. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. GASTRULATION IN SAND DOLLAR 291. Figure 5. Scanning electron micrographs of the cells in the ectoderm and invaginated archenteron during early stages of gastrulation: Hemicentrotus pulclierrimus (A-D): Scaphechinus mirabilis (E-H). In A-D, arrowheads indicate the bending point: in E-H they indicate the boundary between animal and vegetal cells. Arrows in D and G indicate bottle cells. Ectodermal cells of 5. mirabilis (F-H) were more elongated in the apico-basal direction than those of H. piilcherrimus (B-D). Columnar and ske
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. GASTRULATION IN SAND DOLLAR 291. Figure 5. Scanning electron micrographs of the cells in the ectoderm and invaginated archenteron during early stages of gastrulation: Hemicentrotus pulclierrimus (A-D): Scaphechinus mirabilis (E-H). In A-D, arrowheads indicate the bending point: in E-H they indicate the boundary between animal and vegetal cells. Arrows in D and G indicate bottle cells. Ectodermal cells of 5. mirabilis (F-H) were more elongated in the apico-basal direction than those of H. piilcherrimus (B-D). Columnar and skewed cells were frequently observed in both species. Wedge-shaped cells were also observed in both species, especially in the vegetal half. In H. pulcherrimus. two to three wedge-shaped cells were observed just at the bending point of the epithelium (B. C). Such wedge-shaped cells are distributed more broadly apart from the blastopore in S. mirabilis (F, G). The scale bar indicates 10 jum. about 209r as invagination progressed; in S. mirabilis, the ratio remained constant at a rather higher level. Columnar cells were barely observed in S. mirabilis (Fig. 6D), whereas this type of cell increased in H. pulclierrimus at the end of primary invagination (Fig. 6B). Secondary mesenchyme cells at the archenteron tip Figure 7 shows the secondary mesenchyme cells ob- served at the archenteron tip of the midgastrulae. In H. pulcherrimus, these cells were globular and formed long thin filopodia. Several SMCs were located between the archenteron tip and the future oral opening region. In 5. mirabilis, SMCs were flattened to some extent and formed broad ruffled membranes. No cells were observed between the archenteron tip and the future oral opening region. Although more than 200 gastrulating S. mirabilis embryos were examined, an image that showed direct contact be- tween the filopodia of the SMCs and the inner surface of the apical plate could not be obtained. Shape of archenteron cells dur
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