212 CHARLES R. STOCKARD 



modifications and defects of the lesser components are in every 

 sense identically the same as the defects which have been recorded 

 and illustrated as occurring in single individuals. 



The types of defects most commonly found, such as those of 

 the eyes, are also the most frequently observed anomalies in 

 single individuals. Therefore, not only the kinds of defects, 

 but the frequencies of their occurrence are the same among these 

 lesser components as among single deformed specimens. 



The fact that these malformed components are developing 

 in intimate union with larger normally formed components makes 

 it evident that the causal factors for the malformations are to 

 be sought in some difference that exists between the develop- 

 mental processes of the two. And, further, since the malforma- 

 tions of the one component are identical with those in single 

 specimens, the difference in conditions found between the larger 

 and smaller component may also furnish the clue to causes of 

 malformed structures in general. We shall attempt beyond, 

 in section 7, to give a logical explanation of abnormal structure 

 from this standpoint. 



c. The components in double human specimens 



In order to demonstrate that the conditions above described 

 as existing in double specimens of fish are in no way limited to 

 this class of vertebrates, I wish briefly to consider several very 

 interesting degrees of double development in human specimens 

 that have recently come into my laboratory. 



All of these specimens have been examined by my colleague 

 Doctor Morrill for conditions of situs inversus viscerum, and the 

 left component in one case of anterior duplicity, as reported by 

 him ('19), shows a reversal in the position of the viscera just as 

 was found in certain of the components among the double fish. 



The three specunens seen in plates 3 and 4 to form again a 

 graded series. The series begins with a double individual pre- 

 senting two heads and anterior portions on a single pelvis, seen 

 in the upper photograph, plate 3, and passes on to a completely 

 double specimen with the components strongly united through 



