The elements of embryology. / By M. Foster ... and Francis M. Balfour.

  • Michael Foster
Date:
1874
Catalogue details

Licence: Public Domain Mark

Credit: The elements of embryology. / By M. Foster ... and Francis M. Balfour. Source: Wellcome Collection.

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

    182/336 (page 156)
    over the neural canal. Fresh lines of segmentation then appear in the intervertebral portions, which run in such a way that each ganglion is now more closely associated with the vertebral portion in front of it than with that behind it, though the latter sprang in part from the same original protovertebra as itself 13. Meanwhile from the fourth to the sixth day im- portant changes take place in the notochord itself On its first appearance the notochord was, as we have seen, composed of somewhat radiately arranged but otherwise perfectly typical mesoblast-cells. On the third day some of the central cells become vacuo- lated, while the peripheral cells are still normal. The vacuo- lated cells exhibit aroimd the vacuole a peripheral layer of granular protoplasm in which the nucleus lies embedded, whilst the vacuoles themselves are filled with a perfectly clear and transparent material, which in an unaltered con- dition is probably fluid. Towards the end of the day the notochord acquires a delicate structureless sheath which is no doubt a product of its peripheral cells. According to His there is a cavity in tliei centre of the notochord on the third day. We have never observed this, and it is denied by Miiller {Ucbtr dtn Bail, der Chorda Borsalis. Jenaische Zeitschrift. Bd. ¥i. 1871). On the fourth day all the cells become vacuolated with the exception of a single layer of flattened cells at the peri- phery ; and the vacuoles themselves become larger. At the point where the nucleus lies there is generally rather more ]3rotoplasm than round the remainder of the circumference of the cells. On the sixth day all the cells are vacuolated. In each cell the vacuoles have so much increased at the expense of the protoplasm that only a very thin layer of the latter is left at the circumference of the cell, at one part of which, where there is generally more protoplasm than elsewhere, the starved remains of a nucleus may generally be detected. Miiller {loc. cit.) considers that the cells have a membrane. This however is probably merely a hardened external layer of the protoplasm j and is stained by reagents. Dursy {Zur EntwicJdungsgeschichte des Kopfes des Menschen und der hdheren Wirbelthiere) believes that what we have spoken of as vacuoles in the cells are really intercellular spaces. So that according to his view the notochord is composed of stellate cells with large round intercellular spaces filled with transparent intercellular matter. Superficially viewed a section of the noto-