Quarterly Journal of Microscopical Science, Vol s3-97, 109-121, Copyright © 1956 by Company of Biologists

The Thickness of the Cortex of the Sea-Urchin Egg and the Problem of the Vitelline Membrane

¹ Department of Zoology, University of Edinburgh, and the Stazione Zoologica, Naples

The gelated cortex of a sea-urchin egg can be seen as a granular layer at the edge of the hyaline zone in a centrifuged egg. Measurements were made of the thickness of this layer in the eggs of Paracentrotus lividus and Arbacia lixula at various stages of development from the unfertilized egg to the first cleavage. The thickness was roughly 2 µ in living eggs, and 1.15-1.35 µ in sections of fixed eggs. There were no appreciable changes in the thickness up to the first cleavage, and it is concluded that a value of 1.5 µ can be taken as an approximate figure for all stages.

The cortex is usually invisible in normal eggs either living or in section, but in the case of sections of fertilized Arbacia eggs it can be seen as a vacuolated layer. The thickness of this layer was found to be 1.13 µ at the sperm aster stage and 1.50 µ at cleavage. In these eggs at cleavage, there were no signs either of differences in thickness at different regions of the surface or of a differentiated region of the cytoplasm ahead of the furrow.

There were no clear indications of the presence of a vitelline membrane either in living or fixed eggs.

A layer about 0.8 µ thick, pale with iron haematoxylin, was found at the edge of sections of unfertilized eggs which had been fixed in Bouin, but not with those which had been fixed in Flemming. This is probably the outer layer of the cortex which normally contains the cortical granules.