G90 The Outline of Science 



that each part of the body is in some way dominant to all the 

 parts that are posterior to it. If a cut is made on the side of the 

 body, the tissues at the cut are often so stimulated by the shock 

 of the operation, that they escape from their bondage to the 

 dominant head, and produce a new head on their own account. In 

 other cases the cut may be so made that new growth will occur 

 at the cut surface, but the new tissue will still be under the 

 influence of the old, and so a new but supernumerary tail will 

 be produced. In this way extraordinary forms may be arti- 

 ficially produced, with extra heads and tails, or consisting of 

 little else than two opposite-facing heads, like the Roman 

 Janus. 



It has even been possible to alter the whole polarity of an 

 animal. A piece of the stem of a polyp, for example, will in 

 ordinary circumstances produce a head only, or first, at its 

 anterior end. But, if it is exposed to dilute poisons or narcotics, 

 the whole piece will lose its differentiation and revert to a shape- 

 less lump ; when this is replaced in pure water it will regenerate, 

 but the head will not form at either of the original ends but from 

 the upper surface, where there is the most abundant supply of 

 oxygen. 



But perhaps the most remarkable power of regeneration is 

 exhibited by many sponges and polyps, which can be dissociated 

 into their component units without losing the power of regrowth. 

 If a sponge be chopped into small bits, and the pieces then 

 strained through the finest bolting-silk, nothing will come through 

 the meshes save the cells of which the sponge is composed, either 

 singly or in groups of ten or a dozen. The bottom of the dish 

 into which they are strained will be covered with a film of these 

 microscopic units; these will join up to form a number of little 

 spheres, about the size of a sponge embryo; in each of these the 

 cells will rearrange themselves in proper order, and the sphere, 

 reorganising itself into a miniature sponge, will thus perform a 

 veritable miracle of vitality. 





