STRUCTURE AND DEVELOPMENTAL RATE 209 



ment of the viscera in one component, though it by no means 

 always occurs, would seem in some manner to be associated with 

 the double condition. This reversed visceral arrangement also 

 occurs very rarely among man and other mammals in single 

 individuals. Its remarkable frequence among these double speci- 

 mens would lead one to suspect very strongly that when a reversal 

 of the visceral arrangement occurs, the apparently single indi- 

 vidual is in reality a twin. All such specimens should be care- 

 fully examined for twin or embryonic inclusions as positive evi- 

 dence of their double nature. Failure to find such inclusions 

 would not, however, disprove the above suspicion, since the 

 inclusions might be represented by structures so minute as to 

 be readily overlooked. 



4. The small component and certain theories of teratoma. An- 

 other much-debated problem may be somewhat illuminated by 

 this study of double specimens. I refer to the various ideas of 

 the possible origin of so-called teratomata or embryonal tumors. 

 Such formations occur with greatest frequency in the lower 

 abdominal or pelvic region. Certain pathologists have thought 

 them to arise from a development of misplaced or arrested blasto- 

 meres, others have thought it possible that they might arise 

 through some form of parthenogenetic development, and still 

 others have looked upon them as a type of twin inclusion. The 



Figs. 24 to 27 A continuation of the twin trout series shown in figures 20 to 

 23. In this group the smaller member is still more inhibited in size and more 

 completely defective in structure. The larger component is perfect in all. 



Fig. 24 The smaller twin is little more than half the size of the larger with 

 an amorphous head containing one defective eye and the body is twisted. 



Fig. 25 The smaller twin is here greatly twisted or coiled, its head is de- 

 formed, possessing a large defective left eye, and the right eye consists of a tiny 

 choroid vesicle indicated by the dark spot, E. 



Fig. 26 The lesser component is still smaller in size, short and twisted with a 

 considerably suppressed eyeless head. 



Fig. 27 The larger twin is a beautifully normal specimen, while attached to 

 the opposite surface of the yolk-sac is a small individual represented by a badly 

 deformed head with no mouth or gills and one defective eye. Almost the entire 

 body of this component is absent and the tail is represented by a conical mass 

 with no caudal fin. Should such a specimen attain adult size the smaller indi- 

 vidual would be attached to the ventral abdominal wall of the larger as a nodular 

 twin. 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 28, NO. 2 



