![]() ![]() Development in many of the approximately fifty orders of fishes remains unknown. Comparative studies also help us to identify model systems that are perhaps better suited than more common ones to answer difficult biological questions. Phylogeny and ontogeny support each other in this context. ![]() Common themes and significant differences in developmental patterns should appear through the study of as many vertebrates as possible, chosen from a list of diverse types. Differences ought to reflect either adaptations for various environmental conditions or nonadaptive (not maladaptive) traits that appeared in conjunction with genetic drift, quantum speciation, or punctuated equilibrium. A monophyletic ancestry for the vertebrates, with the possible exception of cyclostomes, implies that some common inherited mechanistic themes exist for development. Such comparisons set the stage for a broader understanding of the mechanisms for development in these organisms, and of the evolutionary relationships between them.Įmbryological literature includes a broad array of descriptions of development in one organism or another. ![]() Lepisosteus is unique among the Neopterygii with respect to this character state. The posterodorsal pit and dorsal lip are reminiscent of similar features in the Chondrostei. Meroblastic cleavage and the ysl in the garpike show an affinity to those character states in the teleosts, though not with Amia, the other neopterygian fish. These are meroblastic cleavage, a well-defined yolk syncytial layer (ysl), and a pit at the posterodorsal edge of the blastoderm, which defines an overhanging dorsal lip. Three features of development are especially noted that compare or contrast with other members of the Neopterygii, and with the Chondrostei. Diagnostic structural characteristics are cited for each stage, and the rate of development is indicated. The embryonic period of life of this fish is divided, as required for experimentation, into 34 stages, from fertilization to exhaustion of the yolk supply. The present study concerns the gars, of which the garpike, Lepisosteus osseus, is a representative example. Such literature dealing with the gars is to be found in older work, done approximately a century ago. Relatively recent descriptions of development exist for the teleost fishes, bowfin, sturgeon, paddlefish and bichirs. Gaps exist in the modern literature that describes patterns of development in living groups of actinopterygian fishes. ![]()
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