. Analysis of development. Embryology; Embryology. Gut epithel. Nofochord. Fig. 78. Diagrammatic illustration of the morphogenetic movements (a) of normal gastrulation and (b) of exogastrulation. c, Main differentiations in an exogastrulated embryo. (After Holtfreter, '33a.) tem, sense organs or specific epidermal struc- tures. It merely develops into a mass of un- specialized epidermis cells. It follows that the entomesoderm contains determinative factors necessary for any typical ectodermal differentiation. The exogastrulated material, on the other hand, may adopt the shape of an embryo and
. Analysis of development. Embryology; Embryology. Gut epithel. Nofochord. Fig. 78. Diagrammatic illustration of the morphogenetic movements (a) of normal gastrulation and (b) of exogastrulation. c, Main differentiations in an exogastrulated embryo. (After Holtfreter, '33a.) tem, sense organs or specific epidermal struc- tures. It merely develops into a mass of un- specialized epidermis cells. It follows that the entomesoderm contains determinative factors necessary for any typical ectodermal differentiation. The exogastrulated material, on the other hand, may adopt the shape of an embryo and differentiate quite normally in the total ab- sence of ectodermal tissues (Fig. 78c). Thus the displaced marginal zone develops into an axial notochord associated with somites and pronephroi, and furthermore into coelomic cavities, head muscles, connective tissue, blood, some cartilage, heart and smooth in- testinal muscles. The latter two, though not innervated, may perform rhythmic contrac- tions. These mesodermal tissues are wrapped into entoderm which differentiates into the various kinds of glands and epithelia charac- teristic of the normal intestinal system. How- ever, the polarity of these epithelia is in- verted, their secreting surface being turned toward the external medium. series of gradations between complete exo- gastrulae and normally invaginated embryos. Figure 79 gives typical examples of such mal- formations in R. pipiens, which in this case, however, were obtained by a shock treatment with alkali. This series differs from corre- sponding ones in urodeles (Holtfreter, '33d) mainly by the fact that the latter rarely ex- hibit the various degrees of tail duplication and spina bifida which resvilt in anurans from a failure of the lateral primordia to join in the dorsal midline (in contrast to the urodeles, the prospective tail somites in the anuran gastrula are located more laterally). In all cases, however, the extent and type of ectodermal organization can be
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