88 Transactions Texas Academy of Science. — 1906. 
interruption. Such indirect development is very frequent, and is 
shown best in cases where there is incomplete larval development. 
Characteristic of continuous life cycles is the occurrence of generation 
by egg cells only, completion of the cycle in the course of one in- 
dividual, and especially the complete transformation of every part 
of a younger stage into a part of a later one. 
By discontinuous development, on the other hand, we would com- 
prehend life cycles where there are breaks in the progress, in that 
either the cycle consists of two or more individuals, or that a younger 
stage develops organs that do not take part in the construction of 
the adult organism. Some of these cases have been grouped as 
alternation of generations. But that idea has come to combine and 
to some extent to confuse two different conditions, namely, succes- 
sion of different kinds of reproduction, and succession of different 
kinds of somatic individuals. The second of these conditions is poly- 
morphism and it need have no necessary connection with the first, 
because there may be succession of different modes of generation yet 
no polymorphism, as well as polymorphism associated with sexual 
generation only. Polymorphism is indeed a condition quite different 
and sometimes quite independent of occurrence of different kinds of 
generation, and therefore need not be associated with the idea of dis- 
continuous development. 
Three kinds of discontinuous life cycles may be distinguished: 
the metagenetic, the hetero genetic, and the ekdytic. 
Metagenesis we employ in the original sense of Haeckel 21 as a life 
cycle consisting of two or more individuals with alternation of sexual 
and asexual reproduction. To be consistent we should apply the term 
to all kinds of such alternation and then will find it to be a much 
more general phenomenon than is usually supposed. One recalls at 
once the well known examples of budding hydroid and egg-produc- 
ing medusa, the succession of buds and eggs in Bryozoa, Tunicates, 
Sponges, etc., where the distinction of individuals is patent to all. 
But one may also find cases that lead gradually from the state of 
obvious sharp distinction of individuals to one where metagenesis 
is still present even though it is difficult to discover more than one 
individual. Thus in some Cnidaria the process may become more or 
less masked, become hypogenetic to use another of Haeckel’s terms; 
the medusa instead of being abstricted early from the polyp may be 
retained longer and longer, until it finally become reduced to no more 
than a reproductive organ, a gonophore. Yet such hypogenesis is 
only a somewhat obscure metagenesis. Then in a Siphonophore the 
individuals never become fully separated from each other, which has 
21 Generelle Morphologie der Organismen, 1886, Berlin. 
