A text-book of embryology for students of medicine / by John Clement Heisler. With 190 illustrations, 26 of them in colors.

  • Heisler, John C. (John Clement), 1862-1938
Date:
1899
Catalogue details

Licence: Public Domain Mark

Credit: A text-book of embryology for students of medicine / by John Clement Heisler. With 190 illustrations, 26 of them in colors. Source: Wellcome Collection.

Provider: This material has been provided by the Augustus C. Long Health Sciences Library at Columbia University and Columbia University Libraries/Information Services, through the Medical Heritage Library. The original may be consulted at the the Augustus C. Long Health Sciences Library at Columbia University and Columbia University.

    24/466 (page 18)
    been designated the preformation theory. According to this doctrine, the egg or germ contained all the parts of the adult organism in an exceedingly minute condition, and develop- ment consisted in the simple growth or unfolding of already formed parts. As the theory of unfolding implied the pre- formation not only of the immediate but of all subsequent offspring, its votaries were able to compute that the ovary of Eve contained 200,000 millions of human germs. With the discovery of the spermatozoon in 1677 by Hamm, a pupil of Leuwenhoeck, a controversy arose as to whether it was the spermatic filament or the ovum that contained the germ. Those who maintained the former view were known as aniiaalculids ; those who held the latter, as ovists. Accord- ing to the opinions of the animalculists, the spermatozoon was the complete organism in miniature, and it required for its growth the soil or environment which the ovum alone could furnish. The enunciation by Wolff, in 1759, of his doctrine of eplgene- sis completely overturned the preformation theory. Wolff maintained that the germ was imorganized matter, and that the union of male and female material was essential to reproduc- tion. While Wolff's theory was in the main correct, it re- mained for later investigators to show that the ovum did not consist of unorganized matter, as he thought, but that it pos- sessed definite structural characteristics. Thus, the germinal vesicle of the hen's egg was discovered in 1825 by Purkinje, and the germinal spot in 1826 by Wagner. Soon after the enunciation of the cell-doctrine by Schleiden and Schwann, it was seen tiuit the ovum was in reality a typical cell, ])os- sessing all the parts of such a structure. It was not, however, until about the year 1840 that it was shown, by Kolliker, Reichert, and others, that the spermatozoa are the active agents in fecundation. Previously it had been held, since the refutation of the preformation theory, that the seminal fluid performed this function, and that the spermato- zoa were parasitic organisms. The length of time necessary for the develo{)nient of the new individual varies according to the species; in man it