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

HOW INDIVIDUALS ORIGINATE

Fig. 160. Some different kinds of male gametes.

1. Sea-urchin. 2. Bat. 3. Man. 4. Fire-bellied Toad.

5. Field-mouse. 6. Spade-footed Toad. 7. Tortoise.

8. Greenfinch. 9. Flatworm (Procerodes). 10. Spider-crab. 11. Squat-lobster. 12. Frog. 13. A Seaweed (the bladder-wrack). (All very highly magnified.)

of the gametes is more obvious than it is in ourselves. They are almost like a separate generation alternating with the principal body-phase as the various phases of the liverfluke alternate. In many marine invertebrates the sperms and eggs are spawned in vast misty clouds into the sea, where the sperms swim about and unite with the eggs in a perfectly independent manner.

The product of the union of two gametes is

called a zygote. Mr. Everyman, for example, is a zygote, and so are his dog and his canary and the flies on the window of his study and the cabbages in his back garden. All of the familiar living creatures of every day are zygotes. Let us turn to the less familiar gametes with which they alternate. _ The egg, the female gamete, is almost invariably a large cell. Even the smallest eggs, such as those of placental mammals, are large as cells go. The human ovum is almost a thousand times the bulk of an average human tissue-cell. Most eggs when ripe are passive and spherical, their only visible activity being to send out a welcoming cone of protoplasm which engulfs the firstcone sperm and draws it into the egg, and many lack even this.

The size to which the single egg-cell may grow is enormous. ‘There are small eggs like those of some mammals and flowering plants, under a tenth of a millimetre

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(1/250th of an inch) across; those of frogs and newts, the size of small shot; others grow as big as peas, like those of trout or some land-snails, as big as damsons, like those of cuttlefish or pigeons ; and so eventually to the huge eggs of dogfish and crocodiles, of swans and ostriches. In all eggs of birds and reptiles the true ovum is the yolky part; this is a cell packed so full of the stored food we call yolk that practically no living protoplasm remains save for a thin film on the surface, and a concentration in the form of a delicate cap at one pole. The white and the shell are not part of the true egg-cell, but merely its wrappings. In the extinct Aipyornis of Madagascar this one bloated cell attained a volume of about a gallon !

In shape some eggs depart from the spherical, but usually stop short at oval. Comparatively few, like the eggs of mosquitoes and cockroaches and a number of other insects, are elongated or flattened.

The sperm is much more variable in appearance than the egg. It is nearly always active and seeks restlessly for the waiting ovum. Though the actively swimming sperms of all many-celled animals, from polyps to men, are built on the same plan, with single propelling whip-lash behind, a condensed nucleus forming the bulk of the “head,” a middle-piece between the two

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