The Structure and Development of the Dorsal Bristle Complex of Oxytricha fallax and Stylonychia pustulata*

SYNOPSIS. Oxytricha fallax and Stylonychia pustulata possess 6 rows of dorsal bristle units. Each dorsal bristle unit consists of a pair of kinetosomes; the anterior kinetosome has a cilium and the posterior kinetosome a ciliary stub. The kinetosome pair, located at the bottom of a cortical pit surrounding the cilium and ciliary stub, is surrounded by an asymmetrical fibrillar mass. Future rows 1-4 are formed from 2 sets of primordia originating within mature dorsal rows 1-3. Rows 5 and 6 originate from the anterior regions of both right marginal cirral primordia. Old dorsal bristle units utilized in formation of primordia are presumably maintained in the new rows of the proter and opisthe; those outside the primordia are resorbed. The morphogenetic pattern of the Oxytrichidae is similar to those of the Urostylidae and Holostichidae, but quite different from that of the Euplotidae.     

Subcellular Localization of Angiotensin I Converting Enzyme in Rabbit Lung

THE rapidity with which angiotensin I is converted to angiotensin II^1–5 suggests that the subcellular localization of the converting enzyme is important. Bakhle⁶ and Cushman and Cheung⁷ have demonstrated the particulate nature of this enzyme. Subcellular distribution studies and marker enzyme analysis indicate that converting enzyme activity in rabbit lung is most concentrated in the pellet sedimenting at between 1,000 and 25,000g (P₂), subsequently characterized as the light and heavy mitochondrial fraction. To identify this fraction more fully, we have resuspended P₂ and centrifuged it through a discontinuous density gradient, a procedure which separates converting enzyme activity from the mitochondria. Marker enzyme analysis and electron microscopy suggest plasma membrane as the major constituent of that fraction displaying highest specific activity of converting enzyme.     

Differentiation of Reproductive Cells in Volvox carteri*

SYNOPSIS. The life cycle of Volvox carteri was studied in axenic culture using the NB-3 and the NB-7 strains isolated from Nebraska. Vegetative colonies of both strains contain 8–12 asexual reproductive cells (gonidia) which divide to form daughter colonies. During daughter colony formation, the reproductive cells of the daughters are delimited at an early stage of cleavage. Gonidia are delimited at the division from 16 to 32 cells, but eggs and male initial cells are not differentiated until the division of the 32-celled stage. In all instances the reproductive cells are the products of unequal cleavages. Male and female colonies are formed in separate clones. Female colonies contain approximately 20 eggs. Male colonies have approximately 50 male initial cells, each of which forms a sperm bundle containing 64 or 128 sperm. Sperm bundles penetrate female colonies and fertilize the eggs. Zygote formation, zygote germination, and the development of gone colonies is described. Sexual type was inherited in a 1:1 ratio. Male colonies appear spontaneously in the male strain, but female colonies were formed in the female strain only in the presence of a substance produced by colonies from male cultures. This female inducing substance is produced in male cultures primarily, if not exclusively, by male colonies rather than by vegetative colonies. The female inducing substance is heat labile and non-dialyzable. Activity is destroyed by Pronase, but not by trypsin, chymotrypsin or ribonuclease. Gonidia appear to be most susceptible to female induction during the early stages of their expansion prior to cleavage.     

Stomatal Movements, Frequencies, and Resistances in Two Maize Varieties Differing in Photosynthetic Capacity

The Stomatal characteristics of two maize varieties previouslyfound to differ in rates of net photosynthesis were examinedin a controlled environment. Measurements with a ventilateddiffusion porometer showed that one variety exhibited a pronouncedand the other a weak periodicity in stomatal resistance of theadaxial epidermis. At equal illumination the stomatal resistanceof the adaxial epidermis decreased from upper to lower leaves,while the resistance of the abaxial epidermis changed in theopposite manner. Stomata on the adaxial and abaxial surfacesof maize leaves exhibited random not compensatory, movementsin a constant environment. The variety with the lesser stomatalfrequency and higher total leaf resistance to water loss hadnevertheless faster net photosynthesis than the variety withthe greater stomatal frequency, demonstrating the importanceof the so-called mesophyll resistance.     

