. The biology of the amphibia. Amphibians. DEVELOPMENT AND HEREDITY 31 Helff (1926) has shown that the atrophying gills produce a sub- stance which digests two neat holes in the confining cover (Fig. 15). Further, the metamorphosing tadpole owes the formation of its eardrums to the influence of the annular cartilages which come to underly the integument of the ear region. Foreign integument Fig. 16.—The effect of the annular tympanic cartilage on the formation of the tympanic membrane. Skin transplanted from the back to the tympanic region develops a tympanic membrane by the influence of the u
. The biology of the amphibia. Amphibians. DEVELOPMENT AND HEREDITY 31 Helff (1926) has shown that the atrophying gills produce a sub- stance which digests two neat holes in the confining cover (Fig. 15). Further, the metamorphosing tadpole owes the formation of its eardrums to the influence of the annular cartilages which come to underly the integument of the ear region. Foreign integument Fig. 16.—The effect of the annular tympanic cartilage on the formation of the tympanic membrane. Skin transplanted from the back to the tympanic region develops a tympanic membrane by the influence of the underlying tym- panic cartilage. Skin from the tympanic region transplanted to the back fails to develop a tympanum. , back-skin graft; , skin graft from tympanic membrane region transplanted to the back; , tympanic membrane. (After Helff, O. M., Studies on Amphibian Metamorphosis, Physiol. Zool., Vol. I, No. 4, adapted from Plate IV, Fig. 13.) transplanted over an annulus is molded (Fig. 16) into a tympanum (Helff, 1928). Basis of Homology.—The induction of structures by organizers of various grades is probably due to chemical substances, and these may have different positions in such related types as the frog and the newt. In Rana the roof of the archenteron persists as the dorsal lining of the alimentary tract, a median strip of topmost cells becomes the notochord, and the dorsal mesoderm splits off from the roof on either side of this. In the newt, the whole roof of the archenteron in the midline becomes converted into notochord, and the gut is completed dorsally by the ingrowth of yolk cells from the side. Undoubtedly homologous structures such as the gut of the frog and the newt may thus differ consider- ably in their manner of origin. The organ-inducing materials are most probably homologous but their center of activity has been shifted. Similar changes of position of organ-forming substances in the developing embryo may have been responsible for the diffe
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Keywords: ., bookcentury1900, bookpublishernewyorkmcgr, booksubjectamphibians
