Fig. 530.—Diagram of the formation of the dorsal organ in Hydrophilus. A, cross-section through an egg, whose primitive streak is still covered over by amnion (a) and serosa (s). B, amnion and serosa are grown together in the middle line, then separated and drawn back to form a fold on each side. C, by the contraction of the serosa (s),which becomes converted into the dorsal plate, the folds become drawn up dorsally. D, the contracted serosa becomes partly overgrown by the folds. E, the folds grow together to form the dorsal tube. F, the mid-gut has closed over dorsally and enclosed the dorsal tube (s): a, amnion; d, yolk; ec, ectoderm; h, heart; l, body-cavity; m, rudiment of the mid-gut; n, nervous system; s, serosa (in C and D = dorsal plate, in E and F, dorsal tube); tr, the chief tracheal stem.—After Graber and Kowalevsky, from Lang, and Korschelt and Heider.
The gnathal or post-antennal segments at first bear but a small part in completing the tergal region of the head, but shortly before hatching the mandibles and their muscles enlarge, giving fulness to the upper and back part of the head.
i. Dorsal closure and involution of the embryonic membranes
Fig. 531.—Schematic figure of the formation of the dorsal tube by invagination of the dorsal plate (transformed serosa); following after stage Fig. 520, C, and Fig. 521, D; am, amnion (now forming the provisional dorsal closure); r, dorsal tube, whose cells are already breaking away.—After Korschelt and Heider.
In most other Arthropoda (Crustacea, Arachnida, Myriopoda, etc.) development goes on by the formation of a so-called primitive band, but without the appearance of peculiar embryonic membranes. The outer surface of the entire egg becomes, then, in part covered by the band-like embryonic germ, and partly by a portion of the blastoderm which remains unchanged. The dorsal region is formed by the widening and spreading of the primitive band over the greater part of the surface of the egg, while the area of the unchanged section of the blastoderm continually becomes more restricted. It is generally accepted that the latter is concerned in the dorsal closure, because, together with a histological transformation, it becomes involved in the formation of the ectoderm of the primitive band.
A similar form of retrograde structure possibly occurs in the embryos of Poduridæ, in which a dorsal organ has been observed to develop in an early embryonic stage, which bears some relation to the cuticula enveloping the embryo, but whose significance is in general rather obscure.
In most insects the relations are more complicated, since in such cases, the amnion-folds rise on the edges of the primitive band and of the unchanged section of the blastoderm, whose retrograde development is intimately connected with the closure of the back.
A very simple case of dorsal closure, but which certainly is not a primitive one, occurs in Muscidæ and certain other Diptera whose amnion-folds are developed in a rudimentary way. In this case (according to Kowalevsky and Graber), the amnion-folds become smoothed out again. Amnion and serosa become then a simple epithelium, which throughout corresponds to the unmodified type of blastoderm of Crustacea, Arachnida, and Myriopoda, and here seems to share in the formation of the back. More complicated and very manifold relations of dorsal closure and involution of the embryonal membranes occur in other insects, of which Korschelt and Heider distinguish four different types: