Palaeoecology of coralline sponge-coral meadows from the upper jurassic of Portugal

TheComophyllia polymorpha-Crispispongia cf.expansa association of the Kimmeridgian Alcobaça Formation occurs in a 5–10 m thick unit that can be followed for at least 10 km in the vicinity of Alcobaça (Estremadura). Corals, coralline sponges (mainly Calcarea), cryptalgal crusts and, to a lesser extent, crinoids are the dominant constituents of the autochthonous community relic which can be grouped in framebuilders, framework encrusters, frame binders, reef-dwellers, and reef destroyers. These organisms formed low meadows in a shallow, fully marine environment subject to low rates of sedimentation and moderate to low energy levels punctuated by rare high energy events. The abundance of coralline sponges in reefs and reef-like communities is uncommon in the Jurassic and appears to be restricted to very shallow water environments.     

Morphogenesis of the hatching gland of Atlantic halibut (Hippoglossus hippoglossus)

Summary The halibut hatching gland (HG) cells are first observed as a cellular disc in front of the embryonic head around the midpoint of intra ovo development. The disc is subsequently transformed into a loop of increasing diameter as the HG cells migrate over the anterior part of the yolk sac. When the HG disc is transformed into a loop, the density of HG cells is highest at the migratory front. Some HG cells lag behind the migrating front at the early stages of HG development. At maturity, all cells are contained in a narrow belt which is about 10 cells wide. The HG belt structure consists of a monolayer of HG cells, and is maintained while the cells migrate between the two epidermal cell layers. Migration is halted about 2 days before normal hatching when the HG cells reach a destination at about a right angle to on the embryonic axis. Under the scanning electron microscope, the differentiating HG cells protrude as a ridge the yolk sac surface. The HG cells immunostain with antiserum to hatching enzyme when the HG is observed as a crescent structure around the embryonic head. By counting the number of immunostaining cells in composite photos of the entire yolk sac membrane, we found that the HG belt consists of approximately 2000 secretory cells at maturity. This cell number stays fairly constant throughout the period of HG cell migration. Accordingly, mitoses of the halibut HG cells have generally ceased prior to morphogenesis, and cytodifferentiation is already quite advanced when cell migration starts. Offprint requests to: J.V. Helvik     

über die Verschiebung der Vererbungsrichtung unter dem Einflu\ von KohlensÄure

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Die Vegetation und der Canal auf dem Isthmus von Suez

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Zwei Fehlerquellen bei Merogonieversuchen und die Entwicklungsfähighkeit merogonischer und partiellmerogonischer Seeigelbastarde

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Über die Zweikernigkeit der Leberzellen

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