An introduction to the study of embryology / by Alfred C. Haddon.

  • Haddon, Alfred C. (Alfred Cort), 1855-1940.
Date:
1887
Catalogue details

Licence: Public Domain Mark

Credit: An introduction to the study of embryology / by Alfred C. Haddon. Source: Wellcome Collection.

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

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    to the blood by the hypoblast of the area opaca, and by it conveyed to all the regions of the body of the embryo. To anticipate, as the embryo is being formed, an anterior, and later a posterior, fold in the blastoderm make their appeareance, which mark the anterior and posterior extremities of the embryo ; they are known as the head and tail folds. The head-fold travels backwards and the tail-fold forwards in such a manner as to con- strict the embryo from off the yolk. Less marked lateral folds also appear. Eventually the embryo is quite constricted off the yolk, so that it is merely connected with the latter (or yolk-sac, as Fig. 33.—Surface View of the Pellucid Ai^ea of the Blas- toderm OF A Fowl of Twenty HouKS. Magnified 24 dia- meters. [From Kolliker after His.'] Ao. area opaca; Ap. area pel- lucida; Pr. primitive streak; vAf. head-fold. it is now termed) by a narrow stalk. The development of the embryonic structures known as the amnion and the allantois will be considered in another section (p. 78). The Primitive Streak.—The first noticeable sign of incubation in the blastoderm of the Amniota when viewed from above is the appearance of an opaque band which extends some distance for- wards from the posterior margin of the area pellucida. This is the primitive streak, and its opacity is due to the presence of a greater thickness of cells than occurs elsewhere. Shortly after the primi- tive streak is formed a shallow groove (the primitive groove)