218 REPRODUCTION 



as individuals composed of cells which may exist in a balanced 

 physiological state, as in protozoa, or in an unbalanced state, as 



in metazoa. 



Reproduction and Development. — The distinction between 

 reproduction and development, to which reference was made in 

 the introductory paragraph of the present chapter, is now apparent. 

 Reproduction has been defined as the formation of a new individual 

 ('/. p. 129). In protozoa reproduction may occur asexualhj by 

 cell division of various sorts, and sexually by the conjugation, or 

 syngamy, of gametes. In the latter process a new cell is formed 

 by the permanent fusion of two cells, as in the fertilization of 

 many-celled animals. In metazoa what is called asexual repro- 

 duction occurs by budding and fission, but is confined to the lower 

 many-celled animals (cf. Fig. 121, p. 250); while sexual reproduc- 

 tion is effected by the syngamic union of ova and spermatozoa in 

 fertilization. The zygote, or one-cell stage, which is thus formed, 

 then produces the adult individual by the cell divisions and 

 differentiations that constitute develo-pment. By making this dis- 

 tinction between reproduction and development, the sexual repro- 

 duction of protozoa and metazoa may be exactly compared; and 

 it becomes clear that development in the metazoa has no exact 

 parallel in the protozoa, although the cell division of protozoa 

 and metazoa are obviously comparable as shown by the' table 

 (Fig. 110). To compare development, as thus defined, with 

 the cell development that occurs when protozoan cells increase 

 in size and undergo certain differentiations after fission, as in 

 the growth and differentiation of the two cells resulting from 

 division in a ciliate (Fig. 99, p. 185), is confusing, since one is 

 comparing a single cell with a mass of cells. Yet something 

 may be said for such a comparison along the line of the Organismal 

 Theory. The comparison of cell cycles points to the origin of 

 development as a necessary incident of the evolutionary change 

 from the single-celled to the many-celled state. It is, there- 

 fore, possible to consider the reproductive processes of meta- 

 zoa apart from those of development. This is desirable, because 

 we have now become famihar with specific examples of repro- 

 duction in the protozoa, and because the related processes in 

 metazoa are so similar in all many-celled animals that they may 

 be described in general terms. In our subsequent study of animal 

 types, the reproductive and developmental processes will be 



