The resulting pattern is similar to the embryonic disk described for one species of anuran. At the end of gastrulation, a small archenteron spreading dorsally from the blastopore represents the relatively small and superficial area of the egg where early embryonic axis formation occurs. Cell migration is visible through the transparent blastocoel roof of the gastrula. The ventral lip of the blastopore forms closer to the vegetal pole, and relatively little involution occurs during gastrulation. At the beginning of gastrulation, the blastocoel roof is one cell thick, and the dorsal lip of the blastopore forms below the equator of the embryo. Cleavage is meroblastic until approximately the 16-cell stage (fourth or fifth cleavage). This species has one of the largest eggs known for an amphibian, with a mean ± SD diameter of 6 ± 0.43 mm (range 5.3-6.9 n = 17 eggs). In this paper, we describe for the first time the early development (from fertilization through neurulation) of the plethodontid salamander Ensatina eschscholtzii. Salamanders of the family Plethodontidae are particularly appropriate for such comparative studies because the species have eggs of varying size, including very large yolky eggs. Increased egg size is one factor that could have a strong effect on early developmental processes such as cleavage pattern and gastrulation. Comparative analyses between amphibians, concentrating on the cellular mechanisms of morphogenesis, reveal a large variability in the early developmental processes that were thought to be conserved during evolution.
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