154 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii 



and had to rely, like Riolanus, on "perspectives", or simple lenses 

 of very low power. 



1. There can be no doubt that the doctrine omne vivum ex ovo 

 was a tremendous advance on all preceding thought. Harvey's 

 scepticism about spontaneous generation antedated by less than a 

 century the experiments of Redi. It is important to note that he was 

 led to his idea of the mammalian ovum by observations on small 

 conceptions surrounded by their chorion and no bigger than eggs, 

 for the true ovum itself was not discovered until the time of de Graaf 

 and Stensen. 



2. He identified definitely and finally the cicatricula on the yolk- 

 membrane as the spot from which the embryo originated. 



3. He denied the possibility of generation from excrement and 

 from mud, saying that even vermiparous animals had eggs. 



4. He discussed the question of metamorphosis (preformation) and 

 epigenesis, and decided plainly for the latter, at any rate for the 

 sanguineous animals. 



In addition to these achievements, there are others, perhaps less 

 striking, but equally important. 



5. He destroyed once and for all the Aristotelian (semen-blood) 

 and Epicurean (semen-semen) theories of early embryogeny. This 

 was perhaps the biggest crack he made in the Peripatetic teaching 

 on development; but, in spite of it, Sennertus, van Linde and 

 Sylvius adhered to the ancient views, and Cyprianus, in 1700, 

 had the distinction of being the last to support them in a scientific 

 discussion, though Sterne, as late as 1 759, referred to them in a way 

 that shows they still lived on in popular thought. 



6. He handled the question of growth and differentiation better 

 than any before, anticipating the ideas of the present century. 



7. He settled for good the controversy which had lasted for 2200 

 years as to which part of the egg was nutritive and which was forma- 

 tive, by demonstrating the unreality of the distinction. 



8. He set his predecessors right on a very large number of detailed 

 points, such as the nature of the placenta. 



9. He made a great step forward in his theory of foetal respiration, 

 though here he did not consolidate the gain. 



10. He affirmed that embryonic organs were active, and that the 

 embryo did not depend on external aid for its principal physiological 

 functions. 



