136 HEREDITY IN RELATION TO EUGENICS 



Finally, the peculiar thickening of the palm of the hand 

 and the plantar surface of the foot known as Tylosis seems 

 to follow the same rule as keratosis of which it is only a 

 special case. Both males and females are affected and two 

 normal parents, even of an affected family, rarely transmit 

 the defect (Figs. 110, 111). 



The records of 45 families with this abnormality have 

 been studied by Gossage. In the 39 that can be used, it 

 appears that males and females are equally affected (166 

 to 140) and transmit equally. As affected persons always 

 mate with normals, affected offspring are always simplex 

 and expectation is that half of their offspring shall be ab- 

 normal. In 28 families 222 children are abnormal and 184 

 normal. Only one exception appears to the rule that two 

 normal parents have only normal children. 



30. Epidermal Organs 



Heredity in these organs may be considered under the 

 four heads of glands, hair, nails and teeth. The inclusion 

 of teeth is justified since their true epidermal origin is now 

 recognized; they are equivalent to the scales of fishes, but, 

 in the higher animals, including man, they are confined to 

 the mouth and jaws. On account of the close interrelation- 

 ship of these four types of organs a modification of one may 

 mean a change in all, and so it is not possible in discussing 

 one of them always to avoid a consideration of another. 



a. The Skin Glands are principally the sebaceous and 

 sweat glands, associated functionally with the hair and 

 morphologically with the milk glands. The latter are usually 

 reduced to two in man but cases of supernmnerary mam- 

 mae are not exceedingly rare. This condition is doubtless 

 hereditary for Leichtenstern (1878) refers to the case of a 

 woman with three mammae on the chest who bore a daughter 

 who in turn also had three mammae (though the additional 



