[' ', 'AGE', 'OF', 'MICE', 'AND', 'SKIN', 'CANCER', 'INCIDENCE', '851', 'are', '1].—Times', 'of', 'infiltrating', 'Carcinoma', 'with', 'Numbers', 'at', 'Risk', 'Young', 'gro', 'ips', 'Old', 'groups', 'Number', 'of', 'infiltrating', '9', '-———', 'pa', ' ', 'Weeks', 'from', '=', 'carcinomas', 'occurring', 'Anumals', 'Animal,', 'first', 'treatment', 'in', 'young', 'groups', 'Age', 'alive', 'Age', 'alive', '9', '@', '-', '98', '288', '»', '68', '283', '20', '¢', '2', '273', '68', '204', '40', 'G', '49', '238', '103', '57', '65', '1', '64', '198', '123', 'il', '57', 'I', '66', '190', '126', '4', '60', 'I', '69', '183', '128', '62', '2', '71', '174', '130', '4', '63', '1', 'rp', '168', '131', '3', '64', '1', '73', '158', '132', '3', '65', '1', '74', '156', '133', '3', 'iW', ']', '80', 'ao', '139', 'i', '13', '1', '82', '116', '-', '_', '4', '2', '83', 'bla', '_', '_—', '15', '1', 'a4', '107', '-', '_', '13', '2', '86', '102', '-', '-', '80', '1', '89', '84', '_', '-', 'DISCUSSION', 'Thete', 'are', 'several', 'defects', 'in', 'this', 'experiment.', 'It', 'would', 'have', 'been', 'better', 'had', 'the', 'animals', 'been', 'allocated', 'to', 'the', 'old', 'and', 'young', 'groups', 'by', 'randumisation,', 'since', 'the', 'main', 'comparison', 'of', 'interest.', 'is', 'between', 'old', 'and', 'young', 'painting.', 'It', 'would', 'have', 'been', 'better', 'if', 'the', 'young', 'groups', 'had', 'not:', 'been', 'killed', 'at', 'week', '80', '(whon', 'the', 'main', 'tumour', 'crop', 'was', 'just', 'getting', 'under', 'way),', 'but', 'this', 'was', 'necessitated', 'by', 'an', 'epidemic', 'infestation.', 'It', 'would', 'have', 'been', 'better', 'if', 'all', 'the', 'animals', 'had', 'been', 'painted', 'with', 'the', 'same', 'substance', 'according', 'to', 'one', 'unique', 'regime,', 'since', 'this', 'would', 'have', 'produced', 'homo-', 'geneity', 'of', 'the', 'main', 'groups', 'to', 'be', 'corapared.', 'Despite', 'these', 'defects', 'the', 'results', 'obtained', 'were', 'so', 'striking', 'and', 'so', 'contrary', 'to', 'one', 'uf', 'the', 'hypotheses', 'that', 'the', 'data', 'are', 'nevertheless', 'worth', 'reporting.', 'Since', 'the', 'animals', 'in', 'the', 'old-painted', 'group', 'were', 'at', 'risk', 'during', 'old', 'age', '(when', 'the', 'majority', 'of', 'tumours', 'tend', 'to', 'occur)', 'they', 'should.', 'under', 'the', 'first', 'hypothesis,', 'have', 'produced', 'about', 'four', 'times', 'as', 'many', 'cancers', 'as', 'the', 'young', 'painted', 'groups', 'whereas', 'in', 'fact', 'the', 'old-painted', 'groups', 'produced', 'no', 'cancers', 'at', 'all', 'while', 'the', 'young-painted', 'groups', 'produced', '14.', 'This', 'could,', 'however,', 'still', 'be', 'explained', 'by', 'extending', 'the', 'first', 'hypothesis', 'to', 'posit', 'a', 'constant', 'latent', 'period', 'of', 'a', 'year', 'or', 'more', 'between', 'the', 'induction', 'and', 'the', 'produc-', 'tion', 'of', 'a', 'carcinoma.', 'If', 'thin', 'were', 'the', 'case', 'then', 'the', '16', 'tumours', 'observed', 'between', 'the', 'ages', 'of', '64', 'and', '8?', 'weeks', 'in', 'the', 'young-painted', 'groups', 'were', 'induced', 'before', 'week', '37', 'of', 'their', 'lives', 'and', 'have', 'laid', 'dormant', 'for', 'a', 'year', 'or', 'more,', 'and', 'altheugh', 'many', 'tumours', 'were', 'induced', 'in', 'the', 'old-painted', 'groups', 'these', 'were', 'always', 'prevented', '(by', 'the', 'prior', 'death', 'of', 'the', 'host)', 'rrom', 'being', 'detected.', 'However', 'comparison', 'with', 'previous', 'experimental', 'results', 'of', 'a', 'similor', '‘ype', 'shows', 'thit', 'this', 'cannot', 'in', 'fact', 'be', 'so;', 'the', 'latent', 'period.', 'if', 'constant,', 'cannot', 'exceed', '40', 'weeks', 'sinve', 'in', 'an', 'experiment', 'on', '6000', 'animals', 'under', 'similar', 'conditions', '(Dav.', '1967)', 'the', 'main', 'crop', 'of', 'carcinonias', 'etarted', 'before', 'age', '49', 'weeks.', 'In', "Day's", 'experi-', 'ment', 'j{', 'was', 'found', 'that', 'the', 'incidence', 'rate', 'increased', 'approximately', 'as', '(¢', '—', '19)*-5', 'where', '¢is', 'the', 'age', 'in', 'weeks.', 'Assuming', 'the', 'firat', 'hypothesis', 'to', 'be', 'true', 'and', 'assuining']