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.

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    1 model, wliich he easily can do by spreading a cloth out flat tto represent the blastoderm, placing one hand underneath it, tto mark the axis of the embryo, and then tucking in the tcloth from above under the tips of his fingers. The fingers, c.covered with the cloth and slightly projecting from the level 1 of the rest of the cloth, will represent the head, in front of \ which will be the semicircular or horse-shoe-shaped groove • of the head-fold. At its first appearance the whole 8 may be spoken of as tthe head-fold, but later on it will be found convenient to I restrict the name chiefly to the lower limb of the B. Some time after the appearance of the head-fold, an aaltogether similar but less conspicuous fold makes its ap- jpearance, at a point which will become the posterior end of tthe embryo. This fold, which travels forwards just as the ] head-fold travels backwards, is the tail-fold (Fig. 8, G). In addition, between the head- and the tail-fold two lateral f folds appeal, one on either side. These are simpler in cha- rracter than either head-fold or tail-fold, inasmuch as they jare nearlv straight folds directed inwards towards the axis of :the body (Fig. 8, F), and not complicated by being crescentic iin form. Otherwise they are exactly similar. As these several folds become more and more developed, tthe head-fold travelling backwards, the tail-fold forwards, i^and the lateral folds inwards, they tend to unite in the : middle point; and thus give rise more and more distinctly to the appearance of a small tubular sac seated upon, and • connected, by a continually-narrowing hollow stalk, with that larger sac which is formed by the extension of the rest of the •blastoderm over the whole yolk. The smaller sac we may call the embryonic sac, the ■larger one the yolk-sac. As incubation proceeds the smaller -sac (Fig. 8), gets larger and larger at the expense of the yolk- ■sac (the contents of the latter being gradually assimilated by imitritive processes into the tissues forming the growing walls 'of the former, not directly transferred from one cavity into ► the other). Within a day or two of the hatching of the chick, at a time when the yolk-sac is still of some consider- able size, or at least has not yet dwindled away altogether, and the development of the embryonic sac is nearly com- plete, the yolk-sac (Fig. 8, N) is slipped into the liody of 3—2