214 PHYSIOLOGICAL GENETICS 



few genie actions, controlling one of the just-mentioned variables 



within the parts that grow at a differentia] rate from others, 

 thus producing a pattern of form. We have already met with 

 cases which might be described in this way, when we studied the 

 production of hereditary abnormalities by changes in definite 

 areas of early embryonic stages: the rimipless fowl, the mice with 

 deformed tails, etc. Another example, which we mentioned there 

 briefly, the creeper fowl, shows, however, that here a different 

 type of production of form pattern is involved. According to 

 Landauer and Dunn (1930) and Landauer (1931, 1934), the 

 creeper fowl, a short-legged fowl, is the result of a dominant 

 mutation which is practically lethal when homozygous. The 

 limbs here were shown to be smaller even in the earliest stage of 

 their visibility, and all their further growth occurs at the normal 

 rate. The special pattern of Short-leggedness is then not 

 produced by a change in the variables of growth of the limb 

 but by a change acting only upon the early embryonic Anlage 

 of the limb — a slowing up of differentiation in the early deter- 

 mination field of the limb. (But there is also a smaller effect 

 upon growth in general.) The various bones are not equally 

 affected. The more distal bones in the limbs w r hich are deter- 

 mined later are more affected; the proximal ones, wdrich have 

 at least partly been determined before the retardation within 

 the Anlage set in, are less affected. The fact that the longer 

 bones are more affected indicates that tissues with the larger 

 growth intensity are more affected by the retarding action in 

 the Anlage. In agreement with this interpretation is the fact 

 that the homozygous embryos which die at an earlier stage show 

 a large retardation in growth during the first days of incubation, 

 which most strongly affects those organs that are found at this 

 time in their most active stage of growth. Landauer, there- 

 fore, concludes that a general retardation of growth in early 

 stages, w r hich become localized by the more ready response of 

 parts in most active growth, is responsible for all phenomena. 

 If this interpretation is correct, the case does not actually belong 

 to the group of patterning by differential growth, because the 

 pattern is produced by a change in a general rate process, which 

 automatically affects more strongly such parts as are in active 

 growth. Probably the same explanation applies also to the 

 other cases of a similar type (reported on page 47ff.), according 



