About the Cover
Cover
The horseshoe crabs are among the oldest living chelicerates, and they show many ancestral characters during embryogenesis. Therefore, Beate Mittmann (Humboldt-Universität, Berlin) has been examining early neurogenesis in Limulus polyphemus and using the results to shed new light on our understanding of phylogenetic relationships among the arthropods.
In this issue of The Biological Bulletin (p. 221), Mittmann-using immunocytochemical techniques-describes the formation of neurons and ganglia from the ventral neuroectoderm of Limulus embryos that have been developing for about 10 to 12 days since fertilization. The mode of gangliogenesis is similar to that reported for other chelicerates (especially spiders), but is distinct from that in crustaceans and insects.
The central image on the cover of this issue shows an embryo (about 1.5 mm long) that has been developing for an additional 5 to 8 days. The yolk mass has been excised from this embryo, and the parallel ventral nerve cords, joined ladder-like by connectives, are clearly visible. Nerve cords in this location and of this structure are a feature among many protostome invertebrates. The areas containing the limb buds, and their innervation by a thick bundle of nerve fibers, are also obvious in this image, as is the stomodeum, which is near the anterior end of the embryo. Embryos of this stage will be ready to hatch in about 10 days. Two such hatchlings (3-5 mm long) also appear on the cover; they are called trilobite larvae because of their similarity to the trilobites-an ancient group of arthropods, now extinct and known only from fossils.
The cover images were made by Beate Mittmann. The embryo was treated with a fluorescent antibody to |ga-tubulin and examined with a Pascal confocal microscope (Zeiss), which produced a projection of a stack of 64 images. This picture was the winner, in Scientific Imaging, of the 2002 Marine Biological Laboratory Photography and Photomicrography Contest. The cover design is by Beth Liles, MBL.
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Copyright © 2003 by the Marine Biological Laboratory.
