. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 706 Comparative Animal Physiology When the anterior lobe and the pars tuberaHs are extirpated, the melano- phores become maximally dispersed and show no background response. When the posterior lobe is removed, the animals are maximally pale. The source of the darkening hormone appears definitely to reside in the intermediate lobe.^' 1^- Removal of the whole gland leaves the pigment slightly dispersed and non-responsive. All these facts fit the hypothesis of the existence of two factors, with the pars tuberalis r
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 706 Comparative Animal Physiology When the anterior lobe and the pars tuberaHs are extirpated, the melano- phores become maximally dispersed and show no background response. When the posterior lobe is removed, the animals are maximally pale. The source of the darkening hormone appears definitely to reside in the intermediate lobe.^' 1^- Removal of the whole gland leaves the pigment slightly dispersed and non-responsive. All these facts fit the hypothesis of the existence of two factors, with the pars tuberalis responsible for the W-substance and the pos- terior or the intermediate lobe for the B-substance. Support for the presence of two factors also comes from the relative ef- fects of injection of B-substance into completely hypophysectomized Xeno- fus"'^ as compared with its effects in normal animals or animals with only the posterior lobes removed (Fig. 271). As would be expected, a larger dose Total removal A. Fig. 271. Responses of the melanophores of Xenopus on a white background to in- jections of equivalent doses of pituitary extracts into totally hypophysectomized specimens, specimens with only posterior lobe removed, and normal specimens. From Hogben and Slome.'^ is required in the last two types of recipients to bring about a given re- sponse, while in those with complete hypophysectomy a much smaller dose has an equivalent action. This latter observation finds a most logical explana- tion in terms of the resulting absence of an antagonist secreted by the pars tuberalis. Fishes. The fishes, which have probably been more intensively investi- gated than any other group, with respect to their chromatophore system and color changes, have several types of chromatophores. The most common and conspicuous type is the melanophore. Other common types contain yel- low pigment (xanthophores), red pigment (erythrophores), and white pig- ment (leucophores). In addition, many fishes
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