704 



Comparative Animal Physiology 



response involving stimulation through the eye, immediately on hatching. 

 Such species are Bonihinator and Hyla,^^ and Xenopus.^-'^ In the secondary 

 phase primary responses are still operating but are dominated by the second- 

 ary ones. 



Investigation of the physiology of color changes in amphibians has pro- 

 vided no evidence of any direct nervous control of the integumentary melan- 

 ophores. The striking discovery was made very early that hypophysectom- 

 ized tadpoles remain pale indefinitely."^ This strongly suggested that the 

 pituitary might be the normal source of a melanin-dispersing hormone in 

 amphibians. The relation of the hypophysis to color changes in Rana was 

 carefully investigated by Hogben and Winton.'^"- '^'^' '^^ Upon hypophysectomy 

 the animals were rendered both pale and refractory to further color changes. 

 Removal of the anterior lobe by itself, however, showed no significant inter- 

 ference with the background responses. Extracts of the posterior lobe showed 

 a tremendous capacity to darken pale frogs. All attempts to produce specific 

 chromatophore responses through nerve stimulation or nerve transection 

 failed. Therefore, these workers concluded that the color changes in Rana 

 could be readily accounted for in terms of the activity of a single hormone 



S 20 



10 



__q_<i_2-8— °-»-8"» 8 * ' 



10 12 14 16 18 20 22 

 Days -^ 



Fig. 269. Changes in melanophore state in Xenopus after transfer from white to black 

 backgrounds and vice versa, and from black background to darkness. From Hogben and 

 Slome.'" 



arising in the posterior lobe of the pituitary, whose concentration in the 

 blood was controlled by environmental stimuli operating through the eyes. 

 This conclusion has been confirmed through transfusion of blood from a 

 dark to a light Rawa.^"*'* 



More recently much work has been done on Xenopus,"^ ^'"^^^ in which di- 

 rect innervation of the melanophores seems also to be lacking. It, too, lightens 

 after hypophysectomy and darkens on injection of extracts of posterior lobe 

 of the pituitary. A critical and detailed examination of the characteristics 

 of change of the melanophores of Xenopus after transfer from a white to a 

 black background and the reverse, from black to darkness and reverse, and 

 from white to darkness and reverse (Fig. 269) fails to reveal an adequate 

 explanation in terms of a single principle and leads to the postulation that 

 the hypophysis produces two principles, one with melanin-dispersing action, 

 referred to as the B-substance (probably intermedin), and the other with 



