Papers . ch a swimming stage in 532 hours after theinsemination of the egg; mesenchyme cells begin to push into the Hyhrklizalion of Echhwids. 7 blastocoele from the flattened and thickened posterior pole of theblastula in 8 hours; the process of gastrulation begins in 9 hours;chromatophores appear in 15 to 16 hours; skeletal spicules appear in15 to 16 hours; and the young pluteus stage is reach in 24 hours afterinsemination; it may be reached in as short a time as 20 facts are shown in parallel columns (p. 6). The hours mentionedindicate hours after insemination. The difference in
Papers . ch a swimming stage in 532 hours after theinsemination of the egg; mesenchyme cells begin to push into the Hyhrklizalion of Echhwids. 7 blastocoele from the flattened and thickened posterior pole of theblastula in 8 hours; the process of gastrulation begins in 9 hours;chromatophores appear in 15 to 16 hours; skeletal spicules appear in15 to 16 hours; and the young pluteus stage is reach in 24 hours afterinsemination; it may be reached in as short a time as 20 facts are shown in parallel columns (p. 6). The hours mentionedindicate hours after insemination. The difference in sequence ofstages as well as the difference in rate of development is evident. As has been stated above, there is nothing unusual in the develop-ment of Cidaris, aside from extreme slowness, until the stage whenmesenchyme formation might be expected. The blastulse (fig. la;plate 3, fig. f) have a wall of rather uniform thickness. The cellsat the posterior end seem very slightly larger than those of other. Fig. la, Optical section Cidaris blastula 16 hours old. Drawn from 6xed and stained material. X , Optical section Cidaris beginning gastrulation; 20 hours. Drawn from fixed and stained material. X , Optical section Cidaris gastrula, 22 hours; drawn from fixed and stained material. X 285. parts of the wall. A section of an 18-hour blastula (plate 3, fig. f)shows that these larger cells are about to move or be forced first indication of an archenteron appears in embryos of about20 hours (fig. 16). In these, at the posterior end, there may be seena small, hollow plug of cells, extending into the blastocoele. The cellsforming this plug are quite characteristically rounded, standing outdistinctly from each other like the seeds in a blackberry. The ca\dtyof the archenteron, though shallow, is cylindrical and has a distinctblastopore. The archenteron continues to grow forward into theblastocoele, and 3 hours later has about doubled in length (figs. Ic, 2a;plat
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