Bilateral asymmetry of Diplostomum infections in the eyes of lake whitefish Coregonm clupeaformis (Mitchill) and a computer simulation of the observed metacercarial distribution

Abstract. Fifty-four per cent of the specimens in a sample of 147 lake whitefish ( Coregonus clupeaformis ) from northern Quebec exhibited a pronounced lack of bilateral symmetry in the distribution of metacercariae of the genus Diplostomum . This asymmetry was shown not to be a product of chance. A comparable degree of asymmetry was generated by a computer simulation incorporating a positive feedback mechanism. The simulation and the biological significance of such asymmetry are discussed.     

The frequency distribution of tetracotyles of Apatemon gracilis pellucidus (Yamaguti, 1933) in stickleback Culaea inconstans (Kirtland) populations of homogeneous age and size structure

Abstract. The frequency distribution of tetracotyles of Apatemon gracilis pellucidus in natural populations of brook sticklebacks Culaea inconstans homogeneous for age and size were fitted to negative binomial curves. Values of k were unusually high while the degree of overdispersion was correspondingly low. It is suggested that these properties of the distributions are a function of the homogeneity of the host population.     

Sperm whales associated with Gulf Stream features off the north-eastern USA shelf

Marine mammal surveys were conducted during sum Mer 1990 and 1991 in shelf-edge and off-shelf waters between Cape Hatteras and Georges Bank. Sperm whales were the most frequently sighted large whale in both years, constituting 27.9% and 12.7% of total sightings in 1990 and 1991, respectively. Sighting rates of sperm whales that were within a 9.3 km buffer (5 n.m.) of the Gulf Stream north wall were not significantly different (x²= 2.86, P > 0.05) from those within the Gulf Stream during 1990. In 1991, however, significantly more (x²= 51.0, P < 0.05) sightings were associated with shelf-edge areas where warm core rings were located.     

The formation of chondrules: petrologic tests of the shock wave model

Chondrules are millimeter-sized rounded igneous rocks within chondritic meteorites. Their textures and fractionated mineral chemistries suggest that they formed by repeated, localized, brief (minutes to hours) melting of cold aggregates of mineral dust in the protoplanetary nebula. Astrophysical models of chondrule formation have been unable to explain the petrologically diverse nature of chondrites. However, a nebular shock wave model for chondrule formation agrees with many of the observed petrologic and geochemical properties of chondrules and shows how particles within the nebula are sorted by size and how rims around chondrules are formed. It also explains the volatile-rich nature of chondrule rims and the chondrite matrix.     

The effect of thermal time and soil water on emergence of Ranunculus repens

The effect of temperature and soil available water (AW) on the emergence of Ranunculus repens ␣L. was examined in artificial media in growth chambers. Two seed populations, `arable type' and `wild type', were investigated. Moisture levels were 20%, 40%, 60% and 80% AW, and temperatures were 10/5 °C, 15/7 °C, 20/10 °C and 25/15 °C. Time to onset of emergence varied from 11 to 28 days for both seed populations, primarily as a result of mean temperature. Mean thermal time (TT) required for the onset of emergence of both populations was 251 ± 45 TT. Total emergence of the `arable type' R. repens varied from 3% at 10/5 °C and 20% AW to 30% at 25/15 °C and 60% AW. Total emergence of the `wild type' was greater, ranging from 4% at 10/5 °C and 20% AW to 57% at 25/15 °C and 80% AW. Temperature and AW significantly affected total emergence. A logistic response was displayed for the cumulative emergence of both populations. Seedling emergence models based on TT were developed for both populations at each of the four levels of AW.     

ASSESSMENT OF THE ACCURACY AND PRECISION OF A PATIENT IMMOBILIZATION DEVICE FOR RADIATION THERAPY IN CANINE HEAD AND NECK TUMORS

The positioning accuracy and precision of a head and neck immobilization device for radiation therapy of tumors in the canine skull was evaluated. Nineteen dogs with a spontaneous tumor of the head were enrolled including 12 with an intracranial mass and seven with an intranasal or maxillary tumor. Three hundred thirty-three pairs of orthogonal digital portal radiographs were analyzed to assess patient displacement in the cranial–caudal, lateral, and dorso-ventral directions. The mean systematic displacement was 0.8, 1, and 0.9 mm. The mean random displacement was 1.9, 1.6, and 1.5 mm. These values resulted in an overall displacement of 2.1 mm in the cranial–caudal direction, 1.8 mm in the lateral direction, and 1.7 mm in the dorsal–ventral direction. The mean displacement value of the three dimensional (3D) vector was 2.4 mm with a standard deviation of 2.1. Ninety-five percent of all vectors were <6.4 mm. This study quantifies the precision and accuracy of this particular positioning device. Knowing the limitations and setup variability of the system being used to set patients up for daily radiotherapy is paramount to planning and delivering appropriate radiation doses, especially as more complex treatment methods are used.