PubMed1985
The structure of the hen's egg shell was studied during the incubation. The local differences in the structure of the shell were estimated during the incubation in dependence to the development of the allantochorial vascular system. On the external surface of the shell there is a thin layer, homogenous in the SEM. It remains changeless during the whole period of incubation. The main part of shell is formed by long prisms with their long axes oriented perpendicularly to the external surface of the shell. These prisms are partially twisted round each other, similarly as in the case of the teeth enamel. The inner layer of the shell is formed by a system of fibers forming the three-dimensional network, similar to an air filter. In the middle of incubation, when the embryo's skeleton is still predominantly cartilaginous, is the structure of the shell identical with that one at the beginning of incubation. At the end of incubation, when the embryo's skeleton is already predominantly ossificated, the inner layer of the shell disappears without any trace. By resorption of the shell are the peaks of the prisms of the middle layer of the shell destroyed as well. The above described changes are the best visible in the equator region of the shell. Less developed are they in the region of the sharp pole of the egg. They are not detectable in the region of the blunt pole of the egg. In this region the egg's shell has the same structure during the whole period of incubation. The changes of structure of the shell--the resorption of the inner layer together with partial destruction of the prisms of the middle layer - exactly correspond with extension of the allantochorial vascular system.