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This paper reports experiments with Fundulus heteroclitus to modify larval development by means of ultraviolet radiation. Eggs were exposed to radiation at various intervals after fertilization for varving periods of time. The results accord with previous work on fish teratology, and the developmental types obtained are essentially similar to those produced by chemicals, cold, and hybridization. The deviations from normal development occur in the same body regions as do those in other vertebrates whose early development has been modified by the action of radiation (x-rays, radium, etc.).The results of these experiments indicate that there is a non-specificity i n susceptibility relations with the production of similar types of monsters for widely different reagents.That these eggs are differentially susceptible to the action of ultraviolet radiation is indicated by the fact that those regions which have the highest metabolic activity when a n inhibiting influence is active are the ones most generally affected. Thus modifications of the nervous system, sense organs, circulatory system, tail region, and body axis result, respectively, in the production of varying degrees of cydopia, inadequate circulation, short, stubby or bent, non-motile tails (some bifid), and anterior twinning.As these modifications may be produced by applying inhibiting influences during the first few minutes after fertilization, it is evident that there is in the egg a t this stage some constitutional or physiological basis which determines early differences in susceptibility of its various parts. * National Research Fellow. 939 JOURNAL OF MORPHOLOGY AND PHYSIOLOGY, VOL. 41, NO. 1 :;. Embryonic, rapidly (liffcreiitiatiiig tissues arc more sus-4.weak dosage may acc~elc~ratc~, wliilc~ a stiwiig tlosag:.cb tj. Tlic~ more iiiteiise the radiatioii aiitl the carlicr applic(1, c ep t ibl e t 11 a ii adult ti s s11 e s . tl el a y s, ce 11 act ivi t J-.t h e m o w marlmi the tlcfccts. ' 2 , 3, 4, 3, and 6 appear to hold also for ultraviolet radiation, :IS shown bp the work on fish development.

It has been known for some time that the skeletal muscle of frogs when sensitized by a suitable dye contracts when exposed to visible light of sufficient intensity, although a contracture with long latent period, rather than the characteristic twitch and tetanus of normal stimulation, has been the usual result. Lippay ('29) demonstrated that the effective radiation was in the region of the absorption spectrum of the dye used, and this rule holds also for other types of photodynamic phenomena (Blum, '32). Usually the presence of free oxygen has been found necessary for the production of photodynamic effects (Blum, '32) ; Lippay ( '30) and Spealman and Blum ('33) have found this to be true for the photostimulation of muscle. Some years ago Hinrichs and Lillie ('24) showed that the response of eosin-stained muscle to light was greatly increased when the tissue was immersed for a few minutes in pure isotonic solutions of sodium Salts with a deionizing influence on calcium (such as Natartrate and citrate) were found especially effective ; but the direct stimulating effect of the light was obscured by that of the salts which themselves cause vigorous twitching and tetanus, so that satisfactory quantitative relations could not be obtained. I n the present investigation a further experimental study has been made of the photosensitivity of stained This work was mpported in part by a graut from the Rockefeller Foundation. a

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