522 ANNUAL REPORT SMITHSONIAN INSTITUTION, 196 2 



organisms, and bacteria cannot live alone in nature. If, however, 

 we can once account for tlie origin of an amoeba, evolution from 

 Protozoa to man is relatively easy. 



CELL DIVISION, REPRODUCTION, AND HEREDITY 



Since no form of life is individually immortal, there would be no 

 living thing on the earth today if the first living creatures had not 

 devised a means of multiplication. The simplest type of reproduction 

 is the division of a unicellular animal into two duplicating cells, and 

 cell division is still the basis of reproduction in all higher animals. 

 In the simpler Protozoa it begins with a division of the nucleus, fol- 

 lowed by a constriction of the cell between the daughter nuclei, and 

 ends with a complete separation of the cell into two cells. In others 

 the mechanism becomes variously more complex until it comes to re- 

 semble that characteristic of the Metazoa. In some of the Protozoa 

 the successively formed cells, instead of separating as individuals, ad- 

 here in a mass, forming a many-celled animal of a sort, such as Vol- 

 vox, in which there is some coordination among the individuals, but 

 little differentiation of structure, except that certain cells assume the 

 function of reproduction. With the true many-celled animals, or 

 Metazoa, however, the cells are so closely adherent that they practi- 

 cally lose their individualities, and groups of them differentiate into 

 complex organs of various functions. 



In some of the lower Metazoa the body cells may retain a repro- 

 ductive potential sufficient to regenerate lost appendages or even a 

 large part of the body including the head. Among the higher ani- 

 mals this faculty becomes reduced to wound healing. In general, re- 

 production is the function of special cells set apart for the purpose. 

 The germ cells are differentiated into male and female cells which, 

 with few exceptions, must first unite in pairs before development 

 begins. The combination of the male and female elements in the 

 zygote, or egg, combines hereditary material from two parental 

 sources, and is usually accredited with inducing more structural vari- 

 ations in the adult for natural selection to work on in the evolution 

 of species. Sexual reproduction occurs among the Protozoa, some 

 of which come together in pairs and unite, while others simply ex- 

 change nuclear material. Among the metazoic animals and in the 

 flowering plants it is all but universal. Bisexual reproduction, there- 

 fore, evidently has had a strong evolutionary influence. Only in a 

 few invertebrates, as in the honey bee, does the unfertilized egg 

 develop. 



Since the number of chromosomes in the body cells of the Metazoa is 

 typical for each species, the primary germ cells undergo a reduction 

 division by which the number of chromosomes is halved. Union of the 

 sex cells at the time of fertilization then restores the normal cliromo- 



