STRUCTURE AND DEVELOPMENTAL RATE 201 



b. Double individuals with unequal components 



In every extensive collection of double specimens we not only 

 have those with components of similar size, but also a number of 

 double individuals presenting two components of different size. 

 The discrepancies in size between the two components may be 

 arranged in a graded series beginning with only a slight size dif- 

 ference and finally ending with a very small mass attached to 

 the larger component. Figures 12 to 17 illustrate such a series 

 in cases of anterior duplicities, and figures 20 to 27 show various 

 size differences between the components in completely double 

 specimens. 



Associated in all cases with these size differences are strikingly 

 noticeable and important structural differences between the 

 components. 



Figs. 12 to 17 A series of double-headed trout specimens some time after 

 hatching, and ilhistrating the fact that when the two components of a double 

 individual are unequal in size the larger component is normal in structure and 

 the smaller component is invariably 4efective. 



Fig. 12 The two heads in this individual are equal in size and both are 

 structurally normal. 



Fig. 13 The left head is slightly smaller than the right, and the right eye of 

 the smaller head is defective with a wide coloboma. The right head is entirely 

 normal. 



Fig. 14 The difference in size between the two heads is more marked than in 

 figure 13 and the smaller head is also more decidedly deformed. Its right eye is 

 entirely absent and the left eye is extremely defective, being only a small choroid 

 body with a protruding crystalline lens. The mouth and gills are unopened 

 with considerable structural distortion. The larger left head is in all respects 

 perfectly normal. 



Fig. 15 The left head is normal in size and perfect in structure, while the 

 smaller right head is completely deformed with a twisted irregular shape and no 

 definite outer indications of mouth and gills. The right eye is absent and the 

 left eye is defective. A somewhat different view of the smaller head is shown 

 immediately below the entire figure. 



Fig. 16 A double specimen with the left head still smaller in size and more 

 completely deformed. It has a cyclopean eye, and a narrow tubular brain, and 

 the branchial parts are entirely distorted. 



Fig. 17 Completes the series with a perfectly formed larger component, 

 while the smaller left head is represented by an amorphous mass as seen from 

 surface view. Should this specimen have attained adult size, it would probably 

 have been a normal trout with a small nodule representing the lesser component 

 projecting from its body wall. 



