ABSTRACT
Three talpid mutants have been described in the fowl, all of which are characterized by lethal embryonic abnormalities. As explained in a previous paper (Ede & Kelly, 1964), Cole’s talpid and talpid3 are similar in all essential respects and show striking abnormalities in the head region, whereas talpid2 has a relatively normal head. In all three, embryos surviving to 11 days and over show the following abnormalities in the trunk region: (1) The vertebral column is shortened. (2) The limbs are extremely short and the number of digits on each is greater than normal. (3) The viscera are ectopic, i.e. they protrude through the ventral body wall, which remains open. (4) Feather development is incomplete. (5) There is much subcutaneous oedema and haemorrhage. Instances of inherited polydactyly have always excited interest on account of the light they may throw on the origins and production of the pentadactyl limb pattern in typical vertebrate embryos. The talpid mutants are particularly interesting in this respect since the effect on limb development is much more drastic than in other cases of inherited polydactyly in the fowl, producing a totally different pattern rather than simply duplicating one of the digits as in polydactyly (Po), duplicate (Pod) and diplopodia (dp) (reviewed by Hutt, 1949). Also, unlike these other mutants in which other structures are affected only to a very minor degree, the effect of the talpid genes is extremely pleiotropic, producing widespread developmental disturbances. The unravelling of complex pleiotropic patterns of damage has long been established as a powerful technique of embryological investigation, especially by Grüneberg in many studies on mouse mutants (reviewed by Grüneberg, 1963). In these studies it is postulated that gene action is unitary, and that by establishing a ‘pedigree of causes’ the primary action of the gene, producing the primary defect from which all the others ultimately arise, is revealed. In this paper the development of the abnormalities in the trunk and limbs of talpid3 is analysed with a view to discovering the connexions between them and relating them to the abnormalities of the head region. The basic defect appears to lie in some disturbance of the mesenchyme, affecting its segregation into mesenchymal condensations, and later the formation within it of cartilage and bone. It is hoped that further work will test this hypothesis, and give more precise information about the fundamental developmental disturbances.