of the organism. The parts work together at all stages of the process to maintain the harmonious coördinated development of the whole. How and why the diverse parts are coördinated in their ever changing relations is one of the unsolved problems of biology. As we have stated in the beginning of the chapter, the wonder of development is one of the most striking of all the wonders which living beings exhibit.

REFERENCES

BAILEY, F. R., AND MILLER, A. M., A Text-Book of Embryology. N. Y., Wood, 1921.

HAECKEL, E., The Evolution of Man. N. Y., Putman, 1910.

HERTWIG, O., Text-Book of Embryology. London, Swan, Sonnenschein, 1892.

HOLMES, S. J., The Biology of the Frog (chapter on "Development"). N. Y., Macmillan, 1914.

KELLICOTT, W. E., General Embryology. N. Y., Holt, 1908.

Outlines of Chordate Development. N. Y., Holt, 1908. LILLIE, F. R., Problems of Fertilization, University of Chicago Press,

1919.

MARSHALL, A. M., Vertebrate Embryology. London, Smith, Elder, 1893.

MORGAN, T. H., The Development of the Frog's Egg. N. Y., Macmillan, 1897.

WILDER, H. H., A History of the Human Body (2nd ed.). N. Y., Holt,

1923.

CHAPTER X

THE PERPETUATION OF LIFE

A large part of the business of living is concerned with the production of the next generation. Life is always bent on its own perpetuation, and the process of reproduction is only one of its many devices for cheating death. If the grim reaper overtakes the individual, as he always does in the higher organisms, life wins in the game through the production of new individuals from the bodies of the older ones before they die. The perpetuation of kind is a problem which organisms solve in ways which are almost as numerous and varied as the activities of the organisms themselves. Some of these ways are very simple, others are very elaborate and ingenious. Obviously we can consider only a few of them in the present chapter.

One of the simplest modes of perpetuating life is ordinary binary fission, by which the organism is divided into two similar parts. This is the prevalent fashion of reproduction among the bacteria, the unicellular algæ, and most groups of the Protozoa. A variation of this method occurs in reproduction by budding, which is a sort of unequal fission. Even very low organisms, such as the yeast plants, reproduce by budding, and the method occurs more or less in many other one-celled forms. It is a notoriously common method of propagation among plants. It is common among sponges, and in the hydroids and coral polyps. In the fresh-water Hydra which usually propagates in this way, the buds which protrude from the sides of the body contain both ectoderm and entoderm and a central cavity which connects with the general digestive cavity of the parental organism (Fig. 52). Tentacles push out around the distal end of the bud, a mouth forms between them, and the bud finally constricts off at the base, and becomes an independent Hydra. In the hydroids the

buds usually remain attached to the stem and they may form a much-branched colony, simulating the form of a higher plant, which is developed by a somewhat similar method. A number of worms bud off small incipient individuals at the posterior end of their bodies. The tunicates, coral polyps, and several other kinds of attached, or sessile, animals may form colonies of numerous individuals by a continued process of budding.

What is called spore formation is another variation of fission. In one-celled organisms there is frequently a series of nuclear divisions without an accompanying division of the cytoplasm, after which the cytoplasm aggregates around each nucleus, and the whole breaks up into a number of small, nucleated masses called spores. This is the method of reproduction among the Sporozoa, and it is found also in other Protozoa, and in some of the unicellular plants.

All of the preceding modes of perpetuating the species are examples of asexual reproduction. In sexual reproduction there is a union of gametes, or sexual cells, which precedes, and is often necessary to initiate, the reproductive process. As we have already seen, sexual reproduction occurs in both unicellular and multicellular organisms. It is not quite coextensive with life, because, so far as we know, it is not found in the very simplest of the one-celled organisms, the bacteria and the blue-green algæ. It has also never been discovered in several groups of flagellates. Scientific caution should prevent our asserting positively that sexual reproduction does not occur among these minute forms, and in fact, some investigators hold that there is evidence for its existence even in the bacteria. Any day some assiduous observer may prove that it occurs. Nevertheless, there

has been much careful search for indubitable proof of its existence, but thus far in vain.

The simplest manifestation of sexual reproduction is the conjugation of unicellular organisms. This commonly occurs after a series of generations produced by the ordinary process of fission. In Hæmatococcus it has been found that the free-swimming flagellated cells meet and fuse in pairs, literally becoming one

flesh, the two nuclei, as well as the cytoplasmic bodies, becoming united into one (Fig. 29). There is here no marked difference between individuals of the fusing pairs, so that we cannot speak of either member as male or female. There are other unicellular forms where one individual, the female gamete, is relatively large and passive, while the other, the male gamete, is much smaller and

FIG. 93-Conjugation in Paramecium: A, two Paramecia with their oral surfaces in contact; mac, macronucleus; mic, micronucleus. B, division of each micronucleus. C, each of the two micronuclei resulting from the preceding division again dividing. D, stage with four micronuclei in each Paramecium. E, three of the micronuclei degenerating, the fourth dividing once more; the macronucleus beginning to fragment. F, further fragmentation of the macronucleus, the wandering micronucleus passing from each Paramecium into the other. G, union of the stationary and wandering micronuclei. H, a single Paramocium after conjugation, the macronucleus degenerated, and the micronucleus beginning to divide. (Schematic representation based on the work of Maupas and Hertwig.)

active. Such, it may be recalled, is the condition in the sexual stage of the malarial parasite, Plasmodium, in which the sexual gametes resemble the ova and spermatozoa of the higher animals.

A departure from the usual mode of conjugation occurs in

Paramœcium, and most other Infusoria. In Paramecium, in which the process has been most thoroughly studied, the individuals which pair become united for a time by their ventral, or oral, surfaces. While they are swimming about in pairs, several changes take place in their nuclei. The large meganucleus in each breaks up and degenerates. The micronucleus divides twice, forming four micronuclei, of which three degenerate; the remaining one divides once more into two nuclei, one of which passes over from each Paramecium into the body of the other. The two Paramœcia thus exchange nuclei. In each individual the nucleus which is received from the other fuses with the nucleus which remains. It is perhaps not necessary for our purpose to describe the nuclear changes that take place in the Paramœcia after separation, further than to state that the single nucleus resulting from conjugation gives rise to both the micronucleus and the meganucleus of the individuals that are derived from each member of the conjugating pair.

In this complicated nuclear behavior there are certain features which recall the sexual reproduction of the higher animals. The divisions of the micronuclei preparatory to conjugation suggest the reducing divisions in the formation of the sex cells; and the union of the two micronuclei is, of course, comparable with what takes place in fertilization. Whether the division of the micronuclei actually effects a reduction in the number of chromosomes in Paramœcium is very difficult to ascertain. Unfortunately, little precise information has been gained concerning chromosome reduction in the one-celled organisms, although there are several facts which indicate that such a phenomenon occurs among them, much as it does in higher animals and plants.

Since the phenomena of sex are fundamentally similar in all sexually reproduced forms, it is probable that sex has much the same biological import for the life of the species, wherever it occurs. The problem of the significance of conjugation is but a part of the larger problem of the biological meaning of sex in general. Why should there be two sexes, anyway? Why could not organisms have perpetuated their kind with just females?