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RÉSUMÉ

A fate map for the late blastula stage of a sturgeon is presented for the first time, together with descriptions of the principal morphogenetic movements, and a timetable of involution, based on vital dye experiments on Huso and three species of Acipenser. Massive migration of internal cells from the blastocoel roof to an inner marginal zone precedes the earliest drift of cells at the surface. The definitive endoderm which lines the archenteron is derived from a complete band of cells encircling 360" of the surface of the blastula from the equator up to 20" latitude. Their movement across the lips of the blastopore to the interior is strictly defined as an involution, not an invagination. After involution of the prechordal endoderm, the right and left components of the sub-notochordal endoderm fuse with each other as they pass across the blastopore lip in the midline. Above the band of prospective endoderm on the blastula surface there is a complete band of both surface and inner marginal cells up from 20" to about 30" latitude, which produces notochord and somite mesoderm, also by involution. Mesoderm of the tail has all reached the interior before closure of the blastopore. Head mesoderm, and intermediate and lateral plate mesoderm are not represented at or near the blastula surface. The neural plate is derived from material at and immediately inside the blastula surface lying above 30" latitude and between right and left 90" longitudes.The remarkable contrast between the morphogenetic movements and fate maps of the meroblastic teleost and the holoblastic sturgeon embryos-both actinopterygian fishes-and the close resemblance of sturgeon embryos to those of frogs, are discussed.

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