. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. I2O HOLMES. [VOL. I. .rrr -. D C FIG. 6. — View of the posterior side of an egg in a somewhat later stage. There are seen two pairs of mesomeres in the cleav- age cavity. little later in Crepidula than the stage shown in Fig. 31, other- wise the two eggs are practically identical. The four cells in Serpulorbis are all on the surface of the egg and are not over- lapped so much by the ectomeres as in Crepidula. The upper pair in Fig. 4 is shown in proc- ess of division. Each cell buds off a small cell into the interior of the
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. I2O HOLMES. [VOL. I. .rrr -. D C FIG. 6. — View of the posterior side of an egg in a somewhat later stage. There are seen two pairs of mesomeres in the cleav- age cavity. little later in Crepidula than the stage shown in Fig. 31, other- wise the two eggs are practically identical. The four cells in Serpulorbis are all on the surface of the egg and are not over- lapped so much by the ectomeres as in Crepidula. The upper pair in Fig. 4 is shown in proc- ess of division. Each cell buds off a small cell into the interior of the egg, the spindles diverging anteriorly. At about the same period an almost exactly similar division occurs in Crepidula, the upper pair of cells budding off a small cell into the interior of the egg in very nearly the same direc- tion. At a somewhat later stage in Serpulorbis, shown in Fig. 6, I have found two pairs of small cells lying entirely within the cleavage cavity which probably represent the descendants of the single pair whose origin has just been described. The paral- lelism with Crepidula extends also to this division as the correspond- ing pair of small cells in that form divides at about the same period. This is as far as the cleavage of these cells was carried. These results were worked out before Conklin's paper appeared, and as I did not follow the further history of these cells, as Conklin has suc- ceeded in doing in Crepidula, it seemed uncertain what interpreta- tion of them should be made. It seemed improbable that all of 4 d should form the mesoblast, as it was supposed to do in several forms. The four large cells showed no signs of passing into the interior of the egg, and it is probable that, after the mesoderm is separated from the upper pair, they enter into the formation of the entoderm, as in Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appear
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