Fig. 181. Diagrammatic longitudinal sections of an Insect embryo at two stages to shew the development of the embryonic envelopes.
In A the amniotic folds have not quite met so as to cover the ventral plate. The yolk is represented as divided into yolk cells. In B the sides of the ventral plate have extended so as nearly to complete the dorsal integument. The mesenteron is represented as a closed sack filled with yolk cells. am. amnion; se. serous envelope; v.p. ventral plate; d.i. dorsal integument; me. mesenteron; st. stomodæum; an i. proctodæum.
The typical mode of formation of these membranes is represented diagrammatically in [fig. 181] A and B. A fold of the blastoderm arises round the edge of the ventral plate. This fold, like the amniotic fold of the higher Vertebrata, is formed of two limbs, an outer, the serous membrane (se), and an inner, the true amnion (am). Both limbs extend so as to cover over the ventral plate, and finally meet and coalesce, so that a double membrane is present over the ventral plate. At the same time ([fig. 181] B) the point where the fold originates is carried dorsalwards by the dorsal extension of the edges of the ventral plate, which give rise to the dorsal integument (d.i). This process continues till the whole dorsal surface is covered by the integument. The amnion then separates from the dorsal integument, and the embryo becomes enveloped in two membranes—an inner, the amnion, and an outer, the serous membrane. In [fig. 181] B the embryo is represented at the stage immediately preceding the closure of the dorsal surface.
By the time that these changes are effected, the serous membrane and amnion are both very thin and not easily separable. The amnion appears to be usually absorbed before hatching; but in hatching both membranes, if present, are either absorbed, or else ruptured and thrown off.
The above mode of development of the embryonic membranes has been especially established by the researches of Kowalevsky (No. [416]) and Graber (No. [412]) for various Hymenoptera (Apis), Diptera (Chironomus), Lepidoptera and Coleoptera (Melolontha, Lina).
Considerable variations in the development of the enveloping membranes are known.
When the fold which gives rise to the membranes is first formed, there is, as is obvious in [fig. 181] A, a perfectly free passage by which the yolk can pass in between the amnion and serous membrane. Such a passage of the yolk between the two membranes takes place posteriorly in Hydrophilus and Donacia: in Lepidoptera the yolk passes in everywhere, so that in this form the ventral plate becomes first of all imbedded in the yolk, and finally, on the completion of the dorsal integument, the embryo is enclosed in a complete envelope of yolk contained between the amnion and the serous membrane. During the formation of the dorsal integument the external yolk-sack communicates by a dorsally situated umbilical canal with the yolk cavity within the body. On the rupture of the amnion the embryo is nourished at the expense of the yolk contained in the external yolk-sack.
In the Hemiptera and the Libellulidæ the ventral plate also becomes imbedded in the yolk, but in a somewhat different fashion to the Lepidoptera, which more resembles on an exaggerated scale what takes place in Hydrophilus.
