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.

    121/374 (page 87)
    The anterior fold of the true amnion is certainly absent in the Eabbit, and this may prove to be the case for Mammals generally, now attention has been drawn to the question. At all events, the posterior fold of the amnion is always well developed. By this time the partially vascular yolk-sac ^has gradually diminished in size; and the vascular allantois is greatly increasing in size and importance, and is functionally replacing the yolk-sac. Allantois.—The Mammalian allantois has a similar origin to that in Birds (figs. 77, 79). It extends to a greater or less extent between the amnion and the serous membrane or subzonal membrane. The outer membrane of the highly vascular allantois fuses, as in Birds, with the subzonal membrane, the villi of which become vascular and usually grow more complex. The compound mem- brane thus formed is known as the chorion. That portion of the chorion which enters into immediate connection with the uterus of the mother constitutes the foetal portion of the placenta. As will be shown later (p. 259), the proximal portion of the stalk (urachus) of the allantois persists as the urinary bladder, and it is generally admitted that the urinary bladder (urocyst) of Amphibia is a homologous orsjan with that of the Amniota. It is thus a fair assumption to make that the allantois is merely the precociously developed urinary bladder. In the lower Vertebrates the egg is usually laid in water, and the larva is, as a rule, early hatched, respiration being effected by gills situated on the gill-arches. In Alytes and Notodelphis ovipara and some other Anura, large external gills are developed while the embryo is still within the egg-covering, their function apparently being to give increased facility for respiration to the unhatched young. A similar condition also occurs in some Elasmobranchs. Certain Anura, however, have such an abbreviated larval existence that the young are hatched as small Frogs, and in some of these the external gills atrophy early (Pipa americana), or are said to be entirely absent (Rhinoderma darwinii, Nototrema marsupiatum). In Pipa the long tail of the tadpole functions as a respiratory brgan [Peters], and the same holds good for Hylodes. Boulenger finds that the abdomen of the just-hatched Rana opisthodon is provided with a lateral series of symmetrical folds, which probably have a respiratory function. The above facts tend to show that some Frogs are losing their ancestral larval breathing organs, and are utilising other organs for respiratory purposes ; and it is very significant that this occurs amongst those Frogs which do not deposit their eggs in water. It is then not difficult to imagine that some primitive Amphibian which had acquired an increase of food-yolk (as a few recent Anura have done) would find in the urinary bladder an organ which could be pressed into the service of aerial respiration. If we may assume that some such Amphibian was the ancestor of the Amniota, we have a, clue to the significance of the total absence of even rudimentary gill-filaments on the gill arches of even the youngest embryos of the less specialised Amniota in