FERTILIZATION We understand by fertilization (q.v.) the fusion of two cells and nuclei which are unequal, being differentiated sexually into male and female. The two cells that fuse together are called gametes, the product of their fusion the zygote. In the higher animals the gametes are called eggs and sperm and the zygote the fertilized egg. The latter rapidly divides to form a many-celled embryo, which later becomes a new individual of the species to which it belongs (see EMBRYOLOGY).
Amongst the Protozoa, most can multiply by simple division and many are only to be driven to the act of fertilization by ex ceptional external influences, such as lack of food, overcrowding, accumulation of the products of metabolism and so on. And even then fertilization is sometimes not associated with multiplication but rather with a reduction of numbers, for the gametes are here not parts of individuals but often whole individuals so that at fertilization only one individual is produced by two. The zygote produced develops into an ordinary individual which only then can begin to multiply.
With such protozoans fertilization seems merely an intermezzo, virtually superfluous, which, as we shall see later, can be left out entirely under favourable conditions of life; it may be mentioned here that there are many protozoans in which we know no sexual process and in which there very possibly is none, e.g., euglenoids and trypanosomes. There are protozoans on the other hand as de pendent on fertilization for their reproduction as most of the higher animals. A gregarine for example cannot multiply by plain division ; once mature it shuts itself up, together with another indi vidual, in a capsule and each breaks up by multiple division into a large number of small gametes, leaving over the so-called residu ary body. After this the gametes of the one fuse with those of the other; each of the numerous zygotes formed in this way surrounds itself with a thick membrane : it is now called a spore; inside this membrane the zygote divides into eight elongated cells, the sporozoites. These sporozoites are embryonic gregarines which, as soon as the spores get into the gut of the animal in which the particular gregarine grows—its "host"—slip out and grow into mature animals. Here then sexual reproduction is ap
parently indispensable to the continuance of the species.
It has been supposed, almost down to the present day, that fer tilization is necessary to all protozoans, if not actually as a means of reproduction then for some other reason ; it was thought to have been proved that Protozoa that had for some time multiplied only asexually, gradually aged; that they became weaker, showed all kinds of symptoms of degeneration and finally petered out altogether as a strain if they failed to find an opportunity of fer tilization. After fertilization, on the other hand, the organism's vitality appeared to be restored. Hence it was concluded that for the maintenance of the race mere asexual division was not enough; that after a certain period of this kind of multiplication a process of senescence necessarily supervened and that this danger could only be escaped by sexual reproduction or some other kind of re organization such as parthenogenesis (q.v.). Hence fertilization was called a process of rejuvenation. The observations on which this assumption was based are undoubtedly correct, yet the con clusions that have been drawn from them are only applicable to a limited group of Protozoa—the infusorians—and not even appli cable to all species of these.
Other protozoans of various kinds (Eudorina, Actinophrys) have been kept for years under the most careful observation and it has been determined that so long as they are kept under favour able conditions of life they will reproduce asexually year in and year out, probably indefinitely, without degeneration.
This fact is referred to as the "potential immortality" of the protozoan ; a multicellular animal must die because it can only reproduce by discharging from its system cells which develop into new individuals while its body cells are incapable of indefi nite reproduction or existence. A protozoan, on the other hand, which divides into two and passes into its offspring entire, does not need to die. Such death as it undergoes is not the result of senility but of an accident. It is indeed potentially immortal.