Ciliates were the favourite subjects for isolation culture, since being relatively large – the larger species are easily visible to the naked eye – and freeswimming they are readily observed and manipulated. Before describing the behaviour of the cultures, I shall first briefly describe ciliate reproduction and sexuality, to introduce technical terms that I shall make frequent use of later. More detailed accounts of some crucial processes are deferred to the appropriate sections.

Reproduction

The ciliates that I shall be describing reproduce exclusively by a transverse binary fission, a single cell dividing at right angles to its long axis to give rise to two daughter cells. Because the animal is not symmetrical about the plane of division, the two fission products are not perfectly identical, and for a short period following division anterior and posterior fragments can be distinguished. These differences, however, soon disappear. A more serious complication is introduced by the fact that ciliates possess two different kinds of nuclei. The micronucleus, or ‘generative nucleus’, is a small body usually possessing two sets of chromosomes. During fission it divides by a regular mitosis, so that the chromosomes, organised on spindle fibres, are distributed accurately into the two daughter micronuclei. During vegetative growth the micronucleus appears to synthesize little or no RNA, and therefore plays little or no part in normal somatic function; indeed, viable strains which completely lack micronuclei are known from several species. Protein synthesis is thus mediated largely, if not exclusively, by the much larger macronucleus. This is a highly polyploid nucleus, typically comprising the equivalent of 40–80 or even more haploid sets of chromosomes.