Embryology and genetics / by Thomas Hunt Morgan.

  • Morgan, Thomas Hunt, 1866-1945.
Date:
1934
Catalogue details

Licence: In copyright

Credit: Embryology and genetics / by Thomas Hunt Morgan. Source: Wellcome Collection.

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    DEVELOPMENT AND GENETICS It must have been known, long before there is any recorded human history, that some animals develop from eggs, at least those animals whose eggs are visible to the unaided eye. Birds, lizards, frogs and fish have large eggs; most other animals have smaller eggs, many of them just visible to the naked eye, Fig. i. The human egg measures only one-fifth of a millimeter, and since it develops inside the mother and, therefore, out of sight, the origin of the human embryo remained a mystery until von Baer identified the mammalian egg in 1820. The role of the male in procreation was much more difficult to dis- cover. The need of the semen to start the development of the egg may have been suspected in very early times, but the essential ele- ments in the semen could not have been imagined until the presence of animalculae or spermatozoa in it had been discovered by Hamm and Loewenhoek 'n 1677. That these animalculae were the real agents in development was not generally accepted for a long time. Only after 177 years was the actual entrance of the sperm into the egg observed by Newport (1854). By this time it was recognized that the egg is a cell, and that the spermatozoon is also a cell. The next advance was made by Hertwig in 1875, who observed that the sperm head, after entering the egg, enlarges to become a recognizable nucleus which fuses with the nucleus of the egg. The two combined nuclei then give rise to the nucleus of the dividing egg, which is the forerunner of all the nuclei of the cells of the resulting embryo. Since it was believed that the child inherits equally from the two parents, and since the spermatozoon contributes only its nucleus to the combination—its tail not entering the egg as a rule-