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few hours, whereas other species such as the European eel Anguilla anguilla
Linnaeus, takes three years to complete its larval development (Williamson, 1992).
After a series of cleavages (Fig. 2.1) all metazoan embryos pass through the
blastula stage before hatching. At this stage the dividing cells form a more or less
hollow sphere, with a fluid filled space within called a blastocoel (Fig. 2.2). In most
phyla embryonic development continues further although in many echinoderms the
cells of the blastula are ciliated and the animal hatches in this stage (Williamson,
1992). In the following stage, the gastrula, a tube-like inner layer of cells called the
archenteron, or primitive gut, is formed. The internal end of the archenteron is closed,
and the opening at the vegetal pole is called the blastopore (Figs. 2.2, 2.3).
Ectoderm
Endoderm
Primary
mesenchyme
Fig.2.1. Cleavage in Deuterostomes showing the cell divisions from the original
zygote to the late divisions prior the blastula stage. Abbreviations: an 1 , an 2 refer to
cells derived from the animal half; veg 1, veg 2 refer to cells derived from the vegetal
half (Modified from Barnes et al. 2001).
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