. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. U4 THE. ALLANTOIS AND BELLY-STALK maternal blood, instead of with the external atmosphere, as is the case in the egg-laying forms. Thus, the endodermal portion of the allantois, instead of arising from the intestine and pushing before it a layer of splanchnic mesoderm to form a large sac lying freely in the extra-embryonic portion of the body-cavity, appears in the human embryo before the intestine has differentiated from the yolk-sac and pushes its way into the solid mass of mesoderm which f
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. U4 THE. ALLANTOIS AND BELLY-STALK maternal blood, instead of with the external atmosphere, as is the case in the egg-laying forms. Thus, the endodermal portion of the allantois, instead of arising from the intestine and pushing before it a layer of splanchnic mesoderm to form a large sac lying freely in the extra-embryonic portion of the body-cavity, appears in the human embryo before the intestine has differentiated from the yolk-sac and pushes its way into the solid mass of mesoderm which forms the belly-stalk (Fig. 65, A). To understand the significance of this proc- ess it is necessary to recall the abbreviation in the human embryo of the development of the extra-embryonic mesoderm and body-cavity. Instead of growing out from the embryonic area, as it does in the lower forms, this mesoderm develops in situ by splitting off from the layer of enveloping cells and, furthermore, the extra-embryonic body-cavity arises by a splitting of the mesoderm so formed before there is any trace of a splitting of the embryonic mesoderm (Fig. 38). The belly-stalk, whose development from a portion of the inner cell-mass has already been traced (p. 68), is to be regarded as a portion of the body of the embryo, since the ectoderm which covers one surface of it resembles exactly that of the embryonic disk and shows an ex- tension backward of the medullary groove upon its surface (Fig. 66). The mesoderm, therefore, of the belly-stalk is to be regarded as a portion of the embryonic mesoderm which has not yet undergone a splitting into somatic and splanchnic layers, and, indeed, it never does undergo such a splitting, so that there is no body-cavity into which the endodermal allantoic diverticulum can grow. But this does not account for all the peculiarities of the human allantois. In the birds, and indeed in the lower oviparous mammals, the endodermal portion of the allantois is equal
Size: 1733px × 1442px
Photo credit: © Central Historic Books / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookautho, bookcentury1900, bookdecade1910, booksubjectembryology
