ELECTRON MICROSCOPIC OBSERVATIONS ON THE EXTRA-GRANULAR ZONE OF THE EMBRYO OF THE SEA URCHIN, HEMICENTROTUS PULCHERRIMUS*

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Abstract
The extra-granular zone as proposed by Dan and Dan (1940) was studied by EM. Two stages were noticed during its formation: first, a relatively yolk granule-free zone constituted the egg periphery which corresponds to that observed by Dan and Dan, and later, this zone was occupied by a layer of vacuoles (.apprx. 1.3 .mu.m mean diameter) with highly electron-transparent inclusions which migrated to the egg surface from the inner cytoplasm after fertilization. These vacuoles were scarce in micromeres of 16-cell stage embryos.