. The biology of the amphibia. Amphibians. 28 THE BIOLOGY OF THE AMPHIBIA Amphibia, each evagination which becomes the optic cup of that side induces a lens to form in the overlying ectoderm; foreign ectoderm transplanted over the cup is similarly modified. Dur- ing gastrulation in the urodele, but apparently earlier in some species of frog (Brachet, 1927), but not in others (Schotte, 1930), the potencies for many structures are localized in the ectoderm. Areas which are to give rise to gills, balancer, nose, ear, hypophy- sis, etc., are segregated according to a pattern which seems con- troll
. The biology of the amphibia. Amphibians. 28 THE BIOLOGY OF THE AMPHIBIA Amphibia, each evagination which becomes the optic cup of that side induces a lens to form in the overlying ectoderm; foreign ectoderm transplanted over the cup is similarly modified. Dur- ing gastrulation in the urodele, but apparently earlier in some species of frog (Brachet, 1927), but not in others (Schotte, 1930), the potencies for many structures are localized in the ectoderm. Areas which are to give rise to gills, balancer, nose, ear, hypophy- sis, etc., are segregated according to a pattern which seems con- trolled in the first place by the direction of growth of the dorsal lip tissue. These ectodermal potencies greatly affect adjacent tissues. Thus Harrison (1925) has shown that if ectoderm from the region of the mandibular arch of Ambystoma maculatum embryos is transplanted to other parts of the body just before the appearance of the balancer, it will give rise to this structure in these regions (Fig. 14). The ectoderm takes the lead in balancer formation and seems to condense the underlying intercellular ground substance into a fibrillar mem- brane which gives support to the ectoderm. Harrison sug- gests that this modifying in- fluence may be a type of enzyme action. As develop- ment continues the balancer attracts a twig from the mandibular branch of the fifth nerve. If the balancer rudiment is transplanted to a posterior position it may attract a twig from a more posterior nerve or even from similar nerves in a frog tadpole. Epigenesis.—Many other striking cases of the effect of one tissue on the growth of another tissue have been demonstrated by experimental embryologists. The parts of a structure may effect one another during growth, while together they may mold adjacent tissues or be influenced by nutritional or hormonal conditions of the body. For example, the two common sala- manders Amby stoma maculatum and A. tigrinum grow at different rates and the latter reaches a much
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Keywords: ., bookcentury1900, bookpublishernewyorkmcgr, booksubjectamphibians
