Memoirs: The early Embryology of a viviparous Psocid

Abstract

1. The ovum of this Psocid is devoid of yolk-cells and chorion. 2. Cleavage results in the formation of an outer layer of cells lying in the periplasm and an inner group of trophocytes. 3. The primordial germ-cells lie at the posterior end of the embryo. 4. A ventral plate is produced, which forms a median plate and two lateral plates. 5. From the median plate the mesoderm arises by proliferation along its whole length, while the lateral plates give rise to the ectoderm. 6. The endoderm arises from two rudiments--anterior and posterior--from which are proliferated cells; some of these come to line the ventral surface of the nutritive mass above the mesoderm, the rest fill the nutritive mass itself. Together they constitute a middle endodermal mass. 7. The mid-gut is developed from the middle endodermal mass--the ventral cells lining the nutritive mass forming the ventral and lateral walls of the mid-gut, the dorsal wall being produced from cells in the nutritive mass. 8. The original anterior and posterior rudiments form evanescent vesicles. 9. The amnion and serosa arise by the formation of anterior and posterior folds. 10. After disintegration of the amnion and serosa, the dorsal organ develops by arrangement of trophocytes and endodermal cells along an invagination of the ectoderm of the embryo. Later on an opening is effected by which the contents of the dorsal organ come in close contact with the wall of the ovarian tubule. Finally this aperture is closed and the dorsal organ is absorbed along with the nutritive mass. 11. The serosa acts as a temporary organ for obtaining nourishment for the embryo from the wall of the ovarian tubule. 12. Blastokinesis does not take place during development.