SYMMETRY REVERSAL AND MIRROR-IMAGING 169 



monsters (Fig. 57). Each side is a practically exact 

 mirror-image of the other, the inner sides in each partly 

 divided head being less fully developed than the outer. 

 We note another equally beautiful instance of mirror- 

 imaging in the skulls of double-headed monsters. In 

 Figure 58 is shown such 

 a skull in which all outer 

 bones in both compo- 

 nents are perfect and a 

 number of inner bones 

 imperfectly duplicated 

 or single. 



Certain beautiful 

 cases of duplicity of 

 visceral parts are also 

 figured, such as heart and 

 the urogenital system. 

 Especially interesting 

 are the conditions in 

 some oi these hearts in 

 which various degrees 

 of duplicity are shown. 



Figure 59 (p. 170) shows the heart of a normal fish embryo 

 before it has assumed any marked degree of asymmetry. 

 Figure 60 (p. 170) shows a case of partial duplicity, espe- 

 cially complete at the ventricular end. Figure 61 (p. 170) 

 is an example of almost complete duplicity of the heart, 

 only the inner vessels being united, especially the inner 

 vena cava. The two halves are perfect mirror-images. 

 One step farther and the two hearts wdth their main 

 vessels would be entirely separated the one set from the 

 other. Were such a further separation to occur we would 



Fig. 58. — The skull and jaw of a 

 partially double-headed young trout, 

 showing perfect mirror-imaging of 

 twinned structures. (After Gemmill.) 



