202 AN INTRODUCTION TO MODERN GENETICS 



animal. The gene effects the mesoderm as well as the other two layers, 

 and perhaps one might suggest that its primary effect is on the indivi- 

 duation pattern within the organization centre. Another similar case is 

 that of the rumpless gene in fowls.^ Similar malformations of pattern 

 may be caused by genes which affect the pattern-forming properties of 

 the competent tissues. Little and Bagg^ have described a strain of mice 

 in which a gene or chromosome rearrangement causes the production 

 of an excess of cerebro-spinal fluid. This escapes from the neural tube 

 and forms blisters imder the skin of the embryo. Very often a blister 

 comes to lie in the region where the limb buds are forming, and the 

 pattern of the limb becomes distorted, probably by a simple mechanical 

 effect. 



If pattern formation depends on the attainment of an equilibrium 

 between several different processes, we should expect it to be very 

 sensitive to rather unspecific conditions. This seems to be true. Leh- 

 mann^ has recently shown that several poisons can affect pattern forma- 

 tion in particular regions of the amphibian embryo, and has drawn 

 attention to the parallel between such phenomena and the effect of 

 genes like the short-tailed gene in mice. Similarly the effect of the 

 rumpless gene can be imitated by incubation of eggs at a low tempera- 

 ture.^ Many years ago Stockard^ pointed out that there are crucial periods 

 in development when important events are taking place and the embryo 

 is particularly sensitive to non-specific harmful conditions. The time of 

 gastrulation, when the organization centre is active, is one such period 

 and there are many others. A general inhibition, such as a slowing up 

 of growth rate may, if it occurs at the crucial period for a certain 

 pattern, entirely prevent the pattern being formed. 



Perhaps the best example of this type of phenomenon is that of 

 the creeper fowl.** The mutant gene (or deficiency?) is a dominant. The 

 heterozygote shows a condition like human chrondrostrophy, that is, 

 the long bones are malformed, shortened and bent, which gives the 

 animal a characteristic gait from which the name is derived. The 

 homozygote usually dies at an early stage; if it survives it shows still 

 more marked abnormalities of the limbs. There may be a complete 

 lack of calcification or periosteal ossification, resembling human phoko- 

 melia. The pattern of the limbs is completely disorganized. These 

 changes seem to be brought about by a general retardation of growth 



1 Dunn 1925, Landauer 1928. ^ Little and Bagg 1924, Bonnevie 1934. 



' Lehmann 1936. ^ Danforth 1932. 



* Stockard 1931. * Landauer 1932, 1933. 



