THE GENETIC CONTROL OF PATTERN 



203 



rate which begins quite early, at about the 36th hour of incubation, 

 and increases to a maximum at the time of limb-formation, when 

 death usually occurs. All the processes of differentiation which are 

 proceeding most rapidly at this period are affected, and since many of 

 these processes are essential for pattern formation in different organs, 

 the characteristic disorganization is produced. The conditions have 

 been most fully studied in the Hmbs, though other organs (e.g. the 

 eyelids) also develop abnormally. Fell and Landauer^ showed that if 

 normal limb buds are cultivated in vitro in optimal conditions for 



r- 



Fig. 95. Polydactyly in Guinea-Pigs. — A forefoot of normal, 6 of heteroyzgote, 

 C hindfoot of normal, D of heterozygote, £ forefoot of homozygote embryo. 



(After Wright.) 



growth, quite good differentiation is obtained, but that if the cultiva- 

 tion is made in specially growth inhibiting conditions the resulting 

 limbs show the same type of abnormality as is found in creepers. The 

 general inhibition of growth first prevents the formation of hyper- 

 trophic cartilage, and this secondarily prevents periosteal ossification, 

 which normally seems to be induced by the hypertrophy of the cartilage. 

 (b) Reorganization of the Pattern.— Somtxim^s a gene, which in an 

 ordinary genotype produces merely a disorganization, can in a suitably 

 selected geneotypic milieu produce constant and orderly effects which 

 are worthy of being called a new pattern. Wright^ has described a Poly- 

 dactyly gene in guinea-pigs, which was homozygous when lethal, the 

 rare survivors having eight to eleven toes on each foot. The normal 

 guinea-pig has four digits on its forelimbs and three on the hind. By 

 selection Wright built up a race in which the homozygote still had many 



^ Fell and Landauer 1935 



* Wright I935«j Scott 1937. 



