Ultrastructure of the embryonic dorsal organ of Orchestia cavimana (crustacea, amphipoda); with a note on localization of chloride and on the change in calcium-deposition before the embryonic moult.
The transitory dorsal organ of Orchestia cavimana appears simultaneously with the development of the germ layer and is gradually reduced during the last 2-3 days of embryonic development. It represents the only direct connection of the embryo with the chorion or-after the embryonic moult-with the embryonic envelope. The shape is hemispherical and it consists of about 50 bottle-shaped cells, arranged radially around a centre. This centre is filled with different kinds of extracellular material which forms a central plug apically and a central cone below it. The bottle-shaped cells taper apically. The neck region of these cells is characterized by numerous microvilli which project into the intercellular space. This space is filled with an electron dense substance and is in contact with the central cone. In the basal neck region numerous mitochondria are associated with the microvilli. The high density of mitochondria is characteristic for the nuclear region. The cytoplasm of the basal region below the nucleus contains numerous calcium granules. Evidence for the concentration of chloride in the apical dorsal organ is shown. Before the embryonic moult and during the duplication of the egg-volume the number of calcium granules in the dorsal organ and the integument is reduced. Simultaneously calcium granules appear in the now visible periembryonic space. This suggests that part of the calcium is shifted into this space. The function of the dorsal organ is discussed. Besides the probable main function-transport activity of ions-its role before and during embryonic moult and its part in the utilisation of yolk are discussed.
Meschenmoser
M
M
Lehrstuhl für spezille Zoologie, Zoologisches Institut, WWU Münster Hüfferstr. 1, 4400 Münster, FRG.