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 79 - Nematogenic period, any odd number from 3 — 17 inclusive. (Of. p. 39.) 10. 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...
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 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 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 50 - ... germ-cells takes place in one case within , in the other outside, the parent cell. The similarity between the germogen and the axial cell with respect to the formation of germ-cells called forth the following remark from VAN BENEDEN : »Ces germes se forment donc dans les cellules germigenes a peu pres comme les germes des embryons vermiformes dans le corps de la cellule endodermique.
Page 56 - J'ai dit plus haut que les Dicyema s'altèrent, se désagrègent et périssent dans l'eau de mer. Ceci est vrai non-seulement pour les adultes, mais aussi pour les embryons verm i for m es.
Page 52 - It is also certain that the peripheral cells seen in 108 and 109 continue to multiply by division as late as 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.
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...
Page 75 - ... of its structure is more or less the result of the luxurious conditions of life which it enjoys, even if its development furnishes no positive evidence of degeneration. As to the systematic position of the Dicyemids, I see no good reason for doubting the general opinion that they are Flathelminths degraded by parasitism.
Page 70 - 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...

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