Symmetry and Homology in Limbs 115 



ment of parts in an embryo not only as regards the sides, but in 

 the relation of fore and hind parts to each other. The analogy is 

 still more striking between the structure of double monsters and 

 the arrangement of polarized matter about two adjoining magnets. 

 Two parallel magnets brought near together, produce in the matter 

 around them a compound figure, the middle portion of which con- 

 sists of curves from the adjoining sides of the magnets; the 

 particles belonging to either series never pass the line of equili- 

 brium, but are deflected symmetrically upward or downward. The 

 symmetry of the whole figure is thus retained, although the right 

 and left curves of the same magnet have lost their balance. So, 

 in the abnormal development of embryos, where two nervous axis 

 occur side by side, a series of intermediate limbs or organs will be 

 formed where the respective influences of the two axis come into 

 contact, and the two bodies will be symmetrical with each other, 

 but not in themselves. 



Again, if the magnets, touching at one end, are divergent at the 

 other, the isolated ends will have the usual symmetrical double 

 series of independent curves ; but, as the magnets approach each 

 other, these curves will become gradually modified or suppressed 

 until a single symmetrical figure is formed at the end where the 

 magnets unite. So, if the axis of double embryos are inclined, 

 the separated ends will have each a head, but as the two axis 

 converge, the intermediate organs will be altered or obsolete. The 

 body will be provided with two legs, one under the control of the 

 right axis, and the other under that of the left. Precisely such a 

 case occurred in the well-known instance of Ritta Christina. The 

 type of any double monster may be imitated, by the combined 

 action of two magnets. 



Thus a vital force is displayed in the development of animals, 

 which, like a magnetic force on polarized matter, tends, when un- 

 disturbed by external agencies, to produce symmetrical figures. 



Sam. H. Scudder. 



