214 THE CONTINUITY OF THE RACE 



and the term sexual reproduction for reproduction by germ cells (or a 

 germ cell). This will deviate from our everyday usage of the word sexual 

 only in the case of one relatively unimportant type of germ cell reproduc- 

 tion (parthenogenesis) . 



Asexual (Soma Cell) Reproduction in Metazoa 



Among the lower, less highly individualized Metazoa, there is slight 

 development of cell differentiation and division of labor. In these or- 

 ganisms various methods of producing new individuals directly from the 

 soma tissues of the parent individual are an important and often the 

 most common method of reproduction. Such asexual reproduction shows 

 considerable variation in detail among the lower Metazoa, but practically 

 all types may be classified as either budding or fission. 



Budding is the formation of new "daughter" individuals by growth 

 and proliferation of some portion of the parent's body. The parent con- 

 tinues to exist as an individual. Budding is a common and extremely 

 important method of reproduction and propagation in all groups of 

 plants; but in animals it is confined to the lowest, least individualized 

 groups of the Metazoa, especially the sponges and the coelenterates 

 [Hydra (Fig. 13.5), corals, and their kin]. In the sponges a peculiar type 

 of "internal budding" is the chief method of reproduction. 



Fission is the production of two new "daughter" individuals by the 

 self-cutting-in-two of the parent individual. In this process, the parent 

 individual ceases to exist and is replaced by the two new daughter indi- 

 viduals. Fission is not so widely utilized a method of asexual reproduction 

 as is budding, but it is characteristic of the flatworms and a few other 

 forms. 



Limitations to asexual reproduction. Asexual reproduction in the 

 Metazoa is limited to those forms that have relatively little complexity 

 of body parts, in which the tissues are comparatively few and generally 

 distributed over the whole body. In such simple organisms, budding 

 and fission are capable of providing a new individual with all the kinds of 

 tissues and organs that are found in the parent. In the higher Metazoa, 

 with their marked differentiation of soma tissues and highly integrated 

 and interdependent organ systems, asexual reproduction does not occur. 



Other limitations to asexual reproduction are as follows: (1) In many 

 organisms asexual reproduction occurs only during seasons of continu- 

 ously favorable environmental conditions and is stopped by seasons of 

 unsuitable temperature or insufficient food or oxygen, when the species 

 must resort to sexual production of eggs to survive. (2) In several groups 

 that utilize asexual reproduction, it always alternates with sexual, so 

 that each asexually produced individual reproduces by sexual methods, 

 and vice versa. The alternation of sexual and asexual generations that 



