H. S. Jennings

PARAMECIUM BURSARIA: LIFE HISTORY. I. IMMATURITY, MATURITY AND AGE

The life history of clones of Paramecium bursaria shows successive periods: (1) a period of sexual immaturity, during which sexual reactions and conjugation do not occur: (2) a transitional period during which weak sexual reactions occur in a few individuals: (3) a period of maturity, in which sexual reactions are strongly marked and the individuals conjugate readily: (4) a period of decline, ending in many (or all?) cases in death.The length of the period of immaturity and the time of attainment of maturity depend on the cultural conditions. If the animals are kept rapidly multiplying, under the best of nutritive conditions, maturity comes on early; if they are subjected to periods of starvation or other depressing conditions, maturity comes on much later or not at all.Ex-conjugant clones that are kept vigorously multiplying become mature in most cases at the age of three to five months, though cases have been observed of much earlier maturity, the earliest observed age of maturity being 12 days.Ex-conju...

The Numerical Relations in the Crossing over of the Genes, with a Critical Examination of the Theory That the Genes Are Arranged in a Linear Series.

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 The system of relations for which a theory is required to account.. . . . . . . . . . . . . . . . . . . . . 394 Basicpropositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397 The “linear” theory of crossing over. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 400 Obvious coincidences and discrepancies between the relations required by the linear theory, and those shown by the ratios. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401 Relations required by the linear theory.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403 The rules for crossing over when the occurrence of one break does not influence the occurrence of others (no interference). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403 The number of breaks to be expected between any two genes, in crossing over without interference. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411 Comparison of the condition resulting from serial arrangement of the genes, if there were no interference, with those found in nature.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417 The hypothesis of interference. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422 Crossing over with interference, in case the genes are arranged in a series.. . . . . . . . . . . . . . 423 Complete interference.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425 Partial interference. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426 The general relations in crossing over with interference.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 439 CONCLUSION A D DISCUSSION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451 SUMMARY.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453 LITERATURE CITED.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456 PAGE

Causes and Determiners in Radically Experimental Analysis

EVEN where the experimental situation is clear, disagreement often exists as to the causes or determiners of given phenomena. For clearing the mind on such matters, as well as for guiding experimentation, the writer has found useful two rules of thought, which are here submitted. The bald statements of the rules will be followed by a commentary with illustrations. Rule 1. Radically Experimental Thinking.-Test all questions or doubtful propositions as to causation, determination, explanation, by seeking mentally an experiment which, if carried out, would decide the matter. If no such experiment is conceivable, the question is one with which science can not deal. Or: Reduce all questions to an experimental situation. Rule 9. Causation of Differences.-In seeking causes or determiners, compare two cases and discover to what is due the difference between them. A cause or determiner is that which brings about the difference between two specifiable cases.

Recent Work on the Behavior of Higher Animals

groups of chemicals have been discovered which are efficient in the treatment of trypanosome infections. They are: (a) benzicliii dyes, (b) basic triphenyl-methane dyes and (c) arsenical compounds. In experimental animals complete cure has apparently been effected by maximum doses of these compounds. With lesser doses and prolonged treatment he parasites may disappear from the blood for a time, but later on make their appearance again. Those which recur have undergone a pronounced change in their biological characters and constitute a strain resistant to the therapeutic agent employed. Such a strain manifests chenioresistance of a specific character towards the particular substance used to develop it and an increased resistance towards other compounds of the same group. On the other hand, the development of resistance towards one group causes no increase whatever in the resistance towards other groups. By continued experiments, however, a strain has been produced manifesting a triple resistance, specific towards each of substances employed. Chemo-resistance, once acquired, persists unchanged while the resistant trypanosomes are passed through normal animals even for one hundred and forty transfers extending over fourteen months. This has been cited as strong evidence of the transmission of acquired characters. The specificity of the resistance is very striking. After an experimental animal has been inoculated with a mixture of two resistant strains and is then treated with a substance towards which one of the elements is resistant, the other element will disappear from the blood, but the resistant strain will remain and develop unchecked. Indeed, the two strains remain separate and capable of isolation after repeated passages through infected animals. Or, in other words, a strain with double resistance or with modified resistance does not arise as the result of infection with a mixture of two resistant strains.

The Beginnings of Social Behavior in Unicellular Organisms.

Scientific Events: Damage to Scientific Institutions in London; The Classification of Tool Steels; The New Hydraulic Laboratory of the National Research Council of Canada; The Proposed New Cancer Hospital in New York City; Symposium on Malaria; The Pacific Division of the American Association for the Advancement of Science; The Cold Spring Harbor Biological Laboratory; Medalists of the R oyalSociety ............................................................................................. 548