560 MERISTIC VARIATION. [part i. 



It will be remembered that the resemblance between twins is a true case of 

 similar and simultaneous Variation of counterparts. This is clearly proved by the 

 fact that when distinct Meristic Variations are exhibited by one twin they are not 

 rarely present in the other also. Cases of this simultaneous Variation are familiar 

 to all who have studied this subject. A useful list of examples in completely separate 

 twins is given by Windlk 1 . One of the best known cases in twins incompletely 

 separated, is that of the Siamese Twins-, who had each only eleven pairs of ribs 

 (instead of twelve). 



Reference must lastly be made to a particular corollary which may naturally be 

 deduced from the fact that the bodies of incompletely separated twins are grouped 

 as a single system of Symmetry. If the whole common body were bilaterally sym- 

 metrical, one twin must be the optical image of the other. But if the organs of one 

 twin be normally disposed, the organs of the other must be transposed in completion 

 of the Symmetry. This theoretical expectation is in part borne out by the facts. 

 With a view to this question Eichwald 3 examined the evidence as to thoracopagous 

 double monsters (including xiphopagi, &c), and found that in almost every case one 

 of the bodies shewed some transposition of viscera, though to a varying extent 4 . 



There are nevertheless a few cases even of thoracopagi where neither body ex- 

 hibits any transposition 5 . Moreover, contrary to natural expectation, it does not 

 appear that in ordinary cases of completely separate twins either twin has its 

 viscera transposed; and conversely, of 152 cases of transposition collected by 

 Kuchenmeister only one could be shewn to have been a twin 6 . It seems therefore 

 that the frequency of transposition in double monstrosity depends in some way upon 

 the maintenance of the connexion between the twins; and that if the separation be 

 completed early, as it must be supposed to be in cases of homologous twins born 

 separate, then both bodies as a rule develop upon the normal plan, like the bodies 

 of multiple births of other animals. But as the evidence now stands there is no 

 reason to suppose that individuals with transposition of viscera, born as single births, 

 have ever had a counterpart any more than individuals whose viscera are normally 

 placed, tempting as it is to imagine that both may have had some counterpart which 

 in the ordinary course does not develop. 



For the present we need not go beyond the fact that between 

 complete duplicity resulting in " homologous twins," and the 

 least forms of axial duplicity, consisting in a doubling of either 

 extremity of the longitudinal axis almost all possible degrees 

 have been seen 7 . By persons unfamiliar with abnormalities it 



1 Windle, B. C, Jour. Anat. Phys., xxvi. p. 295. 



2 For full abstracts of all evidence relating to this case, see Kuchenmeister, 

 Die angeb. Verlagerung d. Eingeweide d. Menschen, Leipzig, 1883, p. 201. 



3 Eichwald, Pet. med. Ztsch., 1870, No. 2, quoted from abstr. Virch. u. Hirsch, 

 Jahresb., 1871, p. 167. 



4 Eichwald supports the view that in these cases it is the right twin which shews 

 the transposition. As Kuchenmeister (I. c.) points out, this cannot by the nature 

 of the case be a universal rule ; for the relative position of xiphopagous twins may 

 result singly from the way in which they happen to be laid by the mother or the 

 midwife. Of the Siamese Twins, besides, it was Chang, the left twin, in whose body 

 there were indications of transposition. The twins may also remain face to face. 

 The expression " right twin " must always need further definition, and it should be 

 qualified as the right when the livers are adjacent, or when the hearts are adjacent, 

 as the case may be. Whether the rule is wholly or partially true for either of these 

 positions seems to be very doubtful. 



5 For example, Bottchee, Dorpater med. Ztschr., n. p. 105, quoted from V. u. 

 II., Jahresb., I. c. In the specimen Terat. Cat. Coll. Surg. Mus., 1872, No. 114, there 

 is no transposition, but here the hearts were not separate. 



6 1. c., p. 268. One, however, was a child of a mother who had before borne 

 twins, I. c, p. 313. 



7 The fact that some of the degrees are much more common than others has an 

 obvious bearing on the question of Discontinuity, which might with profit be pur- 

 sued. A statistical examination as to the angles at which the bodies are most 

 frequently inclined to each other would also probably lead to an interesting result. 



