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

  • Michael Foster
Date:
1874
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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.

    153/336 (page 127)
    i ^'■] THE MESENTERY. 127 actual termination of the tail, and certain features connected with the development of the allantois, of which we shall ^peak presently, the tail is a counterpart of the head. So rapid is the closure of the splanchnopleure both in ront and behind, that two of the three parts into which Ue digestive tract may be divided, are brought, on this day o the condition of complete tubes. The first division, extending from the mouth to the (luodenuna, is completely folded in by the end of the day; so . likewise is the third division comprising the large intestine ;and the cloaca. The middle division, corresponding to the : future small intestine, still remains quite open to the yolk- i sac below. The attachment of the newly formed alimentary canal to the body above is at first very broad, and only a thin stratum of mesoblast separates the hypoblast of the canal from the notochord and protovertebrjB ; even that may be absent under the notochord (Fig. 41). During the third day, however, along such portions of the canal as have become regularly -enclosed, i.e. the hinder division and in the posterior moiety (of the anterior division, the mesoblastic attachment becomes 1 narrower and (in a vertical direction) longer, the canal appear- ing to be drawn more downwards (or according to the position <of the embryo forwards), away from the vertebral column. In what may be regarded as the pleural division of the general pleuroperitoneal space, along that part of the ali- mentary canal which will form the oesophagus, this with- drawal IS very slight (compare Fig. 32), but it is very marked in the peritoneal space. Here such parts of the digestive canal as are formed come to be suspended from the body above by a narrow flattened band of mesoblastic tissue which ireaches from the neighbourhood of the notochord, and be- ' comes continuous with the mesoblastic coating which wraps iround the hypoblast of the canal. This flattened band is the ^mesentery, shewn commencing in Fig. 44, and much more -advanced in Fig. 47, M. It is covered on either side by a ; layer of flat cells, while its interior is composed of indifferent t tissue. The front division of the digestive tract consists of three iparts. The most anterior part, the cesophagus, still ends Iblindly in front, and reaches back as far as the level of the