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

HOW INDIVIDUALS ORIGINATE

them. ‘The spore-producing fern or fungus is asexual, a neuter creature; but the parthenogenetic greenfly or rotifer on the other hand is definitely a female, even though she lacks a mate.

Moreover the human experimeter has found ways of getting round the need for sexual fusion. One of the most striking discoveries of modern biology was that of Jacques Loeb in 1899, that eggs which normally needed to be fertilized could be made to develop by substituting some mandevised treatment for the natural stimulus of the sperm. In other words, he caused artificial parthenogenesis. Artificial parthenogenesis was first induced by chemical treatment of sea-urchin eggs ; later it was found that almost any eggs which are accessible to the experimenter by being laid into the water—including those of sea-urchins, starfish, worms, snails, and even frogs—can be made to develop unfertilized ; and that the fatherless young thus produced can be as healthy as those arising in the ordinary course of Nature—fatherless star-fish and sea-urchins and tadpoles have been reared to the adult stage. The agencies needed to lift the ban from the unfertilized egg and make it develop differ from animal to animal. Heat or shaking will do it in the starfish, while pricking with a needle dipped in blood (a recipe reminiscent of magic!) is needed for frogs.

In mammals the ovum is inaccessible to the experimenter, so that we do not know whether artificial parthenogenesis is possible. There is no reason to suppose that it is not. Surgery and experimental technique are advancing rapidly, so probably we shall find out very soon.

From these strange facts an important point emerges. It is that a spermatozoon does two things to the egg-cell when it fertilizes it. First of all it activates the eggcell and makes development possible ; this is the step that Loeb and his followers can imitate. Secondly, it, or part of it, blends physically with the egg, contributing material from another stock and so, as we shall learn, affording a basis for father-tochild heredity. The second is the essential sexual process. The first is simply a trick devised by Nature for preventing reproduction until this all-important fusion shall have taken place. Once more it becomes evident

to us that sex is imposed upon reproduction and is in its essence a different thing.

§ 6 Artificial Propagation

Cleared of the complication of sex, reproduction is seen to be simply the detachment of living bits of the bodies of one generation, which grow up into the next,

Now if this goes on under natural conditions, can we imitate it artificially? If the experimenter can make his needles and solutions play the réle of a sperm (or,

Fig. 161. Flatworms Regenerating.

: A Flatworm has been cut so as to make it grow two extra heads

Above : and two extra tails. ¢ of another worm grows head and tail and straightens itself out. Below : A Flatworm has been bisected lengthwise (dotted lines show what has been cut away). Bottom row, left to right : Three stages in the halfworm’s regeneration. As the regenerated tissue (white) grows, the old

Centre : A small oblique piece cut out of the middle

tissue (shaded) shrinks to provide the requisite material.

rather, one of the two rdles of a sperm), can he not make organisms reproduce by simply cutting them up? As a matter of fact he can.

Many segmented worms, especially the little freshwater kinds like Lumbriculus, can be chopped transversely into numbers of pieces, and each piece will grow a new head and tail and become a new whole worm; and most Planarians among the flatworms have this faculty in even greater degree, for they can be cut in all directions, and yet any piece above a very tiny size will turn into a whole worm again. So with many Coelenterates; we can multiply

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