. The elements of experimental embryology. Embryology, Experimental; Embryology. 482 APPENDIX shown in figs. 213, 214, It is of interest to note that the tendency to constriction in the marginal zone (Ch. iii, p. 42) still manifests itself, leading to a waist between the ectoderm and the endo- mesoderm from the earliest stages of gastrulation. Later the waist becomes still further narrowed to a stalk, which can be easily severed, and may break of its own accord. All the mass-movements of the different regions involved in normal gastrulation (p. 43) are still operative in the exogastrulae, thou
. The elements of experimental embryology. Embryology, Experimental; Embryology. 482 APPENDIX shown in figs. 213, 214, It is of interest to note that the tendency to constriction in the marginal zone (Ch. iii, p. 42) still manifests itself, leading to a waist between the ectoderm and the endo- mesoderm from the earliest stages of gastrulation. Later the waist becomes still further narrowed to a stalk, which can be easily severed, and may break of its own accord. All the mass-movements of the different regions involved in normal gastrulation (p. 43) are still operative in the exogastrulae, though their mutual interactions in the altered circumstances are a little different, as indicated by the arrows in figs. 213, 215. For ^^^^. r' Fig. 214 Exogastrulated axolotl embryo, 8 days old. On the right, the epidermis; on the left, the exogastrulated endo-mesoderm: note pharynx with inverted gill- pouches (on left). (From Holtfreter, Biol. Zentralbl. liii, 1933.) instance, the organiser-region, forming the dorsal side of the marginal zone (p. 41), stretches out as a tongue on the dorsal side of the evaginated mass, and subsequently becomes sunk in a groove and finally overgrown by the endoderm. This confirms the view that the dynamics of gastrulation are predetermined in the various local regions of the germ. The final result of exogastrulation is what we may call an exo- embryo (Holtfreter's Exokeim). This consists of two very distinct parts. The ectoderm has flattened down to an irregular wrinkled mass with the blastocoel largely obliterated. It shows no medullary differentiations, notably no trace of neural tube or even of local thickening to form neural plate Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Huxley, Julian, 1887-1975; De Beer, Gavin, Sir, 1899-1972. New York [Hafner Pub. Co. ]
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