Ontogeny of the leg muscle tissue in the crooked neck dwarf mutant (cn/cn) chick embryo.

The crooked neck dwarfism (cn/cn) is characterized, among other anomalies, by a muscular hypoplasia, particularly conspicuous in the tibiotarsal segment. Histological observations were performed between day 6 and day 12.5 of incubation. They show, in the tibiotarsal segment, that the hereditary muscular hypoplasia is not caused by a defect of the normal muscular splitting pattern. Indeed, in the mutant, the splitting of muscle masses proceeds normally up to the last partition (day 7-7.5), but is followed by the secondary fusion of individuated muscles into an unpatterned muscle tissue. Thus the mutant phenotype is the result of an inability of the muscle pattern to become stabilized into definitive structures.