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

Abstract

A method for artificial fertilization of lance let 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 @tm in diameter) are surrounded by a coarsely granular vitelline layer about 1 @m thick and a jelly layer a few micrometers thick. The egg cortex is crowded with a monolayer ofcortical granules, each with an average diameter ofapproximately 3.5 sm. About 20 to 30 5 after insemination, a cortical reaction occurs al most simultaneously over the entire eggsurface. The cor tical granules undergo exocytosis, and part of their con tent evidently forms a dense layer 30 nm thick against the inside of the vitelline layer: both layers together con stitute the fertilization envelope, which begins elevating from the eggsurface. By 80 s after insemination, the jelly layer has disappeared, and beneath the fertilization enve lope the bulk ofthe ejected cortical granule material has become organized into a hyaline layer with a finely fi brogranular consistency. By 20 mm after insemination, the perivitelline space between the fertilization envelope and the egg surface has attained its maximum width of roughly 150 jim, and both the hyaline layer and the vitel line layer component of the fertilization envelope are much attenuated and remain so until hatching about 9 h after insemination. Eggcoats are compared among ma jor deuterostome groups, and the results imply that the ancestral chordate may have been an unspecialized ap pendicularian.