摘要:

AbstractShell‐anchored muscles that extend into the cephalopodium of five species of planktotrophic nudibranch larvae were studied by ultrastructural examination of sequential larval developmental stages. All species, regardless of larval shell type (inflated or non‐inflated), showed a similar basic pattern of shell muscles. The larval retractor muscle (LRM) differentiates prior to hatching and its fibres insert on epithelia of the velum, apical plate, stomodeal region, or mantle fold. Many fibres also connect with subepithelial intrinsic muscles of the cephalopodium. Most but not all LRM fibres Project to left‐sided targets and are innervated from the left cerebral ganglion. Two pedal muscles, which are innervated from the pedal ganglia, differentiate during the post‐hatching larval stage and both insert primarily on pedal epithelium attached to the operculum. The left pedal muscle is anchored to the shell immediately adjacent to the attachment plaque of the LRM and consists of basal and distal tiers of muscle cells. The right pedal muscle arises on the ventral rim of the shell aperture and consists of a single tier of muscle cells. Ontogenic changes in larval retraction behaviour correlate with developmental change in the muscle effectors. Although some interspecific differences were noted, the presence of a common ground plan for larval shell muscles in these five species contrasts with previous indications of marked variability for nudibranch larval shell

ISSN:0001-7272    

DOI:10.1111/j.1463-6395.1995.tb00992.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractAlthough no one has disputed that the piscine progenitors of the tetrapods have a homologue of the human parietal bone, opinions differ as to where in the skull roof this homologue is located. One view holds it to be either of two interorbital bones that together surround the foramen neuroepiphysium (and the so‐called pineal plates); another, that it is each of the two mesial bones which comes next in order, behind the orbital cavities. Both of these views are untenable because neither of the proposed bones has proved to be amenable to conversion to the parietal bone of man. In seeking a solution to this issue thoughts turn to the tentorium cerebelli, whose topographic relationships and comparative morphology place it in the key position as a plausible derivative of the posterior half of the cranium of the tetrapod forerunners. Following this line of reasoning, it can be suggested that the tentorial exoskeleton, here called the pluteal bones, originally was situated in the dermis superjacent to the synotic tectum. In early therian phylogeny, these skull bones were covered by the backwardly expanding cerebral hemispheres which, concomitantly, became overgrown and thus protected by epiotic exoskeleton. It follows that the likely homologues of the human parietal bones are those parts of the skull roof of the piscine progenitors of the tetrapods that lie dorsolateral to the otic capsule

ISSN:0001-7272    

DOI:10.1111/j.1463-6395.1995.tb00993.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe ultrastructure of the skeleton is described in six species of lichenoporid cyclostome bryozoans using field emission SEM. Both interior walls (vertical, interzooidal walls, and brood chamber roofs and floors) and exterior walls (basal walls) are initially secreted as tiny wedge‐shaped crystallites without a strong preferred orientation. These are seeded directly onto pre‐existing crystallites in the case of interior walls, but onto the organic cuticle in exterior walls, the bases of the crystallites forming a tightly packed mosaic against the cuticle. With growth the wedges become longer, broader and relatively flatter, developing into platey crystallites. These crystallites grow predominantly distally (i.e. parallel to wall growth direction) and are closely imbricated in a foliated fabric. Local disruptions to this pattern occur, especially in association with crystallite division along “divergent zones”. The pattern also breaks down in old walls where crystallite growing edges become less evident and imbrication is poorly developed. Although conforming to this general model, some differences exist between species of lichenoporids, and in the patterns found in different parts of the skeleton (e.g. apertural spines). Lichenoporid ultrastructure differs from that of both cinctiporid and hornerid cyclostomes: notably, lichenoporids lack the layer of transverse fibres found in cinctiporids, and their predominant distal growth direction of crystallites contrasts with the proximal direction found in ho

ISSN:0001-7272    

DOI:10.1111/j.1463-6395.1995.tb00994.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe changes in epidermal ultrastructure during the metamorphic cycle ofHyla arboreaare described. The number of cell layers increased from two to four in the late tadpole stages. The cell layers flatten and the process of stratification reaches its peak after the completion of metamorphosis. The mitochondria‐rich cell appears early in the tadpole stages. Numerous flask cells are noticeable in the post‐metamorphic stages.K+–p‐NPPase activity was localized cytochemically in the epidermis ofH. arboreaduring its metamorphic cycle. In the epidermis of the legless tadpole, evidence for K+–p‐NPPase activity was confined intracellularly. During the later tadpole stages, preceding metamorphic climax, the main ATPase activity shifted to the baso‐lateral cell membranes bordering with the intercellular spaces under the surface and later the stratum corneum. This continued after metamorphic climax in the juvenile toadlets, diminishing later in th

ISSN:0001-7272    

DOI:10.1111/j.1463-6395.1995.tb00995.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractEarlier papers dealing with the architecture of the cephalic nervous system of the Eunicea were studied. Thereby, a re‐investigation appeared necessary: the existing literature proved insufficient for detailed comparisons with other polychaete families and many earlier statements and conclusions turned out to be quite contradictory, especially as concerns the homologues of the anterior end appendages. In the present paper, the microanatomy of the brain and the innervation of,inter alia, the antennae, the nuchal organs and the alimentary canal ofHyalinoecia tubicola, Nothria conchylega, Eunice norvegicaandE. pennataare described. The results are summarized in schematic diagrams and tables and compared with corresponding observations in other polychaete families. Thereby, some earlier opinions about the morphological value of the cephalic appendages of the Eunicea are strengthened while others are rejected. On the basis of the present observations and earlier results arrived at by the present author, and as an object for future discussion and criticism, a diagram of the fundamental constitution of the cephalic nervous system of the Polychaeta is tentatively outline

ISSN:0001-7272    

DOI:10.1111/j.1463-6395.1995.tb00996.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractGonadal sex differentiation is temperature‐dependent inAlligator mississippiensis; testis differentiation occurs in embryos incubated at 33°C and ovary differentiation occurs in embryos incubated at 30°C. Laminin and cytokeratin were examined immunohistochemically in the gonads of alligator embryos incubated at these temperatures. The aim of this study was to determine whether these structural proteins show the same sex‐specific expression patterns reported for mammalian embryos, and to assess their usefulness as early markers of gonadal differentiation in species with temperature‐dependent sex determination. Laminin delineated enlarged seminiferous cords in differentiating testes from developmental stage 23 to hatching. Laminin distribution was more diffuse and revealed smaller cords of cells in differentiating ovaries. Cytokeratin was also detected in developing gonads of both sexes. Cytokeratin became concentrated in the basal cytoplasm of differentiating Sertoli cells in developing testes. In developing ovaries, prefollicular cells of the ovarian cortex and cell cords in the medulla stained strongly for cytokeratin. Cytokeratin did not show the same basal distribution in female medullary cord cells as seen in the Sertoli cells of testes, however. These sex‐specific patterns of laminin and cytokeratin distribution in embryonic alligator gonads may serve as early markers of sexual differ

ISSN:0001-7272    

DOI:10.1111/j.1463-6395.1995.tb00997.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY