LIFE-CYCLES: ALTERNATION OF GENERATIONS The successive stages of development of a higher animal such as a bird can be looked upon as a cycle ; if we begin with the fertilized egg the embryo follows, then the young animal, then the mature animal that again produces eggs ; the cycle is complete.
In some animals, such as the green-flies or aphids (q.v.), sev eral generations have to be gone through before this cycle is closed. In each of these instances, in birds and in aphids, we can speak of a life-cycle ; in the aphids we can also recognize an "alternation of generations." Naturally every organism has a life-cycle, even a protozoan that multiplies by division, for here also young animals are pro duced by division that only later come to resemble the creature that produced them. But it has become usual to speak of a life cycle only when some other spe cial process is introduced into the life history. An infusorian can for example multiply by di vision, but it can algo encyst or it can conjugate. Now we speak of an obligatory or of a facultative life-cycle according to whether the succession of events in the life-history is strictly determined, on the one hand, or capable of a certain amount of adjustment, on the other. The obligatory life-cycle is governed solely by the in trinsic properties of the individual, running its course, provided only that the ordinary requirements of life are met; the other, the facultative life-cycle, is profoundly influenced by such external conditions as temperature, nutrition and the condition of the medium in which the organism lives.
There are Protozoa on the other hand that have a clearly facultative career, as they can reproduce by simple fission for an indefinite number of generations; whilst under certain conditions they are forced to copulate or encyst (e.g., Antinophrys).
There is also an obligatory life-cycle in those protozoans that show an alternation of generations. By this alternation we understand a succession of generations that are distinct from one another both in their form and in their method of reproduction. The classical ex example of protozoans with an alternation of generations is the Foraminifera (fig. 13, 14). In most of these two distinct types can be recognized, the so-called micro- and macro-spherical individuals. The micro spherical individuals are the asexual gener ation; as soon as they are mature the protoplasm breaks up into numerous germs, "agametes," which creep out of the shell of the mother-individual and gradu ally develop into new individuals—but in dividuals of the other, the macro-spherical, type. Then the protoplasm of these breaks up, this time into gametes, and after fusion we have the microspherical generation again (fig. 14).