Note 58 — Chapter VI · ANTAGONISM BETWEEN GROWTH AND SEXUAL GENESIS.

To this passage Prof. MacBride appends the remark:—“This is quite proven now, and the statement as it stands is quite correct; but far better and more minutely worked out cases are to be found amongst the Infusoria. In Paramœcium for example, there are normally present a large macronucleus and a small micronucleus lying alongside of it. When two individuals adhere preparatory to conjugation, the macronucleus breaks up into fragments which are absorbed: the micronucleus—which has some time previously divided into two—begins to break up further and eventually forms eight bodies; all of these except one disappear; this last piece then divides into two; of these two one represents a male genital cell, for it passes over into the body of the other Paramœcium and fuses with one of the two corresponding nuclei there; thus each of the two individuals which adhere fertilizes the other. The two individuals then separate and the nucleus (result of fusion of male and female nuclei) in each divides into four. Of these, two move to one end of the animal and two to the other. The animal then divides into two transversely—each of the products thus having two nuclei which form the micro-and macronucleus of it. Thus it appears that the function of sexual union is simply to give increased vigour to all the vital processes including fission. Since as mentioned above (p. 443) if it is prevented, the products of fission are eventually unable to feed themselves.”