. Analysis of development. Embryology; Embryology. Metamorphosis 651 periods even the lowest concentrations might be seen to lead to late metamorphic events. This was clearly recognized by Allen, who stated, "this 'threshold of response' is really a question of time of ; By following the changes in individual animals continuously for long periods, Etkin found that any effective concentration of thy- malities in coordination observed by all earlier workers who gave massive thyroid doses. No single concentration of thyroxine was found effective in inducing a normal "time-
. Analysis of development. Embryology; Embryology. Metamorphosis 651 periods even the lowest concentrations might be seen to lead to late metamorphic events. This was clearly recognized by Allen, who stated, "this 'threshold of response' is really a question of time of ; By following the changes in individual animals continuously for long periods, Etkin found that any effective concentration of thy- malities in coordination observed by all earlier workers who gave massive thyroid doses. No single concentration of thyroxine was found effective in inducing a normal "time- table" of metamorphic events. Etkin was partly, though not entirely, successful in in- ducing a normal timetable in thyroidec-. Fig. 227. Abnormal tadpoles metamorphosing under stimulation from strong doses of thyroid (after Schreiber, '34a). roxine was capable of inducing both early and late metamorphic changes if allowed to operate for a long enough period. The speed of metamorphosis increased with the concen- tration of thyroxine. In low concentrations the first effect observed was an activation of leg growth, and only after the lapse of con- siderable time, during which the legs grew to full size, were late metamorphic events initiated. These later events proceeded in an extremely slow and protracted manner. With low concentrations, therefore, early meta- morphosis is normal in its time relations but the late events of the metamorphic cli- max are unduly protracted. In high concen- trations the events of the metamorphic cli- max were precipitated at their normal rates but not enough time was permitted for early events to undergo their appropriate develop- ment. This is the explanation of the abnor- tomized tadpoles by treating them first with low concentrations of thyroxine and subse- quently raising the concentration (Fig. 228). It would appear from the above results that the responses of the tissues show the ordinary stoichiometric relations common to many chemical
Size: 1764px × 1416px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookpublisherphiladelphi, booksubjectembryology
