Fig. 182. Three stages in the development of the embryo of Calopteryx. (After Brandt.)
The embryo is represented in the egg-shell.
A. Embryo with ventral plate.
B. Commencing involution of ventral plate.
C. Involution of ventral plate completed.
ps. ventral plate; g. edge of ventral plate; am. amnion; se. serous envelope.
In the Libellulidæ (Calopteryx) there is first of all formed (Brandt, No. [403]) a small ventral and posterior thickening of the blastoderm ([fig. 182] A). The hinder part of this becomes infolded into the yolk as a projection ([fig. 182] B), which consists of two laminæ, an anterior and a posterior, continuous at the apex of the invagination. The whole structure, which is completely imbedded within the yolk, rapidly grows in length, and turns towards the front end of the egg ([fig. 182] C). Its anterior lamina remains thick and gives rise to the ventral plate (ps), the posterior (am) on the other hand becomes very thin, and forms a covering corresponding with the amnion of the more ordinary types. The remainder of the blastoderm covering the yolk (se) forms the homologue of the serous membrane of other types. The ventral surface of the ventral plate is turned towards the dorsal side (retaining the same nomenclature as in ordinary cases) of the egg, and the cephalic extremity is situated at the point of origin of the infolding.
The further history is however somewhat peculiar. The amnion is at first ([fig. 182] C) continuous with the serous envelope on the posterior side only, so that the serous envelope does not form a continuous sack, but has an opening close to the head of the embryo. In the Hemiptera parasita this opening (Melnikow, No. [422]) remains permanent, and the embryo, after it has reached a certain stage of development, becomes everted through it, while the yolk, enclosed in the continuous membrane formed by the amnion and serous envelope, forms a yolk-sack on the dorsal surface. In the Libellulidæ however and most Hemiptera, a fusion of the two limbs of the serous membrane takes place in the usual way, so as to convert it into a completely closed sack ([fig. 183] A). After the formation of the appendages a fusion takes place between the amnion and serous envelope over a small area close to the head of the embryo. In the middle of this area a rupture is then effected, and the head of the embryo followed by the body is gradually pushed through the opening ([fig. 183] B and C). The embryo becomes in the process completely rotated, and carried into a position in the egg-shell identical with that of the embryos of other orders of Insects ([fig. 183] C).
Fig. 183. Three stages in the development of Calopteryx. (After Brandt.)
The embryo is represented in the egg-shell; B. and C. shew the inversion of the embryo.
se. serous envelope; am. amnion; ab. abdomen; v. anterior end of head; at. antennæ; md. mandible; mx1. maxilla 1; mx2. maxilla 2; p1-p3. three pairs of legs; oe. œsophagus.