47 In both the Walton and Marsh plankton samples, marked variation in ratios of the different rission and non-fission radioisotopes is evident and cannot be explained on the basis of present evidence. However, at least two factors may be contribut- ing to the variation and each of the two could mask the possible effect of the other. These factors include, (1) heterogeneity in composition of the plankton collected at the various stations, and (2) variation in available isotopes in given areas due to local fallouts from different types of nuclear devices, and from ATION ~] NKTON| differences in time of firing. Plankton is composed of many types of organisms including ‘ATE | 7 dinoflagellates, diatoms, protozoa, copepods, ostracods, euphau- KTON sids, amphipods, coelenterates, siphonophores, worms, pteropods, heteropods, primitive schordates, and the eggs and larvae of fish. The fraction of the total mass contributed by any one group can vary widely in different areas. It is known that species differ- ences in uptake of isotones do occur. Yoshil (1956) 2 observed that radioactive isotoves in copepods and primitive chordates } | piel collected in the spring of 1954 in the vicinity of the Marshall 1 AND _] Kawabata (1956)** stated “Although the mechanisms of the accumuletion of radioisotopes in planktons and their action in the organisms are still vague, it is, by all means, of importance aPERT eT Ce 2 ractions. Islands, differed both in beta energy and rate of decay, and that certain planktons selectively accumulate specific radio- active elements of minute amount in the sea water in their bodies."