DUPLICITY. XV11 



throughout their entire length, but proceeds most rapidly 

 in the dorsal region, so that they appear to grow forwards. 

 The so-called affinity of " soi pour soi " of Geoffroy is 

 probably quite imaginary ; for organs, when differentiated 

 before union, probably in no case become united to homo- 

 logous parts. This is well seen in preparations Nos. 120 

 and 123 to 130, 138, &c. 



It may be said to be a well-defined law that structures 

 which are united before differentiation become double by 

 the development of homologous structures in continuity 

 with each other, a symmetrical double organ being formed ; 

 whilst structures which are first developed and then 

 united become fused irregularly with each other, giving 

 rise to deformed and unsymmetrical structures. In such 

 cases bone usually unites with bone, from the absorption 

 of all intermediate tissues being effected by the pressure 

 to which they are subjected between the two growing 

 bones. Large portions of structures so united are com- 

 monly deficient from absorption or atrophy. 



III. Cases of Heterologous union form the third sub- 

 class. In these the parts of the two embryos are irregu- 

 larly united, the adhesion resulting from the union of un- 

 like structures. In most cases, at least, these malforma- 

 tions result, like those in the two previous subclasses, from 

 the formation of a double embryo on a single yelk. Several 

 extremely complicated results occur from heterologous union; 

 but they are best studied in individual cases. In some few 

 instances of heterologous union it is possible that the second 

 embryo may result from a second ovum, impregnated at a 

 later date, finding its way into the cavity of the mem- 

 branes of an earlier impregnation. No evidence can, 

 however, be adduced in support of this view. 



Twins united in a common choronic or amniotic cavity 

 are probably the normal result of the formation of a double 

 embryo upon one yelk. There is a continuous and unbroken 

 series of forms commencing with dichotomous and double 

 monsters, passing through twins with a common cord and 



b 



