TROPISM (Gr. _trope_, turning), the automatic directing of an organism toward or away from a source of stimulus. UNIT-CHARACTER, a character which behaves as an indivisible unit in heredity. VASECTOMY (L. _vas_, vessel; _ektome_, cutting out), removal of a portion of the vas deferens (duct for conveying spermatozoa). VESTIGEAL (L. _vestigium_, footstep), representing organs which existed once in a more developed condition. VOLVOX (L. _volvo_, roll), a small fresh-water organism occurring in spherical colonies. WASSERMAN REACTION, a test for syphilis, see p. 188. X-ELEMENT, same as sex-chromosome. ZYGOTE (Gr. _zygon_, yolk), the product of the union of two gametes. REFERENCES FOR FURTHER READING AND STUDY BATESON, W., 1909. _Mendel's Principles of Heredity._ Cambridge, The University Press. The best technical account of Mendelism. It contains also a translation of the original papers of Mendel. CASTLE, WILLIAM E., 1911. _Heredity._ New York, D. Appleton and Company. CASTLE, WILLIAM E.; COULTER, JOHN M.; DAVENPORT, CHARLES B.; EAST, EDWARD M.; TOWER, WILLIAM L., 1912. _Heredity and Eugenics._ Chicago, The University of Chicago Press. CONKLIN, EDWIN GRANT, 1915. _Heredity and Environment in the Development of Men._ Princeton, Princeton University Press. DAVENPORT, CHARLES B., 1911. _Heredity in Relation to Eugenics._ New York, Henry Holt and Company. DONCASTER, L., 1911. _Heredity in the Light of Recent Research._ Cambridge, The University Press. DONCASTER, L., 1915. _The Determination of Sex._ Cambridge, The University Press. ELLIS, HAVELOCK, 1912. _The Task of Social Hygiene._ New York, Houghton Mifflin Company. GALTON, FRANCIS, 1869. _Hereditary Genius: an Inquiry Into Its Laws and Consequences._ London, Macmillan and Company. GALTON, FRANCIS, 1889. _Natural Inheritance._ New York, The Macmillan Company. GALTON, FRANCIS, 1895. _English Men of Science; Their Nature and Nurture._ New York, D. Appleton and Company. GALTON, FRANCIS, and SCHUSTER, EDGAR, 1906. _Noteworthy Families_ (Modern Science). London, J. Murray. GODDARD, HENRY HERBERT, 1912. _The Kallikak Family; a Study in the Heredity of Feeble-mindedness._ New York, The Macmillan Company. GODDARD, HENRY HERBERT, 1914. _Feeble-mindedness; Its Causes and Consequences._ New York, The Macmillan Company. HEALY, WILLIAM, 1915. _The Individual Delinquent._ Boston, Little, Brown and Company. KELLICOTT, WILLIAM E., 1911. _The Social Direction of Human Evolution; an Outline of the Science of Eugenics._ New York, D. Appleton and Company. MORGAN, THOMAS HUNT, 1913. _Heredity and Sex._ New York, Columbia University Press. PUNNETT, R. C., 1911. _Mendelism._ New York, The Macmillan Company. The best popular account of Mendelism. SALEEBY, CALEB WILLIAM, 1909. _Parenthood and Race Culture; an Outline of Eugenics._ London, Cassell and Company. SCHUSTER, EDGAR, 1912. _Eugenics._ London, Collins Clear-Type Press. THOMSON, J. ARTHUR, 1908. _Heredity._ London, John Murray.