A. Section through the posterior part of the embryo fig. 176 D, shewing the completely closed amnion and the germinal groove.
B. Section through an older embryo in which the mesoblast has grown out into a continuous plate beneath the epiblast.
gg. germinal groove; am. amnion; yk. yolk; ep. epiblast.
During the later stages of the process last described remarkable structures, eminently characteristic of the Insecta, have made their first appearance. These structures are certain embryonic membranes or coverings, which present in their mode of formation and arrangement a startling similarity to the true and false amnion of the Vertebrata. They appear as a double fold of the blastoderm round the edge of the germinal area, which spreads over the ventral plate, from behind forwards, in a general way in the same manner as the amnion in, for instance, the chick. The folds at their origin are shewn in surface view in [fig. 176] D, am, and in section in [fig. 177] B, am. The folds eventually meet, coalesce ([fig. 178], am) and give rise to two membranes covering the ventral plate, viz. an inner one, which is continuous with the edge of the ventral plate; and an outer, continuous with the remainder of the blastoderm. The vertebrate nomenclature may be conveniently employed for these membranes. The inner limb of the fold will therefore be spoken of as the amnion, and the outer one, including the dorsal part of the blastoderm, as the serous envelope[168]. A slight consideration of the mode of formation of the membranes, or an inspection of the figures illustrating their formation, makes it at once clear that the yolk can pass in freely between the amnion and serous envelope (vide [fig. 181]). At the hind end of the embryo this actually takes place, so that the ventral plate covered by the amnion appears to become completely imbedded in the yolk: elsewhere the two membranes are in contact. At first ([fig. 176]) the ventral plate occupies but a small portion of the ventral surface of the egg, but during the changes above described it extends over the whole ventral surface, and even slightly on the dorsal surface both in front and behind. It becomes at the same time ([fig. 179]) divided by a series of transverse lines into segments, which increase in number and finally amount in all to seventeen, not including the most anterior section, which gives off as lateral outgrowths the two procephalic lobes (pc.l). The changes so far described are included within what Kowalevsky calls his first embryonic period; at its close the parts contained within the chorion have the arrangement shewn in [fig. 178] B. The whole of the body of the embryo is formed from the ventral plate, and no part from the amnion or serous envelope.
Fig. 179. Embryo Of Hydrophilus piceus viewed from the ventral surface. (After Kowalevsky.)
pc.l. procephalic lobe.
The general history of the succeeding stages may be briefly told.
