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 forelimb experimentally 

 produced by an operation on the 

 primordium of the normal limb. Note 

 that the duplicate limb is the mirror 

 image of the normal. (After Harrison.) 



