Comparative anatomy / by C.Th. v. Siebold and H. Stannius ; translated from the German, and edited with notes and additions recording the recent progress of the science by Waldo I. Burnett.

  • Karl Theodor Ernst von Siebold
Date:
1854
Catalogue details

Licence: Public Domain Mark

Credit: Comparative anatomy / by C.Th. v. Siebold and H. Stannius ; translated from the German, and edited with notes and additions recording the recent progress of the science by Waldo I. Burnett. Source: Wellcome Collection.

Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.

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    their chorion contains only loosely-arranged vitelline cells, among which there is seen no trace of one or more germinative vesicles. The vitelline cells always contain, beside a finely-granular albuminous substance, a round nucleus which has a nucleolus. Both the nucleus and the granular sub- stance are shifted from one side to the other of the cell by the very re- markable peristaltic movements of the cell-membrane. After a time, these movements cease, the cell-membrane disappears, and the contents mix with those of other cells which have been affected in the same way: by these means, little collections of vitelline substance here and there are formed, which increase by the addition of other cells, — and finally are transformed into roundish, nicely-defined embryos which become covered with ciliated epithelium. From this time the embryos do not increase as before by the external fusion of cells, but there is a muscular, discoid oesophagus formed upon their periphery, and through this the remaining cells are ingested and assimilated within the animal. Still later, the embryo, hitherto spherical, becomes flat and elongated at two opposite points; — ultimately, and upon the appearance of the eye- specks, it assumes exactly the form of the adult Planariae. The size of the young Planariae depends upon the number of embryos developed in the same egg, for the smaller this number, the larger the embryos at the time of their hatching, and vice versa. The cause regulating the number of embryos in an egg is yet un- known.* (1> 1 See my details upon this subject in the Bericht. ueber die Verhandl. d. Berl. Akad. 1841, p. 83. During the development of Planaria, one can, after a while, ascertain the number of vitelline cells assimilated by fusion and deglutition, by counting their nuclei which are easily seen in the parenchy- ma of the body. According to Focke (loc. cit. p. 201), the eye-specks, and the oesophagus are de- veloped very early in the young Mesostomum Ehrenbergii;—a species with which each egg contains a single embryo only, and which is devel- oped while the egg is in the uterus. * [ End of § 129.] Recent embryological studies have thrown some light upon this point — the alleged plurality of embryos in a single egg. The so-called egg in these cases is almost undoubtedly an ovarian sac. iu which are developed many germs-, some of these germs may perish, and the fewness of those remaining would give the appearance of an egg with many germs. — Ed. f [ § 129, note l.J The development of Plana- ria has been also observed by Schmidt. Die Rhab- The remarkable movements of the vitelline cells in the eggs of the Planariae, and which I was the first to observe, have since been confirmed by Kdl- liker, with Planaria lactea; see Wiesmann's Arch. 1846, I. p. 291, Taf. X. I am unable to say whether or not the spontaneous movements observed by Quatrefages (loc. cit. p. 169, PI. VII. fig. 6-9) upon the larger portions of the vitellus of Polycelis pallidus while in the oviducts, are of the same na- ture this naturalist himself supposes that these portions were the embryos of this Planaria.f doc. Strudelwiirmer, &c., p. 1Y-, by Agassiz (Proc. Amer. Assoc. Advancem. of Sc. 2nd meeting, 1849, p. 438), who made the interesting observation that the Infusoria-genera, Kolpoda and Paramaeciumt are only larvse of Planaria; by Girard (Ibid. p. 398), and by Muller (Muller's Arch. 1850, p. 485). Muller has here some interesting remarks on the relations of the study of these forms to the class Infusoria. — Ed.