2/6 SOME RESULTS OF OSTRICH INVESTIGATIONS. 



with the third digit may be presumed to be equi-potential with 

 those for the second digit as far as the tenth day of incubation, 

 but from this stage onwards they are comparatively weaker, and 

 may ultimately result in the formation of only one or two 

 phalanges, without any claw at the tip, whereas the second digit 

 has always three phalanges and is clawed. The factors for the 

 third claw have wholly disapi)eared from the germ plasm, and 

 those for the second phalanx have almost lost their potency in 

 most individuals, but are fully effective in others. 



Fourth Outer Toe. 



The small uuter toe in the ostrich is the fourth of the penta- 

 dactyle foot, the first, second, and fifth having already disap- 

 peared, though Prof. Broom has shown there are traces of all 

 five in the embryo chick of ten and eleven days' incubation. 

 Following upon the losses which have already taken place, the next 

 in the sequence to disappear will be the fourth, and in the end the 

 middle third will be the only toe remaining. In addition to the great 

 difference in size between the fourth and the third t^e, other 

 conditions represented by the former give every rea.son for 

 thinking that it is at present undergoing slow degeneration, 

 especially as regards the claw and the scales on its upper surface. 

 Though very small in size, the normal munber of ])halanges, 

 i\vQ, is still retained. 



Already the claw on the fourth toe has for the most part 

 <lisappeared, though it is occasionally present, more often on 

 North African than on South African birds. In 25 mixed 

 northern birds it occurred on six specimens, and was wanting on 

 the others, while in 20 mixed southern birds it was found on only 

 one individual. Everywhere it is feebly developed, especially by 

 comparison with the big claw on the middle toe, and is always 

 non-functional, never reaching the ground. Where best formed, 

 it projects for about half an inch from its socket, while at other 

 times it is almost hidden in the integument, and can scarcely be 

 felt with the finger ; but all intermediate sizes can be obtained. 

 Usually it is equally develo])ed on both the right and left foot, 

 though sometimes a difi'erence is observed. (Fig- 3.) 



In crosses where both ])arents are without the claw the 

 progency are also devoid of it. \\ here, however, one parent is 

 clawed and not the other, it appears in about half tlie ])rogeny, 

 showing that the clawed birds are dominant heterozygotes. Out 

 of a total of 36 chicks hatched from breeding })airs, wliere one 

 parent was clawed and not the other, the niuubers were actually 

 equal, 18 chicks were clawed and 18 unclawed. Thus the 

 evidence is conclusive that the ])resence of the claw follows the 

 Mendelian ])ro])ortions for heterozvgotes when breeding takes 

 place. With the small ])roportion of clawed to clawless individuals 

 among both northern and southern birds, it is to be expected that 

 the clawed osl riches will be heterozygous, for a mixed assent- 



