Matrotrophic adaptations and early stages of embryogenesis in the desert scorpionParuroctonus mesaensis (Vaejovidae)

Light and electron microscopy were used to examine the sequential changes in embryos and maternal reproductive tubules in the vaejovid scorpion, Paruroctonus mesaensis. The early stages are described, from oogenesis to the time of embryogenesis just before spiracles and booklungs begin to appear. The oocytes and associated trophic cells originate in the epithelium of the maternal ovariuterine tubules. Each oocyte with follicle (primary trophic) cells forms a protuberance on the tubular surface. After fertilization and cleavage, the morula appears to be pulled into the tubular lumen by tissue invaginated from the wall opposite the protuberance. Secondary trophic cells originating near each oocyte form a trophic layer parallel and connected to the ovariuterine tubules. These cells encircle the tubular epithelium and apparently induce or transform it to a trophic mode. Within the tubules, each embryo develops in a specialized region (uterus) where the embryo abuts a cell mass (follicular placenta). The latter, along with the tubular and uterine epithelia, apparently releases nutrients that are absorbed through the embryo epidermis. The main structures of the embryo body develop from the germinal disc of the blastula. Segments and appendages are formed in an anterior-to-posterior direction. The chelicerae, initially posterior to the stomodeum, are gradually positioned anterior and dorsal. The early abdomen has only a few segments with no separation into meso- and metasoma. The latter structure, when formed, has a ventral flexure and segments of smaller diameter than those in the mesosoma. The telson starts as a broad, bilobed structure, becoming tapered with two openings near the tip. Each pectine appears first as a lobe attached to the body wall. Transverse grooves appear distally and then later along the entire length. In the stages of this study, there was no indication of ventral plates, gills, or sternites in the ventral mesosoma where booklungs and spiracles will eventually form. J. Morphol. 237:187-211, 1998.