The elements of experimental embryology The elements of experimental embryology elementsofexperi00huxl Year: 1963 The endo-mesoderm on the other hand bears a considerable resemblance to an embryo, showing well-marked regions—a head and gill-region with gill-clefts, a trunk-region, and a tail-region. However, it is entirely abnormal in its detailed structure. It is morphologically inside-out; its outer layer is endodermal, and this contains a more or less solid mass of notochord, somites, mesenchyme and cartilage (figs. 213, 216). The endodermal epithelium is, as in the normal embryo, polarise
The elements of experimental embryology The elements of experimental embryology elementsofexperi00huxl Year: 1963 The endo-mesoderm on the other hand bears a considerable resemblance to an embryo, showing well-marked regions—a head and gill-region with gill-clefts, a trunk-region, and a tail-region. However, it is entirely abnormal in its detailed structure. It is morphologically inside-out; its outer layer is endodermal, and this contains a more or less solid mass of notochord, somites, mesenchyme and cartilage (figs. 213, 216). The endodermal epithelium is, as in the normal embryo, polarised: but its outer surface corresponds with that which bounds the gut-lumen in normal ontogeny. This may be compared with the fact noted on p. 250, that spheres composed of gastral layer only (collar- cells), arising in sponge dissociation experiments, have the collars directed outwards, whereas in normal animals they face the gastral cavity. In both sponges and amphibia, one surface of the epithelium orients itself towards the most favourable environment, whether this be an internal lumen or the external medium. Exogastrulation gives us a method by which ectoderm can be totally separated from endo-mesoderm from the first onset of the gastrulation- process. In addition, it provides pseudo- embryos, containing all the derivatives of the endo-mesoderm, in which we can be certain that no nervous tissue is present; and further, in the inversion of the endodermal and mesodermal layers, it provides a natural experiment in ab- normal spatial relations which it would be impossible to duplicate artificially. (See also p. 252 for a comparable case in insects.) As might be expected, conclusions of considerable importance have been arrived at by analysis of the results. In the first place, we 31-2 Fig. 215 Diagrams of transverse sec- tions through Urodele em- bryos, showing the structure and directions of movement of parts in a, normal embryo, h, exo-embryo. site of original vegeta
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