A Contribution to the Embryology,life-history, and Classification of the Dicyemids

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Wilhelm Englemann, 1882 - 89 pages
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Page 70 - La Gastrula des Dicyémides est comparable à cette Gastrula épibolique des poissons osseux. Chez un Dicyémide l'endoderme reste constitué par une cellule unique, pendant toute la durée de la vie. Les Dicyémides sont donc construits sur le type de la Gastrula, et comme celle-ci apparaît dans le cours de l'évolution des Métazoaires avant...
Page 79 - The Infusorigen is a group of cells consisting, at one period fig. 109), of a peripheral layer of cells partially enveloping a large central cell. Its development from a single cell by a process of cleavage, and the epibolic growth of its peripheral layer, give ground for thinking that the Infusorigen represents an individual, comparable with the Gastrula of a vermiform embryo. The peripheral layer may be compared to the ectoderm, the central cell to the axial cell, and the «residual nuclens
Page 67 - Prof. E. VAN BENEDEN has proved that these parasites cannot be dismissed, sans fa con, as retrogressively metamorphosed 'worms'; and though I am not disposed to attach much weight to the absence of a mesoderm, on which VAN BENEDEN insists as a distinction between the Dicyemida and the Metazoa, the manner in which the- contents of the axial cell give rise to germs is so completely unlike anything which is known to obtain in the Metazoa, as, to my mind, to justify the separation of the Duyemida from...
Page 6 - S. officinalis , have each two species of Dicyema. 2. One species of Dicyema occurs in at least two different species of Cephalopod , while another is found in at least three different Cephalopods. 3. Two species, D. Eledones Wag. and D. gracile Wag., found in Cephalopods belonging to two different families (E. moschata and S.
Page 68 - ... Till the further development of the infusoriform embryo is known, it is not possible to arrive at a definite conclusion as to the affinities of this strange parasite. VAN BENEDEN is anxious to form it, on account of its simple organisation, into a group between the Protozoa and the Metazoa. It appears however very possible that the simplicity of its organization is the result of a parasitic existence ; a view which receives confirmation from the common occurrence of the process of endogenous...
Page 52 - ... stage 110, and probably still later. By the time stage 109 is reached, or soon after, the germogen becomes the seat of endogenous cell-formation. How the first cells arise within the germogen, I have not been able to ascertain ; but it is probable that they originate in the first instance by a division of the nucleus of the germogen, as is the case in the vermiform embryo. The successive generations might then arise in the same way, or, more probably, by division of the primary germ-cells. Although...
Page 56 - Quant aux embryons infusoriformes, j'en ai conservé parfaitement vivants dans un verre de montre pendant deux, trois, quatre et même cinq jours, sans qu'ils aient subi , après ce séjour dans l'eau de mer, la moindre altération. Comme je n'ai jamais trouvé dans les reins des Céphalopodes aucune forme de transition entre un embryon infusoriforme et un Dicyema , j'en conclus que les Dicyema ne se multiplient, dans les corps spongieux du Céphalopode infesté, que par les embryons vermilbrmes.
Page 77 - H6;, produce only vermiform embryos; the second produce first infusoriform embryos, then vermiform embryos (secondary Nematogens) (p. 41) . Rhombogen and secondary Nematogen denote two different phases in the life of the same individual. It is still doubtful whether the monogenic and diphygenic individuals are heterogeneous forms. They are alike in origin, development, and adult form and structure ; but their germ cells pursue different courses of development, either because, contrary to appearances....
Page 70 - L'organisation des Dicyema est beaucoup plus simple que celle de tous les Métazoaires connus: ils sont formés d'un fort petit nombre de cellules accolées entre elles et vivant ensemble pour former une individualité de second ordre. Ils ne possèdent aucun organe différencié ni aucune cavité interne. De ce chef, les Dicyémides sont inférieurs aux Métazoaires. Leur...
Page 80 - Gastrula is thus formed cousisting of a small number of ectodermic cells and a single ceutral endodermic cell (figs. 76 and 77). Periods of growth intervene between the successive cleavages, the individual cells attaining, each time, approximately the size of the original germ-cell. The blastopore closes, and the multiplication of cells at this pole soon leads to the pyriform embryo (figs. 80. 81, 87). The pointed end of this embryo is an elongation from the blastoporal region ; the broad end corresponds...

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