The 19th-century Elucidation of Animal Fertilization: Its Relation to the Cell Theory, Embryology, and Cytogenetics

THE 1 9th - CENTURY EL UCIDA TION OF ANIMAL FERTILIZATION: ITS RELATION TO THE CELL THEORY, EMBRYOLOGY, AND CYTOGENETICS HAROLD M. MALKIN* Introduction Francois Magendie (1783-1855), one of the most renowned physiologists of the 19th century, the teacher of Claude Bernard, and one of staunchest mechanists during his lifetime, wrote in the 1855 edition of his popular Précis Elémentaire de Physiologie (translated into English byJohn Revere ) with regard to human reproduction: We may consider it, therefore, as highly probable, that the semen passes either at the moment of coition, or some time afterward, to the ovarium, where it performs its specific action upon the vesicle which is afterward to be developed. But even if it be acknowledged that the semen finds its way to the vesicle of the ovarium, it still remains to be shown how its contact animates the germ. Now this is a phenomenon of which it is impossible that our senses take cognizance. It is one of those mysteries which at present are, and will probably always remain inexplicable [1, p501] That this mystery which had baffled biologists and philosophers since before Aristotle was essentially solved by the end of the 19th century was an amazing accomplishment for mankind. By that time, the cellular characteristics of the egg and sperm were proven, the specific events of fertilization and the essentially equal contributions of each parent to the progeny had been described, and the development of the zygote by segmentation into the basic germ layers and the early stages of the adult animal was established . In addition, some of the major organelles visible with the light microscope in most cells were visualized [2] ; the major function of the largest component, the nucleus, was elucidated; there was final establishment of the Cell Theory, and the field of cytogenetics had begun. In addition to describing the above scientific accomplishments, this paper will attempt to explain the sociological, philosophical, and organizational factors in science during the 19th century both in France and Ger- *250 The Uplands, Berkeley, CA 94705.© 1998 by The University of Chicago. All rights reserved. 0031-5982/98/4201-1080$01.00 Perspectives in Biology and Medicine 42, 1 ¦ Autumn 1998 | 33 many, that could explain the 1855 statement ofMagendie on the one hand, and the unprecedented accomplishments of the suddenly monumental growth of the German scientific community during the second half of the 19th century on the other. Background of the Problem Aristotle, the most respected biologist from ancient times, adopted the idea from Hippocrates that fetuses developed as a result of the combination of male semen and female menstrual fluid. He believed that the form for the fetus was contributed by the semen and the menstrual fluid simply provided material. This belief was accepted by almost all scholars until the 18th century, and only the major controversies were related to the relative importance of each of these fluids, and how the fetus formed. It is of interest that Thomas of Aquino in the 13th century agreed with Aristotle with regard to the importance of the male: The generative power of the female is imperfect compared to that of the male; for just as in the crafts, the inferior workman prepares the material and the more skilled operator shapes it, so likewise the female generative virtue provides the substance but the active male virtue makes it into the finished product. [3] However, by the 1 7th century, the general belief in the fetus forming from the mixing of the semen and the menstrual fluid lost its credibility. This was primarily due to the studies of William Harvey, who in addition to his monumental work on the circulation of the blood, studied the question of animal reproduction, which resulted in his book De Generatione Animalium in 1650 [4]. In his studies Harvey destroyed the semen-menstrual fluid theory of Aristotle by dissecting the uteri of deers following coitus and not finding any of the coagulum predicted by the ancients. Unfortunately, he was not successful in finding a fertilized egg. However Harvey did suspect its existence on the basis ofidentifying small embryos surrounded by chorions. (The true ovum was not found until 1827 by von Baer...