There is a marked and most unfortunate dichotomy in the studies of avian eggs and hence in the application of new findings in commerce. Thus over the past twenty years there has been a renewed interest in the contribu tions of various parts of an egg to embryo development. This is best illustrated by those studies that have explored the diffusion of respiratory gases across the shell and at long last have provided a fundamental definition of a previously nebulous term, porosity. The activity in this general area has led in the past four years to the publication of three major books dealing with many aspects of egg structure, function and embryogenesis. When brows­ ing over these books, two developments are evident. First, the advantages that are to be gained by comparative studies. Thus it is now common to see within a single book articles concerned with the eggs of a range of avian species as well as those of reptiles. Second, it is evident that zoologists and physiologists as well as those employed in large breeding firms are all contributing to an improvement of our knowledge of the egg's role in the breeding biology of birds. Comparative studies are a very uncommon feature of studies concerned with bacterial infection of eggs.