i88 THE PHYSIOLOGY OF TWINNING 



If, however, as in the fishes, the process of bilateral 

 fission happens to halt at a certain definite level, where 

 the alimentary tract is divided pretty well back of the 

 stomach, but remains single in a; considerable part of 

 the intestine, it seems to be a matter of touch and go 

 whether the right-hand component of the conjoined 

 twins will regulate in such a way as to take on the 

 normal left-hand asymmetry of the species {situs solitus) 

 or will continue to behave, with regard to some of its 

 structures, as though it were half of a single bilaterally 

 symmetrical organism . The condition seems to me to be 

 much like that exhibited by bilaterally symmetrical 

 echinoderm larvae in which, the dominance of the left- 

 hand hydrocoele having been reduced, the right-hand 

 half assumes equivalence and both develop equally as 

 bilaterally symmetrical structures. In the cases in which 

 situs inversus viscerum is found in conjoined twins of the 

 fishes, we may interpret the effect as due to a lowering 

 of dominance of the left-hand side over the right only 

 to the point where they are equally dominant, each 

 being to a slight extent influential over the other. In 

 other words a certain degree of the old bilateral integra- 

 tion of the two half-primordia remains to express itself 

 in the mirror-image relations of the viscera. When the 

 co-ordination is completely broken down the right-hand 

 individual, as well as the left-hand one, regulates the 

 normal asymmetry of the species. There appears then 

 to be a very dehcate equilibrium at some period, in 

 connection with bilateral primordia destined to pro- 

 duce twins, between a condition of complete isolation 

 and a condition of partial integration between the two 

 halves. 



