The Embryology of Dacus tryoni 2. Development of lmaginal Discs in the Embryo

The cephalic, labial, wing, haltere, leg and genital imaginal discs of Cyclorrhapha are histologically distinct in the late embryo or early larva (Weismann, 1864; Pratt, 1900; Auerbach, 1936; Kaliss, 1939; Poulson, 1950). Experimental investigation of Drosophila melanogaster further suggests that the determined rudiments of the discs are present in the blastoderm of the early embryo (e.g. Geigy, 1932; Howland and Child, 1935; Howland, 1941; Gloor, 1947). Almost no attempt has been made, however, to describe the embryonic origins of the discs or to follow their development before hatching. Investigation of this matter in Dacus tryoni (Frogg) (Diptera, Trypetidae) has clarified the developmental relationship between the cyclorrhaphan larva and adult. The origin and development of imaginal discs in the embryo of D. tryoni was followed from serial sections prepared in connection with a previous paper (Anderson, 1962). Living embryos were also further examined by the method given in that account.

The artificial production of echinoderm larvæ with two water-vascular systems, and also of larvæ devoid of a water-vascular system

The development of Echinoderms has been characterised, and with justice, as the most remarkable ontogenetic change in the animal kingdom. For the larva is an almost perfect example of a simple, bilaterally symmetrical Metazoon, and the amazing thing is, not that the radially symmetrical adult should develop out of a bilaterally symmetrical larva, but that the axis of symmetry of the radial adult should cut the principal axis of the bilateral larva at an angle which approaches 90°. In the three orders Asteroidea, Ophiuroidea, and Echinoidea, the general anatomy of the early larva is of the same type. In all three groups the larva possesses a simple alimentary canal, consisting of a conical œsophagus opening by a wide mouth, a globular stomach, and a sac-like intestine opening by a narrow anus and directed forwards, so that the whole alimentary canal has the form of a U. On each side of the œsophagus a flattened cœlomic sac is situated; of these, the left sends up a vertical outgrowth termed the pore-canal, which fuses with the dorsal ectoderm, and opens to the exterior by a pore called the madreporic pore. Each cœlomic sac subsequently grows backwards, so that its posterior portion lies beside the stomach, and this portion later becomes separated by a constriction from the rest. Consequently, each sac becomes divided into an anterior and a posterior cœlom.