STRUCTURE AND DEVELOPMENTAL RATE 207 



readily imagine that if this specimen had grown from its present 

 length of 3 cm. up to a size of 30 cm., the small component would 

 have been so outgrown by the larger as to appear a tiny almost 

 unnoticeable nodule on the ventral surface of the large fish. 

 The little component might possibly have become entirely 

 included within the ventral body wall of the larger one. A 

 twin inclusion would thus be formed. 



3. The small component and the frequency of double or twin 

 individuals. The frequency of double and twin individuals is 

 probably much greater than realized. No doubt such speci- 

 mens as the last one considered in the foregoing section might 

 often attain the adult state without being suspected of their 

 twin nature. It is also likely, in view of the fact that a graded 

 series of reductions in the size of the smaller components in 

 double specimens can be arranged down to the conditions here 

 illustrated, that still more decided reductions exist. There 

 probably are specimens with merely a trace of the smaller compo- 

 nent, or it is possible that the small component might entirely 

 disappear. Thus an individual appearing as a typically single 

 specimen might in truth partake of the qualities and nature of 

 the major component of a double individual. 



In connection with such probabilities the condition of situs 

 inversus viscerum is of interest. Morrill ('19) has found in an 

 examination of certain of these double fish that a reverse arrange- 

 ment of the viscera occurs in one of the components with a far 

 greater frequency than has ever been known to occur among any 

 group of single vertebrate individuals. The reverse arrange- 



Figs. 20 to 23 A series of united twin trout, some time after hatching, further 

 illustrating the principle that in double individuals with components of different 

 size the larger one is normal structurally and the smaller is deformed. 



Fig. 20 Twin trout, both of equal size and normal structure. Each twin is 

 fully as large as a single specimen of the same age. 



Fig. 21 The upper individual is the larger and is structurally normal, the 

 lower specimen is slightly smaller with no eye on the right side and the left eye, 

 shown in the small accompanying figure, is deformed with a decided coloboma. 



Figs. 22 and 23 Two views of the same united pair. The upper larger indi- 

 vidual is structurally normal, and the lower smaller twin is eyeless and some- 

 what further deformed, with a twisted caudal region which also causes a twist 

 in the tail of the larger specimen. 



