Researches upon nemerteans and planarians / by Charles Girard. I., Embryonic development of Planocera elliptica.

  • Charles Frédéric Girard
Date:
1854
Catalogue details

Licence: Public Domain Mark

Credit: Researches upon nemerteans and planarians / by Charles Girard. I., Embryonic development of Planocera elliptica. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    what in size, and developed into epithelian cells, according to a general physiological law. If, perchance, they congregate immediately on being detached from the yolk and before they become epithelian, then, still in possession of their own cellular life, they, for a time, will constitute a little spheroid, which will acquire vibrillae, in accordance with the organic law of development of a large portion of the animal kingdom. These little bodies are the so-called Cosmella arachnoides, described by Alex, Nordmann as parasitic animals in the ovum of Tergipes edwardsii, a nu- dibranchiate mollusc.* When this spheroid is of a certain size, it sometimes happens that a small embryo issues from it, with a shape altogether similar to the embryos formed by the entire mass of the yolk. It is my intention to dwell more at length upon these loose cells in another Paper, now in preparation, and in which their physiological signification will be thoroughly examined. X. THE EMBRYO. § 1. Between the period of the division of the yolk and that of the first manifesta- tion of the embryo, there is a period of apparent repose, which lasts from four to five days. The vitelline sphere seems to undergo no process of any kind ; the labor is undoubtedly beyond the reach of our investigations, for, that something is going on in it, cannot be questioned for a moment: life once started, keeps acting until it ceases definitely its actions. During this period the vitelline mass, from opaque (figs. 46 and 47), acquires two transparent diameters (fig. 48), which are transformed into four transparent internal cavities (fig. 49), sometimes a single, but larger central cavi- ty is observed (fig. 50), which is less distinct in others (fig. 52 and 53), probably in the latter the embryonic substance has not yet acquired all its transparency: a phe- nomenon at all events taking place during this period of rest and preceding that of motion. ^ 2. For, now' the embryo begins to'move, the most wonderful sight which an ob- server may behold. When numbers of them are witnessed at once, some may be seen revolving around an ideal axis forty turns a minute, whilst in others the movement is scarcely perceptible. And between these two extremes of motion, there are all in- termediate degrees of velocity. According to the will of the animal, the velocity may be increased or diminished, so that the same individual moves alternatively with dif- ferent degrees of velocity. When passing from one degree into another, this does not take place according to the mechanical law of uniform acceleration, but the twm Essai d’une monographie du Tergipes edwardsii.—Ann. Sc. Nat. 3d Ser. V. 1846, 109, PI. I.
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