MERISTIC PHENOMENA 43 



of hereditary likeness 4 and a proof, if any were needed, that the 

 cause of individual variation is to be sought in the differentiation 

 of germ-cells. The resemblance of identical twins depends on 

 two circumstances, First, since only two germ-cells take part 

 in their production, difference between the germ cells of the 

 same individual cannot affect them. Secondly the division of 

 the fertilised ovum, the process by which they became two in- 

 stead of one, must have been a symmetrical division. The 

 structure of twins raises however one extremely significant 

 difficulty, which as yet we cannot in any way explain. The 

 resemblance between twins is a phenomenon of symmetry, 

 like the resemblance between the two sides of a bilaterally sym- 

 metrical body. Not only is the general resemblance readily 

 so interpreted, but we know also that in double monsters, namely 

 unseparated twins, various anatomical abnormalities shown by 

 the one half-body are frequently shown by the other half- 

 also. 6 The tw r o belong to one system of symmetry How then 

 does it happen that the body of one of a pair of twins does not 

 show a transposition of viscera? We know that the relation of 

 right and left implies that the one should be the mirror-image of 

 the other. Such a relation of images may be maintained even 

 in minute details. For example if the same pattern of finger- 

 print is given by the fingers of the two hands, one is the reverse 

 of the other. In double monsters, namely unseparated twins, 

 there is evidence that an inversion of viscera does occur with 

 some frequency. Evidence from such cases is not so clear and 

 simple as might be expected, because as a matter of fact, the 

 heart and stomach, upon which the asymmetry of the viscera 

 chiefly depend, are usually common to the two bodies. Du- 

 plicity generally affects either the anterior end alone, or the pos- 

 terior end alone. The division is generally from the heart forwards, 

 giving two heads and two pairs of anterior limbs on a common 

 trunk, or from the heart backwards, giving two pairs of posterior 

 limbs with the anterior body common. In either case, though 



4 A good collection of evidence as to disease in homologous twins was lately 

 published by E. A. Cockayne, Brit. Jour. Child. Diseases, Nov., 191 1. 

 B Cp. Windle, B. C. A., Jour. Anat. Phys., XXVI, p. 295. 



