An arbitrary classification of the pore systems in avian eggshells

An arbitrary classification of the pore systems in shells has been based on the observations made during a survey with a scanning electron microscope of the eggs of c. 60 avian species. The classification is intended primarily to aid comparative studies of shell function and field studies of the overall contribution of the shell to the embryo's well‐being. The location and form of the external orifice of the pore was used to define four major types of pore systems: (1) outer orifice open—the pore canal terminates at the surface of the shell; (2) outer orifice occluded—the pore canal terminates at the surface of the shell but the orifice is occluded by amorphous, waxy material; (3) outer orifice capped—the pore canal terminates at the outer edge of the true shell and its orifice is capped and, in many species, plugged by a stratum of vesicles, either organic or inorganic, which clothes the entire surface of the shell, and (4) outer orifice reticulate—the pore canal terminates at the edge of the column layer and its orifice is covered by a calcareous stratum containing a plexus of tubules. Each group was further divided on the form of the pore canals; shells having unbranched pore canals were put into one category and those in which some pores branched into another.