January 10, 1919] 



SCIENCE 



49 



the study of such hirge cells as the amphibian 

 chromatophores a much more satisfactory mi- 

 croscopic preparation may be obtained by 

 mounting pieces of skin entire. 



During the course of experimental studies 

 on the relation of certain of the endocrine 

 glands to pigmentation and growth changes 

 in the frog, striking objective proof has been 

 obtained that the " silvering " which follows 

 hypophysectomy is due mainly to contraction 

 of certain melanophores and not to any marked 

 reduction in the amount of pigment material 

 present. 



The experiments to be described wore per- 

 formed upon larvae of Rana sylvatica. This 

 frog is very darkly pigmented in young stages 

 and the " silvering " which follows hypophy- 

 sectomy is consequently a very striking color 

 change. The hypophysis was removed, follow- 

 ing the methods of Allen and of Smith, at a 

 stage just preceding the appearance of mus- 

 cular activity. The operated larvse were im- 

 mediately returned to large crystallizing dishes 

 containing city water (Ann Arbor, Mich.). 

 Although this water is very hard and may con- 

 tain traces of chlorine, there was not the high 

 mortality among operated tadpoles which Allen 

 noted. The characteristic silvering appeared 

 from the eighth to the tenth day following 

 the operation. As noted by the previous ob- 

 servers mentioned the operated larvse were 

 constantly smaller than controls of the same 

 age and were somewhat more active. 



When a length of from 12 to 14 mm had 

 been attained groups of the silvery larvse were 

 placed in a dilute extract of pars intermedia 

 of beef pituitary or in an emulsion made by 

 shaking a few mgm. of dried pars intermedia 

 in 100 c.c. of distilled water. For each ex- 

 periment a control group of larvae was placed 

 in an equal amount of distilled water. The 

 larvae placed in contact with the pars inter- 

 media substance soon underwent a striking 

 change from silvery to dark, in which con- 

 dition they closely resembled the normal tad- 

 pole. This change began to be apparent in 

 15—30 minutes and attained a maximum in 

 from one to three hours, depending on the 



strength of the extract or emulsion employed. 

 When returned to fresh water the darkened 

 larvae soon regained their silvery appearance 

 (one to three hours). 



This exj^eriment proves conclusively that the 

 silvery appearance of the hypophysectomized 

 larvae is not due primarily to a loss of pigment 

 substance. It indicates rather that it is due 

 to a sustained contraction of pigment-bear- 

 ing cells, which may be caused to expand again 

 by suitable stimuli. 



To test the validity of the latter assumption 

 the experiment was repeated with the larvae 

 held in Clarke's observation chamber so that 

 changes in the pigment cells could be watched 

 under the microscope. In silvery larvae the 

 sub-epidermal melanophores were found to be 

 contracted into dense spherical masses, whereas 

 in normal larvae the same cells were greatly 

 expanded so that processes of neighboring cells 

 were almost in contact. When a silvery larva 

 was subjected to the action of an extract of 

 pars intermedia the contracted pigment cells 

 were seen to expand slowly. In one experi- 

 ment a definite change in the cell under ob- 

 servation could be noted within eighteen min- 

 utes. The cell continued to expand slowly by 

 sending out pseudopodia-like processes into' 

 which pigment granules could be seen to 

 stream. 



To further elucidate the nature of the pig- 

 ment change toto moimts of pieces of skin 

 stripped from normal and exjserimental larvae 

 after fixation in Bouin's fluid, were prepared, 

 as follows: (a) normal, (&) silvery (hypophy- 

 sectomized), (c) darkened silvery (hypophy- 

 sectomized, under influence of pars intermedia 

 extract). 



In addition to the sub-epidermal melanophore 

 with branches so numerous that a typical 

 " mossy " appearance is presented, the skin of 

 the normal frog larva contains another type 

 of pigment cell, with relatively few branches, 

 situated in the epidermis. The latter cells are 

 said not to be contractile (Hooker^). 



Skin mounts prepared from silvery larvae 

 show that the sub-epidermal pigment cells are 



8 Hooker, D., Science, N. 8., Vol. 39, p. 473, 

 1914. 



