Identifier: journalofexperim31harr (find matches)
Title: The Journal of experimental zoology
Year: 1920 (1920s)
Authors: Harrison, Ross G. (Ross Granville), 1870-1959 Brooks, William Keith, 1848-1908 Wistar Institute of Anatomy and Biology American Society of Zoologists
Subjects: Zoology
Publisher: (New York, etc.) Wiley-Liss (etc.)
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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ary to know exactly the wholecell lineage of the germ tract, including the length of time thateach cell remained at rest in every cell generation (or thelength of time that it remained in those parts of the cell genera-tion during which mutation may occur), as well as the numberand distribution of the gametes which subsequently underwentfertilization and development into viable males. We may,however, gain a much clearer conception of how these featuresof the cell lineage influence the numbers in which mutants mayappear among a batch of offspring, if we consider a greatly sim-pUfied diagram, roughly representing the cell-lineage of a parentindividual (Pi female) and her progeny (Fi), during those por-tions of their Ufe-cycles in which the occurrence of a mutationwould lead to a character variation first visible in the offspringgeneration (Fi). The diagram (fig. 1) hence starts with whatmight be called the primordial germ cell of the parent; cell THE MANNER OF OCCUERENCE OF MUTATION 463
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Fig. 1 Diagram to show the effect of cell lineage on the distribution ofmutations. (For explanation see text.) 464 H. J. MULLER divisions of the parent prior to the separation of its somatic(or at least epidermal) tracts from the germinal tract have notbeen shown, inasmuch as mutations occurring then would oftenbe visible in the soma of the parent, and so could not count asmutations discovered in the offspring. The diagram is con-tinued to approximately the same point (primordial germ-cellformation) in the offspring, the first three cell cycles of the latterbeing shown, because a mutation occurring then might still bevisible either as a self or as a mosaic mutant in the individualof this generation. Each separate line in the diagram represents a particular cell,in its passage through its individual cell-cycle; a fork in the linerepresents a cell division, and each line is supposed to be takenas proportional, in its horizontal projection, to the length oftime during which that cell rem

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