i6o THE PHYSIOLOGY OF TWINNING 



to the list of developmental anomalies which bear some 

 lawful relation to the incidence of mental deficiency." 



The author tabulates forty cases of total hemihyper- 

 trophy that have been reported in the literature, 

 and twenty-three cases of partial hemihypertrophy, a 

 considerable proportion of which are associated with 

 mental defectiveness. In discussing the relation of 

 hemihypertrophy to mental defects he says : 



The frequent association of hemihypertrophy and of cranial 

 as3anmetry with mental defect and the consistent preference in 

 both conditions for enlargement of the right side suggests some 

 lawful grouping of causative factors. It is quite possible that 

 the origin of certain altogether obscure cases of secondary 

 amentia may lie in an undetectable but decisive imbalance of the 

 fundamental process of twinning which follows fertilization. 



In addition to the prevalence of mental defects in 

 connection with hemihypertrophy there occur a number 

 of other abnormal conditions always on the hyper- 

 trophied side. Among the commonest of these are 

 various skin peculiarities such as surface dilation of 

 capillaries causing red spots, large hairy moles, blue 

 pigmentation, dark-red pigmentation, excessive hairi- 

 ness. Some other peculiarities of the hypertrophied 

 side are also of interest. The hair may be. colored 

 differently on the two sides, that of the hypertrophied 

 side being longer and darker; the temperature may be 

 as much as two to four degrees higher; the epiglottis shows 

 reversed asymmetry. The fact that about 70 per cent 

 of the cases listed involve hypertrophy of the right side 

 suggests the possibihty of symmetry reversal, but there 

 is no data to show that there ever occurs any true 

 situs inversus viscerum. The only instance of the sort 



