Undersøkelser over Pleuronectider - I. Embryonalutviklingen hos rødspette (Pleuronectes platessa), skrubbeflyndre (Pleuronectes flesus) og deres resiproke bastarder. Kjernestørrelsesberegninger på embryonene og vekst hos larvene

The paper starts by describing the normal embryonic development of the plaice (Pleuronectes platessa) and the flounder (Pleuronectes flesus) up to the formation of the embryo. The development is divided into fourteen stages, a schedule of which is given. The embryonic development of the two species is exactly similar, the only difference being the comparative size of their eggs, blastodiscs and cells. The size proportion between the two species is however not constant, but in the course of the development changes in favour of the flounder. The size proportion between the blastodiscs of the plaice and the flounder which at first is approximately 2:l will thus change to about 3:2 later in the development. The rate of development of the flounder is greater than that of the plaice. The flounder-embryo with optic vescles is formed after about 98 hours, the plaice embryo after about 122 hours. This lead increases during the later stages of the development, since the hatching of the flounder eggs occurs 6-8 days earlier than that of the plaice eggs. The organological and histological development of both the hybrid forms does not differ from that of the parents. But the size of the blastodisc and the cells and the rate of development until the formation of the embryo are in complete accordance with the mother-form. Later the influence of the father on the rate of development becomes noticeable, the hybrid plaice ♀ x flounder ♂ will now develop faster than the mother but slower than the father, and the hybrid flounder ♀ x plaice ♂ slower than the mother but faster than the father. Under the temperature -conditions prevailing during this experiment both the hybrid forms were hatched after a period of 12 days, the flounder after 9 days and the plaice after 15-16 days. Neasurements of nuclei from plaice, flounder and their reciprocal hybrids gave the following results: The nuclear size decreases during the development in all four forms. In the first part of the development until stage 8 the author found especially in the plaice and the hybrid plaice ♀ flounder ♂ great variations in the nuclear size of different individuals from the same stage. These individual variations suggest complications due to a haploid or polyploid development. But probably all of these abnormal eggs die before gastrulation, only normal ones developing further. In almost all stages the plaice has a greater nuclear size than the flounder, a fact which according to studies of equatorial plates, is a consequence not of a greater number, but rather of a greater size of the plaice chromosomes. One should expect the nuclear size of the hybrids to be intermediate between those of the parents, but the curves do not give a clear confirmation of this probably on account of an insufficient number of measurements. But the curves do show that in the latest stages the nuclear size of the hybrids seems to approach that of the flounder. This may indicate that some of the father chromosomes of the hybrids are ejected. Since the cellsize of the hybrids corresponds with that of the mother, but the nuclear size differs from the nuclear size of both the mother and the father, the nucleus-plasma-relation of the hybrids and their parents is different. Until the age of 49-56 days after the fecundation the growth of the fry shows great individual variations. The author did not succeed in breeding the flounder and the hybrid flounder ♀ x plaice ♂, therefore the following growth-dates only concern the plaice and the hybrid plaice ♀ x flounder ♂. The mean length of 355 plaices was 16.58 mm ± 0.6, whereas 114 and 142 hybrids showed a mean length of respectively 17.88 mm ± 0.9 and 16.08 mm ± 0.7. The results indicate that the growth rate of the hybrid plaice ♀ x flounder ♂ does not differ from that of the plaice in the first months of their lives

Analysis of coelom development in the sea urchin Holopneustes purpurescens yielding a deuterostome body plan

An analysis of early coelom development in the echinoid Holopneustes purpurescens yields a deuterostome body plan that explains the disparity between the pentameral plan of echinoderms and the bilateral plans of chordates and hemichordates, the three major phyla of the monophyletic deuterostomes. The analysis shows an early separation into a medial hydrocoele and lateral coelomic mesoderm with an enteric channel between them before the hydrocoele forms the pentameral plan of five primary podia. The deuterostome body plan thus has a single axial or medial coelom and a pair of lateral coeloms, all surrounding an enteric channel, the gut channel. Applied to the phyla, the medial coelom is the hydrocoele in echinoderms, the notochord in chordates and the proboscis coelom in hemichordates: the lateral coeloms are the coelomic mesoderm in echinoderms, the paraxial mesoderm in chordates and the lateral coeloms in hemichordates. The plan fits frog and chick development and the echinoderm fossil record, and predicts genes involved in coelomogenesis as the source of deuterostome macroevolution

A hexamer origin of the echinoderms' five rays

Of the major deuterostome groups, the echinoderms with their multiple forms and complex development are arguably the most mysterious. Although larval echinoderms are bilaterally symmetric, the adult body seems to abandon the larval body plan and to develop independently a new structure with different symmetries. The prevalent pentamer structure, the asymmetry of Loven's rule and the variable location of the periproct and madrepore present enormous difficulties in homologizing structures across the major clades, despite the excellent fossil record. This irregularity in body forms seems to place echinoderms outside the other deuterostomes. Here I propose that the predominant five-ray structure is derived from a hexamer structure that is grounded directly in the structure of the bilaterally symmetric larva. This hypothesis implies that the adult echinoderm body can be derived directly from the larval bilateral symmetry and thus firmly ranks even the adult echinoderms among the bilaterians. In order to test the hypothesis rigorously, a model is developed in which one ray is missing between rays IV-V (Loven's schema) or rays C-D (Carpenter's schema). The model is used to make predictions, which are tested and verified for the process of metamorphosis and for the morphology of recent and fossil forms. The theory provides fundamental insight into the M-plane and the Ubisch', Loven's and Carpenter's planes and generalizes them for all echinoderms. The theory also makes robust predictions about the evolution of the pentamer structure and its developmental basis. *** including corrections (see footnotes) ***Comment: 10 pages, 6 figure