ABSTRACT
Observations were made on chick embryo feet between 6 and 
days of incubation in order to (a) give a precise description of programmed cell death in the interdigital membranes; (b) correlate the evolution of interdigital necrosis with the differentiation of the cartilaginous phalangeal skeleton; (c) measure the relative growth rates of digits and interdigits.
The programme of interdigital cell death proceeds in three phases which comprise the appearance, the extension and the disappearance of necrotic areas. Fourteen distinct stages are described.
The evolution of programmed cell death can be strictly correlated to the chondrogenesis of successive phalanges, in the sense that each stage of necrosis (except two of them) differs from the preceding one by the differentiation of an additional phalanx.
Shaping of the foot results from differential growth rates and proceeds in three phases, the second of which is characterized, between days 8 and 10 of incubation, by the actual shortening (decrease of absolute length) of the interdigital membranes.
These observations are compared to those that were previously made on other avian and mammalian species and the morphogenetic role of programmed cell death is discussed in the light of that comparison.
