EVIDENCE OF GERM CELL SELECTION 389 



index of brachydactyly^ was computed for all chicks that hatched 

 and each booted individual was assigned to one of three grades 

 based on the number of feathers present on the shanks and 

 toes. 



Polydactyly , a well-known condition, seems to be the final 

 manifestation of an early disturbance in the developing rudi- 

 ment of digit I of the foot. The character is quite variable, 

 ranging, in the present material, from a single enlarged hallux 

 to a condition in which the hallux is replaced by two digits with 

 even the occasional indication of a third. The cases may be 

 arbitrarily grouped into three grades, dependent upon the 

 degree to which the peculiarity manifests itself. Rarely Poly- 

 dactyly was found to extend to the wings. The term as here 

 used is a misnomer to the extent that a certain number of in- 

 dividuals which clearly manifest the fundamental character 

 in question really have only the normal number of toes. Poly- 

 dactyly can be recognized with certainty and probably in all 

 cases after about the seventh day of incubation. 



Color in the present paper refers to only two grades, 'black' 

 and 'white'. While the pure-bred birds used were either clear 

 snowy white (Leghorns) or uniform glossy black (Minorcas), 

 a large percentage of the young of mixed ancestry showed some 

 color indication of their hybrid origin. This was particularly 

 true of the white chicks, many of which had one or more small 

 spots of black down. By 'white' may be understood pure 

 w^hite down or feathers or white plumage with only a little 

 black pigment in the form of a few dark spots. 'Black' covers 

 all other shades, even though some individuals classified as 

 black subsequently turned out to be barred or mottled. From 

 the twelfth day of incubation (or even earlier) every chick 

 could be put unhesitatingly in one or the other of these categories. 



^ The index of brachydactyly is an arbitrary value obtained by dividing the 

 sum of the lengths of the two fourth toes by the sum of the lengths of the two 

 second toes. With brachydactjd chicks this gives a value equal to 1 or less, 

 while with normal chicks the value obtained is more than 1. The division is 

 carried to the second decimal place and the quotient then multiplied by 100 

 to eliminate fractions. 



