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Summary The larval tunic of Corella inflata is composed of two cuticular layers, extracellular filaments and ground substance. It lies outside the epidermis and most of it is known to be produced by the epidermis. The dorsal, ventral and caudal fins are specialized parts of the tunic that are essential for larval locomotion. The following hypothesis was tested: Morphogenesis of the larval fins is dependent upon the presence of extraembryonic structures (test cells, chorion or follicle cells) before completion of the late tail bud stage of development. We tested this by dechorionating embryos of Corella inflata and Ascidia paratropa. The operation removes all extraembryonic structures. It was performed mainly on neurula, early tail-bud and late tail-bud stages.Fin formation is inhibited when neurulae are dechorionated but not when late tail-bud or older embryonic stages are dechorionated. Dechorionated neurulae produce all of the major components of the tunic (cuticular layers, filaments and ground substance) but they are unable to form functional fins. At the time of dechorionation, in all experiments, the embryos had no fins.Removal of the follicle cells does not inhibit fin formation. The test cells are known to secrete granular ornaments that attach to the surface of the tunic. The fibrous, acellular chorion may serve to contain the test cells and their products or products of the embryo that are not firmly attached. The test cells may induce or control the morphogenesis of the larval fins in ascidians before the late tail-bud stage of development. We suggest ways of testing this hypothesis and an alternative hypothesis. 相似文献
2.
The presence of two sets of paired appendages is one of the defining features of jawed vertebrates. We are interested in identifying
genetic systems that could have been responsible for the origin of the first set of such appendages, for their subsequent
duplication at a different axial level, and/or for the generation of their distinct identities. It has been hypothesized that
four genes of the T-box gene family (Tbx2–Tbx5) played important roles in the course of vertebrate limb evolution. To test this idea, we characterized the orthologs of
tetrapod limb-expressed T-box genes from a teleost, Danio rerio. Here we report isolation of three of these genes, tbx2, tbx4, and tbx5. We found that their expression patterns are remarkably similar to those of their tetrapod counterparts. In particular, expression
of tbx5 and tbx4 is restricted to pectoral and pelvic fin buds, respectively, while tbx2 can be detected at the anterior and posterior margins of the outgrowing fin buds. This, in combination with conserved expression
patterns in other tissues, suggests that the last common ancestor of teleosts and tetrapods possessed all four of these limb-expressed
T-box genes (Tbx2–Tbx5), and that these genes had already acquired, and have subsequently maintained, their gene-specific functions. Furthermore,
this evidence provides molecular support for the notion that teleost pectoral and pelvic fins and tetrapod fore- and hindlimbs,
respectively, are homologous structures, as suggested by comparative morphological analyses.
Received: 14 July 1999 / Accepted: 4 September 1999 相似文献
3.
Peter L. Forey 《Environmental Biology of Fishes》1991,32(1-4):75-97
Synopsis
Latimeria is the product of a long coelacanth lineage, usually viewed as having changed very little. In this paper a classification of better known coelacanth genera is proposed based on a cladistic computer analysis of 56 morphological characters. Biometrical data are then matched with the classification to explore the possibility of identifying subtle change. It is concluded that throughout coelacanth history there have been changes in the structure of the vertebral column involving an overall increase in the number of vertebral elements, and a consequent crowding of these elements within the abdominal region. These changes may be associated with increasing lobation of the second dorsal and anal fins. In the skull, parameters involving the intracranial joint have also changed in such a way that the anterior part of the skull has lengthened in relation to the posterior part and this may be associated with an increase in length of the basicranial muscle.Abbreviations in text figures Ang
angular
- a.o.r
anterior opening of th rostral organ
- Art
articular
- ba.cr.m
basicranial muscle
- Basi
basisphenoid
- Boc
basioccipital
- bpt.pr
basipterygoid process
- c.p.l
cheek pit line
- De
dentary
- Esc
extrascapular
- eth.sp
etmosphenoid
- f.e
frontoethmoid
- Fr
frontal
- Fr.d
descending process of frontal
- intr. j
intracranial joint
- io.s
interorbital septum
- sc
jugal sensory canal
- L.e
lateral ethmoid
- m.Cor
modified coronoid
- Mm
memtomeckelian
- m.ot.sc
medial branch of otic canal
- Op
operculum
- o.p.l
oral pit line
- ot,occ
otico-occipital
- Pa
parietal
- Pa.d
descending process of parietal
- Par
parasphenoid
- pa.s
parietal shield
- p.Cor
principal coronoid
- Po
postorbital
- Pop
preoperculum
- p.o.r
posterior openings of the rostral organ
- Pmx
premaxilla, Pre-preorbital
- Pro
prootic
- Pro.p
posterior process of prootic
- Rart
retroarticular
- Sc.o
sclerotic ossicle
- So
supraorbital
- Soc
supraoccipital
- Sop
suboperculum
- Sp
spiracular
- spl
splenial
- Sq
squamosal
- Stt
supratemporal
- Stt.com
supratemporal commissure
- Stt.d
descending process of supratemporal
- Par.a.w
ascending wing of parasphenoid
- Te
tectal
- X
level of vagus exit 相似文献
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