Excystment of Didinium nasutum

SYNOPSIS. Excystment of Didinium nasutum is solely dependent upon dense bacterial populations and is independent of the type of medium in which the bacteria have been grown. Beers' hypothesis of the need for a factor from proteose-peptone is thereby not confirmed, but the need for living, intact bacteria as postulated by him is fully shown. The process of excystment is initiated by bacteria, but their presence is not required throughout the excystment. Axenically grown paramecia, shown to be adequate as food organisms for didinia, will not induce excystment. Five types of bacteria including Gram-positive, Gram-negative, aerobic and anaerobic all suffice to induce excystment, suggesting that the initiation of this process is caused by some general metabolic process or product of bacteria.     

Relative age inference in paleontology

Fossil taxa often occur in a regular vertical order in strata, yet, this regularity does not necessarily imply that the taxa succeeded one another in the same order in time. An argument for the time significance of such a regular pattern is strengthened if (1) the taxa involved were preserved in a wide range of depositional environments, and (2) the Eldredge-Gould model of evolution is correct: by contrast, (3) the areal extent of the pattern and (4) the inferred phylogeny of the taxa involved are not relevant. Paleontologists commonly do conclude that a homotaxial pattern, i.e. a definite orderly succession of fossil taxa, results from the taxa having occurred in essentially the same order in time. The conclusion may be justified as follows: If it is false, there should have been some local areas studied where the taxa (or first and last occurrences of taxa) occur in reverse order, or where two or more allegedly sequential taxa occur together in the same strata. But this is not the case: so the taxa probably succeeded one another in essentially the same order in time. The argument is broadly applicable, yet compelling only if (1) certain constraints involving the prior probability of the conclusion apply, and (2) transgression/regression can be ruled out as a cause of the pattern observed. Subsidiary defenses of the time significance of a homotaxial pattern involve other-group, radiometric, marker-bed, magnetic, seismic, or stable-isotope data. The use of fossils for time correlation of strata does not involve circular reasoning.     

Determination in Drosophila Embryos

SYNOPSIS. In this review we describe data of experiments whichinterfere with the formation of the metameric pattern duringembryogenesis. Ligating embryos before blastoderm stage leadsto a gap in the segmentation pattern of the differentiated embryo.The gap can extend up to 6 segments but terminal segments arealways recognizable. In posterior but not in anterior fragmentswe find abnormally large but fewer segments. This increase insegment size results from a different determination of blastodermcells after ligation. During nuclear multiplication stages whena gap can be produced, the zygotic genome is not yet active.Information to develop the metameric pattern in ligated embryosmust therefore have been made during oogenesis. Recently Nüisslein-Volhard and Wieschaus (1980) have describedthree zygotic mutations which form embryos with a gap of segmentssimilar to our ligated embryos. We have discussed these mutantphenotypes in connection with our experimental data. Segmentation is controlled at several levels. During oogenesisthe anterior-posterior and dorsal-ventral axes become established(Nüsslein-Volhard, 1979). Also during oogenesis, but extendinginto early embryonic life, information is generated to subdividethe embryo into blocks of cells forming the metameric pattern.At blastoderm the identity of segments becomes established.     

In the Trenches,Cost Containment Ain't Always Easy: An Illustrative Case Report

A specific case report illustrates the interplay between human considerations and technological potency that makes cost containment difficult in the real world of medical practice.