INTRODUCTION 7 



power of reproduction, the detailed steps in the genesis of new 

 individuals are far from uniform throughout the animal kingdom. 

 Of the varied methods of reproduction encountered among ani- 

 mals, common features permit them all to be classified as either 

 asexual or sexual. Any reproductive process which involves the 

 genesis of new individuals through the functioning of specialized 

 cells, termed the gametes or germ cells, is sexual. Conversely, 

 any reproductive process which does not involve the functioning 

 of germ cells is asexual. 



Asexual Reproduction. — In the simplest forms of animal life, 

 the body of a single individual becomes divided into two or more 

 parts each of which by the growth processes assumes approxi- 





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Fig. 2. 



Fig. 3. 



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Fig. 4. 





Figs. 2-4. — Diagrams to illustrate the methods of asexual reproduction: 

 2, binary fission in Paramecium; 3, multiple fission or spore formation in a proto- 

 zoan; 4, budding in Acanthocystis. 



mately the size of the original organism which produced it. This 

 kind of reproductive process wherein no germ cell functions is 

 termed asexual and depends upon the power of part of an organ- 

 ism to reproduce the whole (Figs. 2 to 4). If the products result- 

 ing from such a division of the body are approximately equivalent, 

 the term fission is applied to the process. Fission is further 

 recognized as simple or binary (Fig. 2) and multiple (Fig. 3) 

 depending upon whether two or numerous individuals result. 

 In binary fission, the direction of the dividing plane is frequently 

 indicated by specifying whether the fission is longitudinal or 

 transverse. Fission is characteristic of many Protozoa and 

 occurs in isolated instances through many metazoan groups, 

 but in the latter asexual reproduction through the formation of 

 buds is more frequent. In budding (Figs. 4 and 36), a relatively 

 small part of the body of a parent individual becomes modified 

 as a starting point of a new individual. Only after development 

 has gone to certain stages is the bud recognizable as similar to 



