202 CHARLES R. STOCKARD 



1. Condition of the larger component. Whenever the compo- 

 nents of a double individual are unequal in size, the larger com- 

 ponent, with one exception in more than seventy such specimens 

 that I have studied, is invariably normal in structure. A careful 

 examination of a large number of illustrations of such specimens 

 through the literature, without exception confirms the above 

 fact. It would seem to be a rule, that the larger component of a 

 double individual is no more likely to be defective inform or structure 

 than is a single individual of the same species developing under a 

 similar environment. 



2. Condition of the smaller component. Whenever the compo- 

 nents of a double individual are unequal in size the smaller compo- 

 nent, in all cases examined, is always abnormal inform and structure. 

 A survey of the figures in the literature also shows this to be 

 constantly the case. 



A study of the types of deformities and defects exhibited by 

 these smaller components is most instructive, and is further 

 extremely suggestive in an analysis of the causes underlying all 

 abnormal development. 



Examining first the cases of anterior duplicities, figure 12 shows 

 two heads of equal size, both structurally normal. In figure 13 

 the left head is only slightly smaller than the right. The right 

 head is normal, but the right eye in the left head is small and de- 

 fective in form, with a ventral coloboma and a protruding crystal- 

 line lens. The size difference between the two heads is slight 

 and the abnormalities shown by the smaller are not of an extreme 

 type. 



In figure 14 one head is decidedly larger than the other, the 

 larger head, as usual, is normal, the smaller is very abnormal. 

 There is only one minute deeply buried eye, E, and the structures 

 of the mouth and branchial arches are peculiarly distorted. Fig- 

 ure 15 shows a still more marked size difference between the two 

 components, and the smaller one here is decidedly twisted, with 

 two poorly developed eyes almost in apposition on the ventral 

 surface of the head. Mouth and gill formations are superficially 

 suppressed, but there are certain contorted structures represent- 

 ing these parts. The brain lacks its usual bilaterality and has a 



