The technique

Mass cultures of protozoans tell us relatively little; division rates are almost impossible to measure and conditions cannot be kept constant. To follow the history of a clone accurately it is essential to study isolated individuals in small volumes of medium; this was Maupas' fundamental technical achievement. Isolation cultures of asexual metazoans are relatively easy to set up: a newborn female is segregated in a chamber, and her offspring removed as they appear, until she dies. This is impossible in protozoans which reproduce by binary fission, simply because there is usually no way of distinguishing between ‘parent’ and ‘offspring’. Isolation cultures of protozoans therefore cannot follow the history of an individual, but are used rather to follow a line of descent. Nor is it possible to follow all of the branches of any such line, since a few dozen generations of binary fission would produce many billions of descendants from a single initial cell; rather, a few cells must be selected in each generation as representatives of their line. The technique which, with unimportant variations, was used by all workers after 1902 was as follows. A culture is maintained until a conjugating pair is found. One of the two products of conjugation is isolated and allowed to divide two or three times, producing four or eight descendants. Four or five of these are in turn isolated, and allowed to divide. The descent of each of these forms a ‘line’, the four or five lines together constituting a ‘series’. At intervals of between one and three days, depending on the assiduity of the investigator, a single individual is reisolated from all the products of fission formed within a line.