Human genetics and its foundations.

  • Whittinghill, Maurice, 1909-1998.
Date:
1965
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Human genetics and its foundations. Source: Wellcome Collection.

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    10 Monohybrid Genie Segregation or any available cells show a plano-convex dark spot in the nucleus when two X-chromosomes are present but no such sign when only one X-chromo- some is present. In a three month old fetus a histological difference can be detected in the primary sex organs, the gonads. At this time the gonads of female specimens show more rapid growth of their epithelial cells leading to the production of egg strings and egg nests, and finally the Graafian follicles present at birth in baby girls. Meanwhile the same ovary shows degenera¬ tion of certain interior gonadal cells which have been present up to the third month of development. In young male fetuses the opposite happens. The epithelial layer regresses, and the rapid growth takes place in the interior sex cords which later elongate, coil, become hollow and are recognizable as the future seminiferous tubules of the adult. The interstitial cells, which are soon to be of importance as secretors of the male hormone testosterone, make their appearance as separate clumps of cells among the seminiferous tubules in fetuses growing from XY human zygotes. After these differences have begun to appear at the histological level, the first anatomical differences between male and female fetuses can be seen. These include the appearance of a rougher surface on the ovary as Graafian follicles develop, and a greater migration of the testes posteriorly as they leave the main coelomic cavity to descend, usually before birth, into a bipartite scrotal sac. Changes which are secondary to the differentiation of the ovaries or of the testes are the abandonment of one of the sets of potential ducts which have been present and growing simultaneously in early fetuses of both sexes. In males the small oviducts degenerate almost completely, and parts of the intermediate kidney with its mesonephric duct persist and enlarge to become an eventual sperm duct on each side of the body. In females, by contrast, these latter ducts largely degenerate during fetal life, while the parallel Mullerian ducts persist and develop to become oviducts anteriorly and fuse and thicken posteriorly to become the uterus and the vagina. These changes are well advanced by the fourth fetal month. Other secondary sexual characteristics are developed during the fetal period. The external genitalia cannot be identified as male or female in the three month fetus, when they are said to be in the phenotypically indifferent stage, although they are composed of chromosomally different cells in males and females. Signs of male determination are the greater growth of the anterior glans with lateral tissues to form the penis and sidewise fusion in the midline to extend the urinary and genital canals as the penile urethra. Signs of female development in the fetal stage are the less conspicuous growth of the labial tissues and the persistence of a wide vestibular opening receiving separate genital and urinary openings. à