566 THE LIFE CYCLE 



occurrence and continuity is more difficult to organize than is that for 

 cycles in the larger, longer-lived Metazoa. 



Furthermore, a certain hesitancy about life cycles in the Protozoa 

 has arisen historically because of the fact that skilled workers in this 

 field have been caught in error by reason of the difficulties above noted. 

 Following upon Biitschli's (1876) and Hertwig's (1889) fundamental 

 analyses of conjugation in the Ciliata and its resemblance to maturation 

 and fertilization in other organisms, there arose a Munich school of 

 protozoologists whose labors brought forth an array of protozoan life 

 cycles fitted to the metazoan pattern. Under the brilliant leadership of 

 Fritz Schaudinn, most of the major groups of Protozoa were subjected 

 to this pattern of analysis, with resulting marvelous conformity to type. 

 Some of these, notably those of Trypanosoma, Endamoeha, and Masti- 

 gella, have not stood the test of subsequent critical reexamination. Others, 

 such as those of Plasmodium, Coccidium, and Paramecium, have, on the 

 other hand, survived and have proved the validity of the basic assump- 

 tion that there are life cycles in the Protozoa, though not necessarily 

 all of the same type. 



The life cycle in the Metazoa starts with the diploid or polyploid 

 zygote, a unicellular stage whose genes, derived from the haploid 

 gametes, determine the characters of all of the varied subsequent stages 

 unfolded in the ensuing life cycle. This cycle in many instances is marked 

 by indirect development with one or more larval stages, followed by 

 metamorphosis into the adult, sexual maturity, gametogenesis, senes- 

 cence, and death. In other instances the development is direct, with 

 adolescence replacing metamorphosis. In both types asexual repro- 

 duction may intervene at different periods in embryonic, larval, and 

 even adult life, giving rise by budding, binary and multiple fission, and 

 sporulation to two or many different functional individuals, all with 

 the original genetic constitution. Parthenogenesis may also intervene and 

 alternate with normal sexual reproduction. There is often considerable 

 change in the external appearance of the successive stages, as in larva, 

 pupa, and imago of the Lepidoptera, though a striking similarity, even 

 continuity, may occur in various organ systems from stage to stage. 



The stages occurring in the metazoan life cycle are brought about 

 by the processes of cleavage, gastrulation, organogenesis and histogenesis, 

 growth, adolescence or metamorphosis, gametogenesis, senescence, and 



