172 THE PHYSIOLOGY OF TWINNING 



toward corroborating my theory that even two unequal 

 components arise through the separation of the bilateral 

 primordia of a single embryonic axis, and that one half 

 must have been more susceptible to retarding agents than 

 the other. A very clear case of typical and complete 

 situs inversus is shown in Figure 63 (p. 1 73) . Several other 

 good cases are figured by Swett. It may also be said that, 

 even in a much smaller collection of double monsters, 



ri-- 



SB L 

 Fig. 62. — Ventral view of trout double monster of the autosite- 

 parasite type, showing partial situs inversus viscerum. The lobe at the 

 left of the figure just below the normal head is the reduced parasite com- 

 ponent representing the inferior or right-hand twin. H, heart; L, liver; 

 SB, swim bladder; S, stomach; /, intestine. (From Swett.) 



Swett found more cases of situs inversus than did Morrill 

 in a larger collection. The difference, I think, is largely 

 one of interpretation. Many slight degrees of reversal 

 are likely to be overlooked. In all probability the cases 

 described by Morrill as uncertain or irregular should be 

 diagnosed as examples of incomplete situs inversus. 



In attempting to interpret the data on situs inversus 

 in fishes two particular conditions need to be emphasized : 



I. There is no reason to expect to find situs inversus 

 except in double individuals derived from the antimeric 

 halves of a single embryonic axis. In the case of 



