The first drawing (fig. 37), viz. that of the whole embryo, shews an embryo of an oval shape, possessing six somites, whilst along the middle of its ventral surface there are two slit-like openings, lying parallel to the long axis of the body, and placed one behind the other. The mesoblastic somites are arranged bilaterally in pairs, six on either side of these slits. The following note in his handwriting is attached to this drawing:
“Young larva of Peripatus capensis.—I could not make out for certain which was the anterior end. Length 1.34 millimetres.”
Balfour's three remaining drawings (figs. 40-42) are, as already stated, representations of transverse sections of the embryo figured by him as a whole. They tend to shew, as he stated in the letter referred to above, that the mesoblast originates as paired outgrowths from the hypoblast, and that these outgrowths are formed near the junction of the hypoblast with the epiblast at the lips of the blastopore.
In fig. 42 the walls of the mesoblastic somites appear continuous with those of the mesenteron near the blastopore.
In fig. 40, which is from a section a little in front of fig. 42, the walls of the mesoblastic somites are independent of those of the mesenteron.
Fig. 41 is from a section made in front of the region of the blastopore.
In all the sections the epiblast lying over the somites is thickened, while elsewhere it is formed of only one layer of cells; and this thickening subsequently appears to give rise to the nervous system. Balfour in his earlier investigations on the present subject found in more advanced stages of the embryo the nerve-cords still scarcely separated from the epiblast[572].
We have since found, in Balfour's material, embryos of a slightly different age to that just described. Of these, three (figs. 34, 35, 36) are younger, while one (fig. 38) is older than Balfour's embryo.
Stage A.—The youngest (fig. 34) is of a slightly oval form, and its greatest length is .48 mm. It possesses a blastopore, which is elongated in the direction of the long axis of the embryo, and is slightly narrower in its middle than at either end. From one end of the blastopore there is continued an opaque band. This we consider to be the posterior end of the blastopore of the embryo. The blastopore leads into the archenteron.
Stage B.—In the next stage (fig. 35) the embryo is elongate-oval in form. Its length is .7 mm. The blastopore is elongated and slightly narrowed in the middle. At the posterior end of the embryo there is a mass of opaque tissue. On each side of the blastopore are three mesoblastic somites. The length of the blastopore is .45 mm.