108 HEREDITY IN RELATION TO EUGENICS 



a. Anomalies of Iris. — Coloboma is a defect in the de- 

 velopment of the optic cup such that it fails to close com- 

 pletely and leaves an open suture running from the pupil 

 to the optic nerve. The commonest external evidence of 



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Fig. 73.— a pedigree of a family affected with coloboma. Black symbols 

 stand for affected persons; all are males. A normal female in the second 

 generation transmits the defect to about half of her children, but her sons 

 alone show the defect. Stkeetfield, 1858. 



this defect is the incomplete iris; but the lens, retina, choroid 

 coat, etc., may be involved. The cause of the defect is con- 

 ceded to be an hereditary defect in the developmental im- 

 pulse (Von Hippel, 1909). 



I ■■p 



Fig. 74. — Pedigree of a family that shows absence of iridae (black s3anbola). 

 Here, too, only males show the defect, except for III, 10 and 11. Hypothesis, in 

 this case, requires that II, 4 and II, 6, shall be related to their consorts and 

 carry germ cells with the inhibiting factor. Gutbier, 1834. 



The method of inheritance is shown by the pedigrees 

 (Figs. 73, 74, 75). These lead to the conclusions that the 

 defect is a positive character and is due to an inhibitor of 

 development; the affected m^-le i^ either simplex or duplex 



