Fig. 534.—Diagram showing the formation of the embryonic membranes in Lepidoptera (A, after Kowalevsky, B and C, after Tichomiroff): k, primitive band; am, amnion: se, serosa; do, yolk; vd, invagination of the fore-gut, ed, of the hind-gut; m, mouth; an, anus; x, dorsal umbilical passage.—From Korschelt and Heider.
The older views on the structure of the layers of the primitive band of insects were thoroughly unsatisfactory. Bütschli first found that in the bee, by a kind of folding process, an inner layer of the primitive band arose. Soon afterwards Kowalevsky, by the employment of section-cutting and thorough researches, laid the foundation of a more exact knowledge of these layers. He found that in Hydrophilus a furrow extended along the whole length of the primitive band (Fig. 515, A, B, r), which, while invaginating or sinking in, gave rise to the inner layer of the primitive band, i.e. the common rudiment of endoderm and mesoderm (Fig. 539, A-C).
Kowalevsky also found similar conditions in the honey-bee (Apis), Lepidoptera, and other forms. The furrow above mentioned must be regarded as a very long gastrula invagination, extending along the entire ventral side of the embryo, and the edges of the furrow as a long-drawnout blastopore. The tube arising in Hydrophilus through the closing of the furrow we may regard as a primitive intestinal canal.
The first rudiment of the gastrula furrow appears in insects as two folds extending along both sides of the median line in the thickened ventral plate (Fig. 536, f), through whose formation a more median section of the ventral plate, the so-called middle plate (m), becomes separated from the side plates (s). As the middle plate curves in and becomes overgrown by the folds forming the edges of the blastopore, the gastrula-tube (Fig. 539, A, r) is formed, and furnishes the rudiments of the lower (inner) layer. The ectoderm, then, according to Heider, arises from the lateral plates of the primitive band. The growth of the edges of the blastopore, by which the closure of the gastrula-tube is effected, takes place latest in the region of the most anterior part of the furrow (Fig. 515, B and C), corresponding to that place in the primitive band in which the stomodæum afterwards develops.
Fig. 535.—Two embryonic stages of a saw-fly (Hylotoma berberidis) in schematic median section: a1–a10, 1st to 10th abdominal segments; bg, ventral nervous cord; og, brain; ol, germ of labrum; sp, salivary gland; ed, hind-gut; x, x′, inner folds of amnion: other letters as before.—After Graber, from Korschelt and Heider.
Fig. 536.—Gastrula stage of the wall-bee (Chalicodoma), so-called flask-shaped stage: f, folds which on each side border the middle plate (edge of the blastopore); m, the partly segmented middle plate (here = rudiment of the mesoderm); s, the segmented lateral plate (becoming afterwards the ectoderm of the primitive band); ve, fore, he, hinder entodermal rudiment.—After Carrière, from Korschelt and Heider.
