Elements of biology, with special Elements of biology, with special reference to their rôle in the lives of animals elementsofbiolog00buch Year: 1933 286 ELEMENTS OF BIOLOGY GILLS or bud of this sort that has the potential to form a definite adult structure is spoken of as the primordium of the adult structure. If a limb primordium is divided before these properties are realized, then each of the divisions possesses the property of limb formation and two or more limbs may develop, depending on the number of divisions of the original limb primordium (Fig. 192). The division can be accomplis
Elements of biology, with special Elements of biology, with special reference to their rôle in the lives of animals elementsofbiolog00buch Year: 1933 286 ELEMENTS OF BIOLOGY GILLS or bud of this sort that has the potential to form a definite adult structure is spoken of as the primordium of the adult structure. If a limb primordium is divided before these properties are realized, then each of the divisions possesses the property of limb formation and two or more limbs may develop, depending on the number of divisions of the original limb primordium (Fig. 192). The division can be accomplished in several ways. Experimentally, the bud may be divided by an incision, or by planting a piece of some other tissue in such position as to divide the structure. Similarly, the origin of double heads or double tails may be re- ferred to accidental separations or divisions in the primordia of these structures before their develop- mental processes have become fixed and determined. Experi- mentally this has been done by mechanical means and by the obliteration of the physiological organization of the organism or a part, that is, by suppression of the dominant-subordinate relations. This has been accomplished in the embryos of lower forms by obliterating the normal differences in rate of metabolism with various agents, low temperature, absence of oxygen, and chemicals that have a depressant action. If the functioning of the leading or organizing region is interfered with during critical periods of devel- opment, dominance no longer is complete and parts undertake more or less independent development; abnormalities result. The principle is illustrated most strikingly by some experimental work with adult hydra. If hydra are placed in a weak solution of alcohol, they even- tually lose their vase-shaped form; the tentacles and mouth region are resorbed and the body appears to melt down to form a flattened NORMAL LIMB DUPLICATE LIMB Fig. 192.—A salamander larva with a duplicate f
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