Once fertilized, the sea urchin egg starts to cleave at a very high frequency. Figures 2.7 and 2.8 describe in detail the pattern of cleavage and the morphogenetic movements which bring about the formation of the larva in the Mediterranean species Paracentrotus lividus, when cultured in sea water at 20 °C with gentle stirring. Very similar developmental patterns are observed in other sea urchins: see Mortensen (1921), for the earliest descriptions, E. B. Harvey (1956) for the development of Arbacia lixula; Stephens (1972a, b) for that of Strongylocentrotus purpuratus; Gustaf son and Wolpert (1967) for a critical description of the development of Psammechinus miliaris, and Amemiya and coworkers for the early development of Anthodaris crassispina, Hemicentrotus pulcherrimus and Pseudocentrotus depressus (Amemiya et al., 1982a, b; Akasaka et al., 1980). Schroeder (1981a), on the other hand, has described the different developmental pattern of a primitive sea urchin (Eucidaris tribuloides), which shows peculiarities in the micromeres and hyaline layer, absence of the primary mesenchyme and reduction of the skeleton (Fig. 2.1).
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