Page:Popular Science Monthly Volume 85.djvu/135

From Wikisource
Jump to navigation Jump to search
This page has been proofread, but needs to be validated.

pairs, leaving the X chromosome unpaired (Fig. 39, B) and in the reduction division the pairs separate, while the X chromosome goes entire into one of the daughter cells, which consequently contains 23 + X chromosomes, whereas the other daughter cell contains 23 chromosomes (Fig. 39 C and D). The former gives rise to spermatozoa with 24 chromosomes, the latter to spermatozoa with 23 chromosomes. In the female there are probably 48 chromosomes, according to Winiwarter, there being two X chromosomes, one from each parent, and after the reduction divisions every egg contains 24 chromosomes. If an egg is fertilized by a sperm containing 24 chromosomes an individual with 48 chromosomes, or a female, is produced; if fertilized by a sperm with 23 chromosomes an individual with 47 chromosomes, or a male, results (Fig. 39).

PSM V85 D135 Diagrams of sex differentiation in man.png

Fig. 39. Diagrams of Sex Differentiation in Man. A, spermatogonium with 47 chromosomes one of which is the "sex" chromosome. B, spermatocyte showing 23 synaptic pairs and a single unpaired "sex" chromosome. C, reduction division in which the synaptic pairs separate while the sex chromosome does not divide, consequently the second spermatocytes D and D′ contain respectively 23 X and 23 chromosomes. E and E′, second maturation division in which every chromosome divides, giving rise to two equal classes of spermatids and spermatozoa, one of which has 24 chromosomes and the other 23. If an egg containing 24 chromosomes is fertilized by a sperm with 24, a female with 48 chromosomes is produced; if an egg with 24 chromosomes is fertilized by a sperm with 23, a male results. (From Morgan after Winiwarter.)

It must be said that other investigators, notably Guyer and Montgomery, have not found 47 chromosomes in the spermatogonia of man, but 23. Since both the latter investigators worked on negroes whereas Winiwarter worked on white men it has been suggested quite recently by Morgan and Guyer that there may be twice as many chromosomes in the white race as in the black. A similar condition in which one race has twice as many chromosomes as another race of the same species is