Some Studies on the New Zealand Oysters
[Introduction]
The micromere cells of the gastrula come together forming a smooth outer surface while the megamere divides and the resulting cells tend to occupy the central space of the gastrula. The cleavage cavity opens to the exterior on the undersurface by the blastopore. Shortly after gastrulation, long tufts of cilia appear
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on the broader, anterior end of the embryo. These cilia are between 80μ and 100μ tall and encircle the polar bodies if the latter are still present. This anterior region forms the prototroch which later develops into the velum (Text-fig. 2, E and F). The invagination forming the mouth represents the ventral region. The next noticeable event observed was the formation of the shelled larva and thus the trochosphere develops into the veliger larva.
Text-fig. 3.—The relationship of height to length in free-swimming larvae of Crassostrea virginica, Ostrea edulis and O. lutaria. Figures for C. virginica and O. edulis taken from Yonge (1960, pl. IV and V)