greater motility and in- 

 tricately adjusted bodies. That means bio- 

 logical success. Along with all of the bene- 

 fits derived from specialization, however, 

 there has been at least one rather severe 

 penalty, and that is natural death. 



Recall that single cells reproducing by 

 binary fission, barring accidental death, live 

 on forever. The amoeba observed under 

 your microscope has been alive since the 

 dawn of life on the earth. Had death oc- 

 curred along the road somewhere the one 



