. Analysis of development. Embryology; Embryology. Metamorphosis 637 entiation among different species of anurans as reported by these workers suggests that httle confidence can be placed in the cyto- logical correlations that have been adduced in specific instances. Unpublished studies by the writer of the growth and histology of the anterior pituitary during the develop- ment of the tadpole in R. pipiens likewise failed to reveal any change in growth rate or histological picture indicative of a marked change in activity related to metamorphosis. It may be concluded that, although the activat
. Analysis of development. Embryology; Embryology. Metamorphosis 637 entiation among different species of anurans as reported by these workers suggests that httle confidence can be placed in the cyto- logical correlations that have been adduced in specific instances. Unpublished studies by the writer of the growth and histology of the anterior pituitary during the develop- ment of the tadpole in R. pipiens likewise failed to reveal any change in growth rate or histological picture indicative of a marked change in activity related to metamorphosis. It may be concluded that, although the activation of the thyroid at the beginning of metamorphosis is clearly dependent upon a thyrotrophic hormone from the pituitary, it cannot be stated whether the initiation of thyroid activity results from an increase in thyrotrophic hormone production or from other factors. CONTROLLING FACTORS IN INSECT METAMORPHOSIS The metamorphosis of hemimetabolous in- sects is commonly described as taking place in stepwise fashion, a small step with each larval* molt. In holometabolous forms, on the other hand, transformation from larva to adult is said to take place in one stage, the pupal stage, at the end of larval develop- ment (Fig. 220). However, it must be real- ized that in all insects there is a process of remodelling taking place at all stages. In hemimetabolous insects noticeable progress toward the adult condition is made with each molt. Nevertheless the last molt, some- times called the metamorphic molt, usually presents a considerably greater amount of change than any of the earlier ones. In that respect this last molt is similar to the pupal stage of holometabola. Also, although the holometabolous animal does not seem to change toward the adult condition in any obvious way during larval molts, yet the imaginal discs may undergo considerable microscopic change. It is to be expected, therefore, that the basic mechanisms of meta- morphosis would be similar in both types of insects. Th
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Keywords: ., bookcentury1900, bookpublisherphiladelphi, booksubjectembryology
