. Analysis of development. Embryology; Embryology. Metamorphosis 635 lated between the absence of the pulmonary arch and neoteny. The results may possibly be explained by secondary effects which in- fluence tissue sensitivity. It may be concluded on the basis of the evidence summarized in this section that the thyroid through its hormone is the pri- mary determinant of metamorphic transfor- mation in amphibians. Except for the hypophysis, which acts through the thy- roid as discussed below, no other organ is known to exert significant control over the process. The evidence implies, though it d
. Analysis of development. Embryology; Embryology. Metamorphosis 635 lated between the absence of the pulmonary arch and neoteny. The results may possibly be explained by secondary effects which in- fluence tissue sensitivity. It may be concluded on the basis of the evidence summarized in this section that the thyroid through its hormone is the pri- mary determinant of metamorphic transfor- mation in amphibians. Except for the hypophysis, which acts through the thy- roid as discussed below, no other organ is known to exert significant control over the process. The evidence implies, though it does not definitely prove, that the thyroid hormone acts directly without the media- tion of other organs in the initiation of metamorphosis. ROLE OF HYPOPHYSIS Hypophysectomy in amphibians prevents metamorphosis. This was first found by Adler ('14), who hypophysectomized tadpoles by cautery. He secured a few survivors of this drastic operation and these showed, among other effects, a failure of metamorphosis. However, the relation of the hypophysis to metamorphosis was not satisfactorily clari- fied until Allen ('16) and Smith ('16) inde- pendently developed the techniqvie of re- moving the primordium of the pituitary from the frog embryo (Fig. 219). These investigators found that such hypophysecto- mized tadpoles remain in the larval condition indefinitely. Their rate of growth is retarded but this does not account for the absence of metamorphosis, since the animals may even- tually attain or even exceed full size. The development of the hypophysectomized tad- pole parallels that of thyroidectomized ani- mals. The hind legs in the two types, for example, develop to the same degree (Allen, '17). These findings have been extensively confirmed (see, for example, Blount, '35; Etkin, '38). The explanation of the manner of action of the anterior pituitary was soon made apparent by studies of Smith (see summary in Smith, '20) and Allen ('32a). They showed that, in the absence of the
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