The study of malformations "by the company they keep".

Our study of individuals with thalidomide embryopathy reaffirms many of the principles of clinical teratology concerning sensitivity of developing structures and specificity of action. It also highlights important features of ocular teratology that differ from those of other organ systems. Because malformations of the eye and associated structures do not significantly threaten the survival of the embryo, fetus, or neonate, any variation in prevalence in older individuals is not due to the nature of the ocular malformation but may be significantly modified by "the company they keep," the systemic malformations occurring at the same time as the original insult, whether it be an environmental, a genetic, or a local disturbance. Additionally, the presence of clusters of malformations caused by a teratogen at a specific time supports the concept of a common pathogenic mechanism and provides suggested avenues for further research into the cause of these malformations.