. The biology of dragonflies (Odonata or Paraneuroptera). Dragon-flies. 232 EMBRYOLOGY [CH. form large cells known as vilellophags, whose function it is to liquefy the yolk and carry it to the developing embryo. They grow to a great size, and possess enormous nuclei. They are not all used up by the time of hatching. Those that are left become finally enclosed, ^vith their store of yolk, in the mid-gut of the young larva, where we hav^e already met with them (p. 71). The Ventral Plate (fig. 110, and fig. 109 a). The first sign of the Dragonfly embryo is a slight thickening of the blastoderm on
. The biology of dragonflies (Odonata or Paraneuroptera). Dragon-flies. 232 EMBRYOLOGY [CH. form large cells known as vilellophags, whose function it is to liquefy the yolk and carry it to the developing embryo. They grow to a great size, and possess enormous nuclei. They are not all used up by the time of hatching. Those that are left become finally enclosed, ^vith their store of yolk, in the mid-gut of the young larva, where we hav^e already met with them (p. 71). The Ventral Plate (fig. 110, and fig. 109 a). The first sign of the Dragonfly embryo is a slight thickening of the blastoderm on the posterior portion of the ventral surface of the egg. This thickening, known as the ventral plate O'p), is caused by proliferation of the blastoderm cells, which here become several layers deep. The ventral plate grows forwards, and quickly becomes a very distinct oval patch, occupying half or more of the ventral surface of the egg. Near the posterior end of the plate—in that position, in fact, where the thickening first began—there next appears a kind of pitting-in or in- vagination of the blastoderm. According to Heymons, this invagination in Libellula (fig. 110) takes the form of a primitive groove ipg) along the mid-longitudinal line of the ventral plate, running forwards from the point where it first began (known as the hlastojwre). A shght enlargement of the anterior end of this groove marks the position of the future stomodaeal opening {st). According to Brandt, in Calopteryx the invagination takes place by a sinking-in of the ventral plate at the blastopore in a gradual manner, beginning with the posterior end (fig. 109 b). The result is the same in either case. The cavity formed by the sinking-in of the ventral plate is known as the amniotic cavity. As the ventral plate passes in by the upper lip of the blastopore (b, c), and as undifferentiated blastoderm is drawn in along the lower lip, the walls of the amniotic cavity are dissimilar. The dorsal wall is formed
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