Outlines of zoology / by J. Arthur Thomson.

Date:
1892
Catalogue details

Licence: Public Domain Mark

Credit: Outlines of zoology / by J. Arthur Thomson. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

    70/748 (page 46)
    CHAPTER IV. THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS. I. Reproduction. In the higher anmials the beginnings of individual life are hidden, within the womb in mammals, within the egg-shell in birds. It is natural, therefore, that early preoccupation with those higher forms should have hindered the recog- nition of what seems to us an evident fact, that almost every organism, whether plant or animal, arises from an egg-cell or ovum which has been fertilised by a male-cell or sperm- atozoon. The exceptions to this fact are those organisms which multiply by buds or detached overgrowths, and those which arise from an egg-cell which requires no fertilisation. Thus Hydra may form a separable bud, much as a rose-bush sends out a sucker; thus drone-bees have a mother but no father, for they arise from parthenogenetic eggs which are not fertilised. Apart from these and similar cases, the ovum theory, which Agassiz called the greatest discovery in the natural sciences in modern times, is true,—that each organism begins from the division of a fertilised egg-cell. We can easily see this simple beginning in the frog-spawn from the ditch, in the eggs of salmon, in those of the pond- snail {Lymnaus), and in hosts of other cases. History.—Perhaps we can realise this discovery better if we con- sider its history. For a long time, on into the ])resent century, what was called the doctrine of preforination ])revailed. According to this theory, development was merely an imfolding (evolution) of a pre- formed miniature which lay within the germ. The ovists found this miniature model of the future organism in the egg; the animalculists found and even figured it within the spermatozoon. There is no becoming, said Ilaller, no jiart of the body is made from another, all are created at once. But this was not all. The germ was more than