lyi THE HISTORY OF BIOLOGY 



already had time, however, to make important contributions to biology in 

 both the anatomical and the physiological sphere. His doctor's dissertation 

 deals with the pancreatic secretion. In it he shows how, by introducing a 

 canula into the duct of the pancreatic gland in a live dog, it is possible to 

 obtain some of the secretion for the purpose of closer examination — a 

 method which has since then been generally adopted in physiology. 



The work on which de Graaf's fame principally rests, however, is his 

 study of the sexual organs, both male and female, but chiefly the latter. The 

 ovaries, of course, had already been described before, of both the higher and the 

 lower vertebrates; that they produced eggs in birds was known, but a great 

 many contradictory theories had been advanced on the subject of what kind 

 of function they possessed in man and the other mammals. The Aristoteleans 

 naturally supported their master's doctrine that the sexual product of the 

 woman is the menstrual blood and that otherwise the male semen is the 

 essential origin of the embryo, which the woman then nourishes and pro- 

 duces. De Graaf, on the other hand, after making a comparative study of the 

 ovaries of mammals and birds, came to the conclusion that the cell-like 

 protuberances already observed by Vesalius and Fallopio in the ovary of 

 mammals corresponded to the egg of the bird ovary, and that the process of 

 fertilization is similar in every animal type; just as a bird's fertilized egg in 

 the ducts of the ovary acquires albumen and shell, the egg of the mammal 

 becomes fertilized through the Fallopian tube, finds its way to the uterus, 

 and there develops further. The very word "ovary" was suggested by him; 

 hitherto the female sexual gland as well as the male had been called testis, 

 a word which he still employs alternatively with the new one. He definitely 

 rejects the assertion of the Aristoteleans that the embryo originates from the 

 man alone; in disproof of that assertion he cites many cases in which demon- 

 strably purely external characteristics have been inherited by the embryo 

 from the mother, both in human beings and in animals; even cases of extra- 

 uterine gestation are cited by him as proof that the embryo is derived from 

 the ovaries and not from outside. Likewise in regard to several other details 

 in the structure of the sexual organs he records valuable fresh observations. 

 These investigations of de Graaf's proved of fundamental importance, 

 although he was wrong in his assumption that the follicles in the ovary, 

 which now bear his name, correspond to the eggs in the ovary of a bird — 

 the true eggs of mammals were not discovered until a century and a half after 

 his death. Nevertheless, the explanation he gave of the actual phenomenon 

 of fertilization was of decisive significance for the future development of the 

 knowledge of this phenomenon. It was impossible, however, either for his 

 own or for the immediately succeeding age to reconcile his claim as to the 

 significance of the egg in embryonic development with the important part 

 that the spermatozoa should be assumed to play in the same process. And so 



