A nine-penned discussion (Maynard Smith et al, 1985) of developmental constraints gave the following definition:
"a developmental constraint is a bias on the production of variant phenotypes or a limitation on phenotypic variablilty caused by the structure, character, composition, or dynamics of the developmental system".
The idea is that different groups of living things have developed different developmental mechanisms, and that the way an organism develops will influence the kinds of mutation it is likely to generate. A plant such as this fern, for example, may be more likely to mutate to a new form with more branches than would a vertebrate (in the vertebrate, new 'branches' might be extra legs, or perhaps having two heads), because it is easier to produce that kind of change in the development of a plant.
An example of a developmental constraint is pleiotropy, which occurs when a gene influences the phenotype of more than one part of the body.
The philosopher Daniel Dennett explains the importance of developmental constraints.