MODES OF REPRODUCTION 219 



further illustrated, and development will be examined in detail as 

 it occurs in vertebrate animals. 



Modes of Reproduction ^ 



Asexual Reproduction. — As the name indicates, this form of 

 reproduction involves nothing that may be called sexual, since 

 it concerns only one individual and is not dependent upon the 

 existence of sexual differentiation. In the asexual reproduction 

 of protozoa, the cell may either divide into two cells of the same 

 size, binary fission (Fig. 99, p. 185); bud off smaller cells, budding 

 (Fig. 89 B-3) ; or form many small cells by simultaneous division 

 of the cytoplasm about nuclei that have been produced by division 

 of a single original nucleus, spondation (Fig. 101 J). The asexual 

 reproduction of metazoa occurs by vegetative processes, Hke 

 the budding of hydra (Fig. 121, p. 250) and the fission that occurs 

 in some Flatyhelminthes and Annulata. Modifications are 

 seen in the budding of hydroids (Fig. 133, p. 275), where the 

 new individuals remain attached to the parent and produce 

 colonies; in the internal budding, known as gemmidation, which 

 occurs in fresh- water Forifera and Molluscoida; and in cases of 

 fission, where the individuals remain attached until completely 

 developed, forming temporary chains which resemble linear 

 colonies. In all these instances among metazoa, new Individuals 

 are produced without any relation to the phenomenon of sex, and 

 hence such modes of reproduction may be called asexual. 



Sexual Reproduction. — As the table shows (Fig. 110), the con- 

 jugating cells in protozoa are obviously comparable with the germ 

 cells in metazoa, and conjugation is equivalent to fertilization 

 insofar as it involves a union of two Unes of descent in the pro- 

 duction of a new individual. It is true that the gametes of pro- 

 tozoa may be isogamous and thus without the sexual differences 

 that characterize the gametes of metazoa, but in many protozoa 



* The distinction that is here made between sexual and asexual reproduction 

 is open to certain objections. Other classifications of the modes of reproduc- 

 tion might be used, for example: uniparental as compared with biparental 

 reproduction ; reproduction by syngamy as compared with reproduction by cell 

 division. The authors have chosen the conventional distinction between 

 sexual and asexual reproduction, because it is perhaps as good as any other and 

 because confusion always results when an elementary presentation departs 

 too widely from the usage in current books of reference. 



