A convenient, reliable method for chromosome delineation of animal cells grown as monolayers on glass has been applied to human, opossum, and Chinese hamster cells.

Tissue cultured cells from 5 different, normal organs of 7 different human subjects uniformly displayed the expected chromosome number of 46 and showed no variations in morphology or number other than the expected sex differences and a small incidence of polyploidy.

The chromosomes of normal cells from the American opossum were as uniform as those of human cells. Cells of the inbred Chinese hamster demonstrated appreciable karyotype variability, the cause of which is under investigation.

The chromosome number and morphology of cells from normal human tissues have remained constant after more than 5 months of continuous, rapid growth in tissue culture involving scores of vessel transfers and a number of generations equivalent to many billions of progeny.

By the use of routine recloning, even cells of malignant, aneuploid constitution have been maintained in active growth for 3 years and hundreds of generations, with stable chromosomal and metabolic characteristics.

The cells of the American opossum and Chinese hamster which possess only 22 chromosomes have been established in vitro and are especially suitable for genetic studies. The readily recognizeable Y and X chromosomes of the male opossum are particularly favorable as cytological markers.

Photomicrographs of the chromosomes of the various cells employed are presented.

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