STRUCTURE AND DEVELOPMENTAL RATE 215 



the same capacities for polydactylous development should occur 

 at one time in this mother. The expression of Polydactyly in a 

 family in which it is hereditary, is most variable. Neither the 

 father nor the mother of these twins were said to have polydac- 

 tylous hands or feet, and it was claimed that they had another 

 child with an ordinary number of fingers and toes. This, how- 

 ever, was a case of a 'seven-month' stillbirth in a New York 

 hospital, and it proved impossible to obtain a family history of 

 positive value. 



These twins show further deformities that are almost identical 

 in the two. All of the four kidneys are cystic and the left kidney 

 in each individual shows the cystic condition in a more exag- 

 gerated form than does the right. The two heads have posteri- 

 orly protruding meningoceles, one being slightly larger than the 

 other. The meningeal hernias are probably due to the action 

 of some environmental influence that produced closely similar 

 responses in these two individuals of identical germinal composi- 

 tion. In this case both members of the pair are malformed in 

 addition to their unusual characters of genetic origin. 



In every way these twins are structurally about the same with 

 the exception that the one on the left is smaller than the one on 

 the right to just the extent carefully shown in the two figures. 



This is a most positive case of identical human twins and 

 would certainly seem to leave no reason for question as to the 

 occurrence of such individuals. 



From these examples it is probable that the two equal compo- 

 nents in double human individuals are in about the same relation- 

 ship to one another as are the equal components of other double 

 vertebrate specimens. 



The double human specimens seen in many musemns as well 

 as those illustrated in the literature in which the two components 

 differ considerably in size also follow the rule found for similar 

 fish specimens. In double human specimens the larger compo- 

 nent is usually normal in structure and the smaller component 

 is always deformed. Extreme cases of this type are exhibited 

 among the freaks in 'side shows.' In these living specimens 

 the larger component has a well-formed human body with the 



