Abstract
Heterochromosomes have now been reported for the male germ cells of the following mammals: man and rat (Guyer, '10); armadillo (Newman and Patterson, '10); opossum (Jordan, '11); guinea-pig (Stevens, '11); and bat (Jordan, '12). Winiwarter and Sainmont, '09, report a longitudinally split “monosome” in the oöcyte of the cat.
A comparative study of mammalian spermatogenesis reveals the absence of typical heterochromosomes in mongoose,∗ cat, squirrel, rabbit and pig. Heterochromosomes are clearly present at synapsis and prophase in the primary spermatocytes of the following forms: white mouse, sheep, horse, mule, dog and bull. Regarding dog, rabbit, and the monkey, the evidence is not yet decisive.
At certain stages the heterochromosomes (chromosome-nucleoli) appear single (accessory; monosome), at others double or bipartite. The latter appearance suggests a pair of idiochromosomes; but the body is more probably a split accessory.
The absence of discernible heterochromosomes in the male, and their conspicuous presence in the female, of the cat indicates their presence in one or the opposite sex in all forms. If this hypothesis can be supported by evidence from the oocytes of mongoose, squirrel, pig, rabbit and similar forms, cogent additional confirmation is given to the idea of a special significance of heterochromosomes, probably in connection with the determination of sex.
A simple explanation of sex-determination suggested by these and other facts -and one in apparent accord with a large body of experimental and cytological data -would seem to be to regard the heterochromosome-complex or “X-element” (Wilson), contributed by the spermatozoön, as an inhibitor to male sex. Regarded in terms of Mendelian concepts, however, an apparent contradiction results in that the presence of a determiner (inhibitor to maleness) would here have to be recessive to its absence. But in terms of a quantitative interpretation two X-elements in the zygote would prevent, one X-element permit, the development of male sex.
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