Smithsonian miscellaneous collections . s and antennae. In his figure 57 he shows the left sac exactly as allother writers have depicted the frontal sacs, and yet he says embry-onic studies reveal no such pouches. Furthermore, Ludwig attributesto Pratt (1901) the absurd statement that the common opening of thesacs is drawn forward and downward, and then posteriorly throughthe mouth, and on this assertion he bases a criticism of Pratts work. no. 3 METAMORPHOSIS OF A FLY S HEAD SNODGRASS 17 However, Pratt makes no such statement, or anything like it. Moreconcisely than does Ludwig himself, Pratt
Smithsonian miscellaneous collections . s and antennae. In his figure 57 he shows the left sac exactly as allother writers have depicted the frontal sacs, and yet he says embry-onic studies reveal no such pouches. Furthermore, Ludwig attributesto Pratt (1901) the absurd statement that the common opening of thesacs is drawn forward and downward, and then posteriorly throughthe mouth, and on this assertion he bases a criticism of Pratts work. no. 3 METAMORPHOSIS OF A FLY S HEAD SNODGRASS 17 However, Pratt makes no such statement, or anything like it. Moreconcisely than does Ludwig himself, Pratt describes in Melophagusovinus the origin of the frontal sacs (dorsal head discs) as dorso-lateral thickenings of the epidermis of the embryonic head. Early intheir history the discs begin to invaginate in the form of crescenticslits (fig. 5 D, FS), and later they move dorsally to the back of thehead, where their outer parts unite in a single, transverse depression,while the inner parts increase in length and extend separately into the. Fig. 6.—Head of stratiomyid larva and sections of embryonic head of Calliphora. A, larval head of Ptccticus trivittatus (Say) partly ensheathed in fold of pro-thorax, dorsal. B, lengthwise section of larval head and prothorax of Odonto-myia (combination diagram from Jusbaschjanz, 1910, relettered). C, crosssection near base of head of 15-hour embryo of Calliphora (from Ludwig, 1949).D, cross section of head lobe of 16-hour embryo of Calliphora, overhanging thelabium (from Ludwig, 1949). body cavity as a pair of stalked sacs that lie in contact with the cere-bral ganglion. Now there takes place, from behind the mouth of thesacs (fig. 4 C, c), the formation of the dorsal fold (Jif), which growsforward over the head. Since the inner lamella of the fold becomesclosely adherent to the f rontoclypeal plate, it thus comes about that inthe larva the sacs appear to be attached to the posterior end of thelarval sucking apparatus (figs. 2 B, 5 E, FS). Th
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Keywords: ., bookcentury1800, bookdecade1860, bookidsmiths, booksubjectscience
