Stage C.—In the next stage (fig. 36) the features are much the same as in the preceding. The length of the whole embryo is .9 mm.
The following were the measurements of an embryo of this stage with five somites, but slightly younger than that from which fig. 36 was drawn.
| Length of embryo | .74 mm. |
| Length of blastopore | .46 mm. |
| Distance between hind end of blastopore and hind end of body | .22 mm. |
| Distance between front end of body and front end of blastopore | .06 mm. |
The somites have increased to five, and there are indications of a sixth being budded off from the posterior mass of opaque tissue. The median parts of the lips of the blastopore have come together preparatory to the complete fusion by which the blastopore becomes divided into two parts.
Stage D.—The next stage is Balfour's stage, and has been already described.
The length is 1.34.
It will be observed, on comparing it with the preceding embryos, that while the anterior pair of somites in figs. 35 and 36 lie at a considerable distance from what we have called the anterior end of the embryo (a), in the embryo now under consideration they are placed at the anterior end of the body, one on each side of the middle line. We cannot speak positively as to how they come there, whether by a pushing forward of the anterior somites of the previous stage, or by the formation of new somites anteriorly to those of the previous stage.
In the next stage it is obvious that this anterior pair of somites has been converted into the præoral lobes.
The anterior of the two openings to which the blastopore gives rise is placed between the second pair of somites; we shall call it the embryonic mouth. The posterior opening formed from the blastopore is elongated, being dilated in front and continued back as a narrow slit (?) to very near the hind end of the embryo, where it presents a second slight dilatation. The anterior dilatation of the posterior open region of the blastopore we shall call the embryonic anus.
Lately, but too late to be figured with this memoir, we have been fortunate enough to find an embryo of apparently precisely the same stage as fig. 37. We are able, therefore, to give a few more details about the stage.