The production and preservation of trilobite resting and furrowing traces

The production and preservation of trilobite resting and furrowing traces is discussed with special reference to specimens from Upper Cambrian strata in North Walcs, Spain, and Poland. Resting excavations are attributed largely or completely to animal activity, and do not normally rcsult from currcnt scouring. Resting excavations, furrows, and many other trace fossils are formed at the sediment-water interface and not bencath a thin sand layer or by burrowing between beds. The factors which allow the initial production of the traces on the ocean floor muds (e.g. cohesion, shear strength, low pore water pressures) will also favour their preservation during deposition of the casting sand.     

On the Origin of Thymic Lymphocytes

In the leopard frog (Rana pipiens), thymic lymphocytes do notoriginate from blood-borne stem cells that migrate into thethymus anlage; rather they arise in situ from elements in thethymic rudiment itself. After thymic differentiation, the lymphocytes(or their descendants) leave the thymus and extensively seedthe peripheral lymphoid organs. Indeed, virtually all the lymphocytesin the spleen, kidney, and bone marrow are ontogenically derivedfrom thymic cells. In postmetamorphic life, the thymus representsan organ in which lymphopoiesis is genuinely self-sustaining.Throughout the juvenile life of the frog, there is no indicationof an inward afferent stream of cells entering the intact thymus.     

Downwind migration of the African army worm moth, Spodoptera exempta, studied by mark-and-capture and by radar

Abstract. 1. About 166,000 African armyworm moths, Spodoptera exempta (Walk.), were marked at an emergence site near Nairobi when they fed at night on trees baited with dyed molasses.
2. Six marked moths were captured in pheromone traps, including one at 90 km after flying for only one night, and another at 147 km.
3. Moth flight trajectories deduced from radar and from marking showed that migration was downwind.
4. During migration, moths become dispersed; hence the high densities that lead to outbreaks must be produced by concentration.
5. Some moths were ready to mate on the same night they completed their long-distance flight.     

Cell Interactions in the Differentiation of a Melanotic Tumor in Drosophila

The cellular events in the formation of melanotic tumors in the tu-W mutant larva of Drosophila melanogaster are described. The first step is the differentiation of spherical hemocytes to flattened cells, the lamellocyte variants. Subsequently, the surface of the caudal fat body undergoes changes to which the hemocytes respond by forming cellular capsules. The hemocytes utilize two mechanisms in this process: (1) phagocytosis of small participate materials escaping from the adipose cells, (2) adhesion to form a multilayered wall of lamellocytes.
Differentiating hemocytes in the vicinity of the tumor-forming site extrude membrane-bound vesicles that tend to adhere to the hemocyte surfaces. These vesicles are trapped between the lamellocytes as they pile in layers to form the capsule wall. It is suggested that the vesicles play a role in lamellocyte-to-lamellocyte adhesion during the initial stages of hemocyte aggregation at the tumorforming site.     

The Membranes of a Eukaryotic Cell

The evolution of internal membrane systems has introduced manyadditional control steps into pathways that, although basicallysimilar in prokaryotic cells, are less sophisticated and leavemany aspects up to chance. Temporal and spatial control of secretion,quality control of the proteins secreted or inserted into theplasma membrane, and exquisite control over the selective degradationof macromolecules, for example, are apparently indispensiblerequirements in multicellular organisms, but are relativelyunimportant in bacteria. Although complicated in detail, mostmembrane traffic in the eukaryotic cell can be reduced to afew basic principles. Such a reductionist's view provides aconceptual framework that allows the reader to organize an otherwiseoverwhelming amount of data on cellular membrane architectureand dynamics.     

Does Late-acting Self-incompatibility (LSI) Show Family Clustering? Two More Species of Bignoniaceae with LSI:Dolichandra cynanchoides and Tabebuia nodosa

By means of hand pollination experiments and fluorescence microscopythe breeding systems and pollen tube growth were studied inDolichandra cynanchoides and Tabebuia nodosa, two bignoniaceousspecies of the Chaco woodland of NE Argentina. Both speciesabscised all selfed pistils within 4–8 d although selfpollen tubes grew successfully to the ovary and penetrated manyovules. Both species therefore show ‘late-acting self-incompatibility’(LSI) or ‘ovarian sterility’ (OS) type control ofselfing. LSI-OS has been reported in 17 species of the Bignoniaceae,and is also clustered in families such as the Bombacaceae-Sterculiaceae,Fabaceae and Myrtaceae. The implications of such family clusteringare discussed, with regard to current hypotheses for the actionof LSI-OS phenomena. Copyright 1999 Annals of Botany Company Bignoniaceae, Dolichandra cynanchoides, Tabebuia nodosa, Chaco, late-acting self-incompatibility, ovarian sterility.     

Covert Categories and Folk Taxonomies

Much of the recent work in ethnoscience has been concerned with the nature of folk taxonomies, an often stated definition of which requires that all folk taxa be monolexemically labeled. This paper offers evidence that unlabeled categories may also be of crucial taxonomic significance, and we feel that it is inappropriate to treat such categories apart from the named taxonomic entities of the system. More importantly, evidence presented indicates that by recognizing unnamed taxa one may gain an understanding of the structure of a particular semantic domain that is actually obscured if one focuses solely on lexically labeled units.