The American journal of anatomy . 3i Figs. 28-31 cells now lying at the edge of the blastopore are derived by divi-sion from cells which lay at the equator at an earlier stage. Theroof of the blastocoele is thinner, especially above the blasto]) mitotic figures are present just above the floor-cells of the blastocoele are migrating singly or in groupstowards the roof against which they place themselves. This ^ The roof of the egg shown in fig. 30 is unusually thick. DEVELOPMENT OF SPELERPES BILINEATUS 197 migration of yolk-cells, disregarded in recent years, appears,
The American journal of anatomy . 3i Figs. 28-31 cells now lying at the edge of the blastopore are derived by divi-sion from cells which lay at the equator at an earlier stage. Theroof of the blastocoele is thinner, especially above the blasto]) mitotic figures are present just above the floor-cells of the blastocoele are migrating singly or in groupstowards the roof against which they place themselves. This ^ The roof of the egg shown in fig. 30 is unusually thick. DEVELOPMENT OF SPELERPES BILINEATUS 197 migration of yolk-cells, disregarded in recent years, appears,according to Moquin-Tandon (76), to have been first noticed byStrieker in a paper I have not seen. In fig. 31, the blastocoeleis filled with migrating cells, which cause the surface of the livingegg to appear mottled. The archenteron has become enlargedbut there has been little or no more invagination at the dorsal. Figs. 32-35 lip. Opposite it, a short cleft, c, has appeared while the exposedsurface of the yolk has decreased. Fig. 32 is drawn from an eggof the same age as fig. 31. It is more developed in some partsalthough less in others. Gastrulation, or better, invagination is nearly completed in theegg shown in fig. 33. The ectoderm, reduced to a single layer ofcells above, is still several layers thick at the blastopore. The 198 HUBERT DANA GOODALE anterior part of the archenteric roof is formed of large yolk cells,which become smaller posteriorly, although their granules remainlarge. Close to the lip, granules of intermediate size are far below the blastopore, the ventral mesoderm, vm, fig. 33,can be seen among the yolk cells which are now almost entirelyinvaginated. Finally, just before the embryo appears, the formation of thearchenteron is complete (fig. 34, cf. fig. 26). The ectodermcells are smaller and arranged in a single layer except around theblastopore. The triangular mas
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectanatomy, bookyear1901