Early Evolution of Immunoglobulin Genes

Considerable progress has been made in the characterizationof immunoglobulin genes from several lower vertebrate taxa.Isolation and identification of immunoglobulin genes in phylogeneticallyprimitive species is based predominantly on heterologous crosshybridization.The unit, clustered organization of heavy chain segmental elementsobserved in the germline of the horned shark (Hinds and Litman,1986) has also been found in another elasmobranch. Studies todetermine whether the clustered organization is universal throughoutthe entire cartilaginous fish assemblage are ongoing. In contrast,the ray-finned (bony)fishes appear to possess a mammalian-typeheavy chain gene organization. Additionally, immunoglobulingenes are being characterized in two relict fish species whoseexact systematic relationships are unknown. Isolation of putativeimmunoglobulin genes from the phylogenetically- ancient hagfishis being attempted using a PCR-based approach. Other ongoingor future research efforts involve characterization of lowervertebrate light chain genes, heavy chain isotype evolution,and the divergence of the immunoglobulins and T-cell antigenreceptors     

Water Relations of Chelonian Eggs and Embryos: Is Wetter Better?

The exchange of water between a chelonian egg and its subterraneanenvironment is influenced by numerous factors, the most importantof which are (1) structure of the calcareous layer of the eggshell,(2) water potential and temperature in the nest, and (3) fractionof the eggshell that actually contacts soil in the nest cavity.Eggs with relatively porous shells tend to absorb large quantitiesof water from cool, moist environments and to lose large amountsof water to warm, dry ones. Net water-exchange by such eggsalso tends to be more favorable (= positive) when soil contactsthe entire eggshell than when a large fraction of the shellis exposed to air trapped inside the nest cavity. In contrast,eggs with relatively impermeable shells usually exchange onlysmall amounts of water with their environment, regardless ofthe physical conditions that prevail inside the nest. The patternof net water-exchange, together with size (and water content)of the freshly laid egg, determines the amount of water thatis available to sustain the embryo. An embryo having accessto a relatively large reserve of water will consume more ofits yolk and grow to larger size before hatching than will anembryo having access to a smaller reserve of water. Large, well-hydratedhatchlings may survive better than small, dehydrated animalsduring the trek overland from nest to water. If so, a cooler,wetter nest will also be a better nest.     

On the Politics of Theorizing in a Postmodern Academy

The authors outline a politics of academic theorizing that seeks pluralism and egalitarianism without relativism and nihilism. Their goal is to help transform the academy by knocking down the walls around institutionalized technoscience without dissolving the enterprise of academic theorizing. Postmodernist critiques have blurred the cultural boundaries between technoscience and popular science practices, calling into question the absolute legitimacy of the former. Yet the positive proposals of postmodernism have been less valuable than the critiques. The authors hope to recover some of the imaginative power of postmodernist criticism by formulating a different politics of academic theorizing, built around the language and practice of "partnering." In the process, they hope to build exchange relations between postmodernism and other forms of theorizing, such as feminism.     

Self-fertilization in the white-lipped land snail Triodopsis Albolabris

The breeding system of the polygyrid land snail Triodopsis albolabris was studied in laboratory colonies through the use of allozyme-genetic markers. Isolated virgin individuals self-fertilized only after several months of apparent self-sterility. Isolated pairs appeared to reproduce solely by outcrossing. Overall, the normalized reproductive success of paired individuals was about 86 times greater than that of isolates.
Nine natural populations of this snail were surveyed at eight allozyme loci. All populations were highly polymorphic, and heterozygosity was high and agreed with Hardy-Weinberg expectations. This indicates that selfing is not common in established populations. We conclude that T. albolabris avoid inbreeding by selfing if at all possible. However, if the probability of finding a mate is low (assessed by several months without finding one), lone individuals self-fertilize.