EGG CASE ULTRASTRUCTURE IN THE NARROW-CLAWED CRAYFISH ASTACUS LEPTODACTYLUS ESCHSCHOLTZ, 1823 (DECAPODA, ASTACIDAE)

The egg case is an extracellular structure that surrounds an egg cell and the embryo after fertilization. Its basic function is to protect the embryo from potentially harmful aspects of the external environment. Changes in the structure of the egg case at different stages of embryogenesis in the crayfish Astacus leptodactylus were studied, with observations made at the beginning of egg incubation, during incubation, and at the time of hatching. We studied the changes using images taken with transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The egg case is made of three layers that are all close together. The outer layer was 4.59 +/- 0.23 mu m thick and composed of an electron-dense material; its external structure was rough. The middle layer was the thickest at 8.94 +/- 0.29 mu m and was slightly folded. It was made of polyhedral grains, interspersed with multiple aeropylar areas and channels. This second (middle) layer consisted of two sublayers of equal thickness but with variable grain size and aeropyle structure. The third and innermost layer was 3.14 +/- 0.33 mu m thick and made of tightly packed grains, smaller than those in the middle layer. It included a few, small aeropylar channels.