X^S THE HISTORY OF BIOLOGY 



to natural-scientific studies and became professor of philosophy, first at Mo- 

 dena and later at Pavia. He applied himself to experimental research, particu- 

 larly in regard to regeneration and fertilization, and left all his predecessors 

 far behind him, in both method and results. In the amphibians, especially 

 salamanders and tritons, he found suitable subjects for the study of regenera- 

 tion even among the vertebrates, and he made as exhaustive a use of them 

 as was possible under the conditions in which he worked; he studied the 

 re-formation of the tail, extremities, and jaws, and this not merely for the 

 purpose of establishing the fact, but by means of dissection and microscopic 

 investigation he followed the re-forming of the various components of the 

 body: muscles, nerves, and bones. He observed the time that the regenera- 

 tion lasted and endeavoured to influence the process by means of altering 

 the conditions of food and temperature. He even experimented with the 

 phenomena of fertilization; by filtering the sperm of particular animals he 

 proved that the presence of the spermatozoa was essential if fertilization 

 was to take place; nevertheless he could not be induced to assume a direct 

 influence of these components upon the egg, but believed that the accompany- 

 ing fluid was the substance that stimulated the egg's development. For he 

 adhered as stubbornly as Bonnet to the preformation theory: he closely 

 studied the development of the frog's egg and followed the formation of 

 the backbone channel, but merely for the purpose of seeking evidence of 

 the entire animal's having been ready-formed in the egg. Eventually he be- 

 lieved that he had discovered incontestable proof thereof, when he saw the 

 frog's egg increasing in size within the body of the mother animal and before 

 it had been fertilized, and as growth is not possible without organs, the 

 larva of the frog must have been ready-made in the egg before fertilization. 

 Just as Spallanzani was thus convinced that he had found an undeniable 

 argument in favour of the preformation theory, another scientist published 

 a treatise which was to form the basis of a new conception of embryonic 

 development. 



Caspar Friedrich Wolff was the name of the naturalist who led the 

 science of embryology into fresh paths. He was born in Berlin in 1733, ^^^ 

 son of a tailor. He went through a course of medical training at the College 

 of Medicine there and thence proceeded to Halle, where he studied philos- 

 ophy after the system of Leibniz and his pupil Christian WolfF,^ and finally, 



^ Christian Wolff (1679-1754) was professor of mathematics at Halle, whence he was 

 ejected through the intrigues of the Pietists (Stahl seems to have taken part in the persecution 

 directed against him) and then became professor at Marburg, but later he returned to Halle 

 and there worked as professor of philosophy. Under the general title Vernunftige Betrachtungen he 

 published a series of essays covering many different fields of human knowledge, in which he 

 expounds Leibniz's theories in a popularized form, in particular maintaining that everything 

 that happens must possess adequate reason for doing so, because otherwise something might 



