J. E. DUERDEN 181 



throughout life. But where structural parts hereditarily disappear 

 during ontogeny we appear to be justified in supposing that some 

 factorial weakening or loss is indicated, leaving aside any modifying 

 inhibitory influence. 



The loss of plumage from the head and legs takes place only several 

 months after hatching. At the beginning the germinal factors con- 

 cerned are present in all the members of the race, but persist in their 

 full expression for about three months and six months respectively, after- 

 wards undergoing some change which in the end leaves the areas 

 featherless. We have a factorial germinal change taking efifect within 

 the life-time of the chick, but whether from atrophy, or inhibition, or a 

 factor for baldness is not determined ; the last is however suggested 

 seeing that in the case of the bald patch the loss is dominant in crosses. 

 A normal, late ontogenetic expression of factors is not unknown in the 

 ostrich as in other animals, for we may recall the changes in the plumage, 

 body colours, and general activities which coincide with the advent of 

 sexual maturity. Though held to be determined by secretions from the 

 gonads the true distinctive nature of these is dependent upon genetic 

 factors late in expressing themselves, in the same manner as for the 

 cephalic and crural feathers. 



As regards the loss of plumage from the wings the germinal factors 

 concerned become wholly lost to the individual in the end, no evidence 

 of the plumes appearing in the embryo. They have undergone their 

 slow, phylogenetic degradation in the mitotic passage from one genera- 

 tion to the other, and are absent from the beginning of ontogeny, that 

 is, from the zygote. The claws and scales appear at a rather late stage 

 within the egg, and persist in the same relative degree of development 

 throughout ontogeny, but the factors undergo reduction in the passage 

 from generation to generation. Apart from Mendelian recessiveness, 

 failure of somatic expression for feathers, scales, and nails, is however no 

 certain proof of the germinal absence of the factors. As they undergo 

 gradual degradation a time will necessarily come when they reach the 

 margin of somatic expressibility, but when this is passed the factors will 

 presumably still continue their degenerative process, generation after 



•' Weismann still adheres nevertheless to his mosaic theory of development, but as before 

 stated the modern work on development does not support this interpretation of develop- 

 ment. His view assumes disintegration of the germ plasm when the body cells are pro- 

 duced in order to account for the localization of characters ; the other view, following the 

 experimental results and microscopical observations, assumes, so far as the chromosomal 

 materials are concerned, that all of the hereditary factors are present in every cell in 

 the body." 



Journ. of Gen. ix 12 



