PubMed · 4448466
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R L Blokhra, N Sakhuja. 1974. Phenomenological & diffusion coefficients in bio-fluids.. https://pubmed.ncbi.nlm.nih.gov/4448466/
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Nucleotide sequence comparisons were used to investigate ordinal and familial relationships within the class Amphibia. Approximately 850 base pairs of the mitochondrial 16S ribosomal RNA (rRNA) gene from representatives of 28 of the 40 families of extant amphibians were sequenced. Phylogenetic analyses of these data together with published data of the 12S rRNA gene for the same families and both genes for three more taxa (approximately 1,300 base pairs total for 35 taxa) support the monophyly of each of the three amphibian orders: Anura (confidence value with the interior-branch test: P(c) = 99%), Caudata (P(c) = 100%), and Gymnophiona (P(c) = 99%). An analysis using the four-cluster method cannot discriminate significantly between all three possible unrooted trees involving the three orders of amphibians and an outgroup. Within the Anura, there is support for the monophyly of the two suborders: Neobatrachia (P(c) = 100%) and Archaeobatrachia (P(c) = 97%); the latter was believed to be paraphyletic on the basis of morphology. Within the Archaeobatrachia, the following pairs of taxa cluster: Pelobatidae + Pelodytidae (P(c) = 99%), Pipidae + Rhinophrynidae (P(c) = 99%), Ascaphus + Leiopelmatidae (P(c) = 89%), and Bombina + Discoglossidae (P(c) = 99%). The latter six taxa cluster (P(c) = 94%) such that Pelobatidae + Pelodytidae forms a basal lineage within the Archaeobatrachia. Three major lineages are distinguished within the Neobatrachia: the superfamily Bufonoidea sensu Duellman (P(c) = 86%), the superfamily Ranoidea sensu Lynch (P(c) = 99%), and the Sooglossidae. Basal within the Bufonoidea, Myobatrachidae + Heleophrynidae cluster at P(c) = 96%. The enigmatic Dendrobatidae clusters with the bufonoid families (P(c) = 92%) and is excluded from the ranoid families (P(c) = 99%). (ABSTRACT TRUNCATED AT 250 WORDS)
The innervation of the musculature of the tongue and the hyobranchial apparatus of caecilians has long been assumed to be simple and to exhibit little interspecific variation. A study of 14 genera representing all six families of caecilians demonstrates that general patterns of innervation by the trigeminal, facial, glossopharyngeal, and vagus nerves are similar across taxa but that the composition of the "hypoglossal" nerve is highly variable. Probably in all caecilians, spinal nerves 1 and 2 contribute to the hypoglossal. In addition, in certain taxa, an "occipital," the vagus, and/or spinal 3 appear to contribute fibers to the composition of the hypoglossal nerve. These patterns, the lengths of fusion of the contributing elements, and the branching patterns of the hypoglossal are assessed according to the currently accepted hypothesis of phylogenetic relationships of caecilians, and of amphibians. An hypothesis is proposed that limblessness and a simple tongue, with concomitant reduced complexity of innervation of muscles associated with limbs and the tongue, has released a constraint on pattern of innervation. As a consequence, a greater diversity and, in several taxa, greater complexity of neuroanatomical associations of nerve roots to form the hypoglossal are expressed.
The tail bud of amniote embryos comprises a mass of apparently undifferentiated mesenchymal cells located at the caudal limit of the embryo, representing the remains of Hensen's node and the primitive streak. These cells have the potential to give rise to a variety of different tissues including the posterior or 'secondary' neural tube, the tail gut, and somites and their derivatives. This seemingly homogeneous accumulation of cells therefore has the capacity to differentiate into tissues which in more cranial regions of the embryo are derived from cells of different germ layers. In this review, the tissue contributions of the tail bud in various vertebrate classes are discussed, with particular attention to the mesenchymal-to-epithelial transformation that characterizes the process of secondary neurulation, and which distinguishes it from the epithelial folding that occurs during primary neurulation in more cranial regions. Recent studies suggest that the transformation is accompanied by extensive changes in the cell surface oligosaccharide complement of the differentiating cells, and that the sialyted form of N-CAM is expressed both temporally and spatially in a manner that suggests a role for it in the process. The pluripotential nature of the tail bud mesenchyme may be revealed experimentally by grafting the tissue ectopically, or by culturing it on different substrata. In the latter case, the mesenchyme can be demonstrated to give rise to myocytes, chondrocytes, neuroepithelium and neural crest derivatives such as melanocytes, depending on the nature of the culture substratum. It is concluded that the tail bud mesenchyme represents a developing system which is readily amenable to experimentation and should provide insights into the general mechanisms of cell differentiation and transformation.