. The elements of experimental embryology. Embryology, Experimental; Embryology. THE FUNCTIONAL PERIOD OF DEVELOPMENT 427 thyroid throwing its stored secretion into the blood.^ In this re- spect, the Urodele may be contrasted with the Anuran, where the thyroid becomes progressively more active during larval life, with- out any such extreme change in its activity. The morphogenetic effects of hormones are varied. Some of the most marked are those concerned with amphibian metamorphosis, in which the growth or differentiation of some organs and the. Fig. 207 Sharply delimited fields in a thyroid-
. The elements of experimental embryology. Embryology, Experimental; Embryology. THE FUNCTIONAL PERIOD OF DEVELOPMENT 427 thyroid throwing its stored secretion into the blood.^ In this re- spect, the Urodele may be contrasted with the Anuran, where the thyroid becomes progressively more active during larval life, with- out any such extreme change in its activity. The morphogenetic effects of hormones are varied. Some of the most marked are those concerned with amphibian metamorphosis, in which the growth or differentiation of some organs and the. Fig. 207 Sharply delimited fields in a thyroid-treated frog tadpole. Section showing on the left the epidermis of the fore-limb bud, on the right the lining of a branchial cleft. The former has reacted to the thyroid hormone by growth (mitoses, crowded nuclei); the latter by degenerative changes (vacuolation, shrunken nuclei). The limit (/.) between the two zones is clear-cut, without transition. (From Champy, Arch. Morph. Gen. Exp. iv, 1922.) atrophy of others will only take place under the influence of the thyroid hormone. All gradations are to be found, however, be- tween such marked morphogenetic effects and effects of a transitory physiological nature. The morphogenetic effect of hormones may be linked with the pre-existence of qualitatively different fields. in the frog, one region of epidermis will proliferate, and another degenerate, under the influence of thyroid- (fig. 207). 1 See Huxley, 1923; Uhlenhuth, 1922. ^ Champy, 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. ]
Size: 2222px × 1125px
Photo credit: © Paul Fearn / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookcollectionbiodiversi, booksubjectembryology
