The upper end is the anterior. gg. germinal groove; am. amnion.
Fig. 177. Two transverse sections through embryos of Hydrophilus piceus. (After Kowalevsky.)
A. Section through an embryo of the stage represented in fig. 176 B, at the point where the two germinal folds most approximate.
B. Section through an embryo somewhat later than the stage fig. 176 D, through the anterior region where the amnion has not completely closed over the embryo.
gg. germinal groove; me. mesoblast; am. amnion; yk. yolk.
The segmentation has not been studied, but no doubt belongs to the centrolecithal type (vide pp. [110‑120]). At its close there is an uniform layer of cells enclosing a central mass of yolk. These cells, in the earliest observed stage, were flat on the dorsal, but columnar on part of the ventral surface of the egg, where they form a thickening which will be called the ventral plate. At the posterior part of the ventral plate two folds, with a furrow between them, make their appearance. They form a structure which may be spoken of as the germinal groove ([fig.] 176 A, gg). The cells which form the floor of the groove are far more columnar than those of other parts of the blastoderm ([fig. 177] A). The two folds on each side of it gradually approach each other. They do so at first behind, and then in the middle; from the latter point the approximation gradually extends backwards and forwards ([fig. 176] B and C). In the middle and hinder parts of the ventral plate the groove becomes, by the coalescence of the folds, converted into a canal ([fig. 178] A, gg), the central cavity of which soon disappears, while at the same time the cells of the wall undergo division, become more rounded, and form a definite layer (me)—the mesoblast—beneath the columnar cells of the surface. Anteriorly the process is slightly different, though it leads to the similar formation of mesoblast ([fig. 177] B). The flat floor of the groove becomes in front bodily converted into the mesoblast, but the groove itself is never converted into a canal. The two folds simply meet above, and form a continuous superficial layer.
Fig. 178. Sections through two embryos of Hydrophilus piceus. (After Kowalevsky.)