The science of life : fully illustrated in tone and line and including many diagrams

BOOK 4

members of conjugating pairs are usually from different parents, and so it affords a method for the actual blending of living material from different stocks.

From these lowly plants we can trace a series of stages leading up to the state of affairs that is found in ourselves. In all the higher animals and plants the essence of the process is the same—the reproductive cells have to melt together before they can give rise to new individuals; they are called, therefore, gametes, or marrying-cells. These gametes are usually of two kinds —active smaller male gametes or sperm-

QIVISION

SS 75

Fig. 159. magnified.

Centre: A typical Copromonas with nucleus, flagellum, and gullet. Left: A series of stages in its Right :

reproduction ; one Copromonas divides into two. Copromonas conjugate to form one.

cells and passive larger female gametes or egg-cells. There is every gradation in this inequality. We may have quite equal cells conjugating or we may find very considerable inequality in the size of the conjugating cells. Thus, in the simplest phase of the sexual process there is neither male nor female ; there is a sexual process without distinction of sex.

But as a further development of the appearance of the differentiation of the conjugating elements into larger and smaller there presently appears a differentiation of the parent bodies into those producing active and those producing larger conjugating cells. These are the incipient phases of sexual differentiation.

272

THE SCIENCE OF LIFE

ye ip

ae a

The life-history of Copromonas, a simple flagellate Protozoan, highly

CHAPTER 2

In plants, and in the lowest animals in which sex is thus entangled with reproduction, the sexual method exists, as an alternative to asexual, but in higher forms such as vertebrata reproduction is exclusively sex-ridden. Before we can produce new individuals there has to be this actual blending of the substance of two parents. Even in our own species this is plainly an anti-reproductive thing, for if we could proliferate asexually it would take only one to do what now needs two, and we could multiply twice as fast. Thus, in the course of Evolution the two originally distinct and antagonistic processes have

6 Nb

gether and become inCONJUGATION separably blended. Evidently there is a riddle here, and one of very profound importance. There must be something very important about this sexual process, this mixing of the stuff of different stocks. What it is we shall presently discover, when the basal principles of heredity have

ee

Stages in its sexual process; two in (Modified from Dobell.) been set forth. § 4 The Gametes, or Marrying Cells

In such great animals as we are there is a rhythm in the life-history, an alternation between two phases. First there is the human body that we know as a familiar fact of every day. Then, proliferated inside the body, are the gametes, that is to say the sperms and eggs, which are to unite in pairs to form new people. The proliferation of the gametes is our method of multiplying ; their subsequent union is the anti-reproductive process which, as we have just seen, is here entangled with propagation.

In many other animals the independence