Fine Structural Study of the Cortical Reaction and Formation of the Egg Coats in a Lancelet (= Amphioxus), Branchiostoma floridae (Phylum Chordata: Subphylum Cephalochordata = Acrania)

¹ Marine Biology Research Division, Scripps Institution of Oceanography, La Jolla, California 92093

A method for artificial fertilization of lancelet eggs is described, and the egg coats are studied for the first time by transmission electron microscopy. Large, ovarian oocytes and spawned, unfertilized eggs (which are about 140 µm in diameter) are surrounded by a coarsely granular vitelline layer about 1 µm thick and a jelly layer a few micrometers thick. The egg cortex is crowded with a monolayer of cortical granules, each with an average diameter of approximately 3.5 µm. About 20 to 30 s after insemination, a cortical reaction occurs almost simultaneously over the entire egg surface. The cortical granules undergo exocytosis, and part of their content evidently forms a dense layer 30 nm thick against the inside of the vitelline layer: both layers together constitute the fertilization envelope, which begins elevating from the egg surface. By 80 s after insemination, the jelly layer has disappeared, and beneath the fertilization envelope the bulk of the ejected cortical granule material has become organized into a hyaline layer with a finely fibrogranular consistency. By 20 min after insemination, the perivitelline space between the fertilization envelope and the egg surface has attained its maximum width of roughly 150 µm, and both the hyaline layer and the vitelline layer component of the fertilization envelope are much attenuated and remain so until hatching about 9 h after insemination. Egg coats are compared among major deuterostome groups, and the results imply that the ancestral chordate may have been an unspecialized appendicularian.