SYMMETRY REVERSAL AND MIRROR-IMAGING 171 



In the cases of typical situs inversus the left-hand com- 

 ponent shows the normal situs and the right-hand 

 component the reversed situs of all asymmetrical struc- 

 tures. Morrill states, however, that mirror-imaging, 

 even in the types of duplicity described as most favorable 

 for its appearance, is by no means the rule, but that the 

 majority of specimens show the normal situs in both 

 components. On the basis of this finding he concludes 

 that reversal of symmetry is rare in fishes. 



Somewhat more recently Swett (192 1) has studied 

 the subject in another collection of somewhat more 

 advanced trout embryos. He also finds that situs 

 inversus is confined to monsters showing a certain 

 limited range of dupKcity. Swett says: 



It will be seen that those whose vertebral columns are fused 

 anterior to the dorsal fin and those that are nearly separate show 

 at best only doubtful mirror-imaging. It is further noted, as 

 was also found by Morrill, that not all the animals falHng within 

 the limits of doubhng apparently most favorable for transposition 

 of the viscera show this phenomenon. Thus there are indications 

 that the reversal of asymmetry is not a necessary consequence of 

 any condition of doubling, but may or may not occur, depending 

 on some other factor capable of operating within these limits. 



Certain additional facts appear in Swett's paper. 

 He finds that mirror-imaging of the digestive tract exists 

 only w^hen the point of union of the twin tracts occurs at 

 some point between the pylorus and the point of exit 

 of the intestine from the abdominal cavity. Another 

 point of considerable interest, in view of the interpreta- 

 tion given by Stockard of the nature and relationship of 

 autosite and parasite in double-monster fish, is that in 

 Swett's material there is a case of partial situs inversus 

 in the parasite (Fig. 62, p. 172). This finding goes far 



