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

BOOK 4

to carry the centrosome, and a cap covering the front of the head, and probably containing the substance needed to galvanize the egg into development, yet this plan is much varied in its details.

The head may be rounded as in the sperm of bats or starfish, or flattened symmetrically as in man, or flat and set slantwise as in squirrels ; it may be a long, thin cylinder as in tortoise, toad and spider, a mere ball like that of amphioxus, or a corkscrew as in finches. The middle-piece may be tiny, or longer than all the rest of the sperm put together. The tail may be a simple whiplash, long or short, or be equipped with a vibrating fin-membrane of transparent protoplasm. The only exceptions to this general plan are found among flatworms, some of which have sperms with two whiptails instead of one.

In plants, however, the single-tailed sperm seems never to occur. Those of many lower plants, including mosses, have two flagella, and in some seaweeds these two-lashed sperms are not to be distinguished in appearance from free-living single-celled flagellates. Fern-sperms have usually a corkscrew nucleus, and a great many flagella instead of only two; often they carry with them that part of the general protoplasm which is not needed for production of head and whiplashes, only to cast it from them before entering the egg. Animal sperms have gone a stage beyond this, for they shed all their unwanted protoplasm before embarking on independent life.

Both among animals and plants, however, are to be found sperms which have abandoned their primitive motility. The gametenuclei of the flowering plants, which are merely floated along the pollen-tube into the egg, are the most.familiar examples ; they have abandoned both motility and independence. But all the higher Crustacea have unmotile though independent sperms, strange stiff structures which look like tripods or calthrops or catherine wheels. In some water-fleas the sperms cannot swim but crawl like amoebe. ‘The round-worms have almost motionless sperms, mere lumps of protoplasm, and mite-sperms and millipedesperms have also lost the power of swimming.

Thus these gametes present themselves in various forms, but always doing the same things, alternating with zygotes, begotten of zygotes and uniting to form zygotes again. We cannot better summarize the position than by quoting Professor Punnett’s little book on Mendelism, in which he says :

“People generally look upon the human

274

THE SCIENCE OF LIFE

CHAPTER 2

species as having two kinds of individuals, males and females, and it is for them that the sociologists and legislators frame their schemes. ‘This, however, is but an imperfect view to take of ourselves. In reality we are of four kinds, male zygotes and female zygotes, large gametes and small gametes, and heredity is the link that binds us together. If our lives were like those of the starfish or the sea-urchin, we should probably have realized this sooner. For the gametes of these animals live freely, and contract their marriages in the waters of the sea. With us it is different, because half of us must live within the other half or perish. Parasites upon the rest, levying a daily toll of nutriment upon their hosts, they are yet in some measure the arbiters of the destiny of those within whom they dwell. At the moment of union of two gametes is decided the character of another zygote, as well as the nature of the population of gametes which must make its home within him. The union once effected, the inevitable sequence takes its course, and whether it be good, or whether it be evil, we, the zygotes, have no longer power to alter it.”

85

Some Evasions and Replacements of Sexuality

In most of the higher animals, as we have noted, reproduction becomes inseparably entangled with sex. But Nature is a tricky worker ; as living things have evolved she has vacillated and often enough she has changed her mind and gone back on her previous acts. One or two animals are curious in that they have returned to sexless methods of reproduction ; evolved from purely sexual stocks, they have found ways of dodging the entanglement. In many species there exist females which produce eggs as if for sexual reproduction, but nevertheless dispense with males. The eggs develop without fusion with a sperm ; the offspring are fatherless. This “ virgin reproduction ”’ or parthenogenesis is found in a number of organisms, such as_greenfly, water-fleas, rotifers, and occasionally flowering plants. From one point of view it is like spore-production, since a single cell is detached and grows into a new organism. But it is different in its origin. Spore-production is among the main original methods of reproduction. Parthenogenesis is secondarily simple, having arisen from true sexual reproduction by dispensing with the need for spermatozoa and with the male that bears