498 THE GENERAL PRINCIPLES OF ZOOLOGY 



for the student of medicine. The chief zoological advances of 

 the century were due to the improvement of the compound 

 microscope, which had been invented in the last decade of the 

 sixteenth century by the Dutchmen, Hans and Zacharias Jans- 

 sen. By means of this, the Italian scientist, Malpighi of 

 Bologna (1628-1694), made many discoveries in the minute 

 structure of the organs of the higher animals, some of which 

 still bear his name, and was the first to contribute to the knowl- 

 edge of insect anatomy in his " Dissertatio de bombyce," pub- 

 lished in 1669. The Dutchman, Schwammerdam ( 1637-1680) of 

 Leyden, in his work, "The Bible of Nature," described with 

 remarkable accuracy the structure of many insects and mol- 

 lusks. Leeuwenhoek of Delft (1632-1723) made extensive use 

 of the microscope and thus discovered many histological struc- 

 tures, such as blood corpuscles and the striations of muscle 

 fibers, as well as many minute animals, notably numerous 

 Protozoa and Rotifera. 



One of the most interesting microscopic discoveries of the 

 century was made at Leyden by the student von Hammen or 

 Hamm, in 1677, when he found for the first time the spermato- 

 zoa of animals, whose real significance was not recognized for 

 some time ; they were then called the sperm animalcules. Pre- 

 viously the ovum had been considered the real essential to 

 reproduction, the spermatic fluid being regarded as furnishing 

 merely the requisite stimulus for its development. There were 

 further two theories as to the method of development : accord- 

 ing to one, the theory of epigenesis held by Harvey, the embryo 

 arose from the ovum by its gradual differentiation into the organs 

 of the adult, a theory which we know to-day to be correct; 

 according to the other, called then the evolution or preformation 

 theory (not evolution as we use the term technically to-day), 

 which was held by Malpighi, the adult existed in miniature 

 completely formed in the ovum and needed merely to increase 

 in size to reach maturity. The discovery of the spermatozoa 

 gave rise to two factions, the ovulists or ovists, and the sperma- 

 tists or animalculists. The former held as previously that the 

 embryo arose from the ovum alone ; the latter maintained that 

 the head of the spermatozoon contained a miniature model of 

 the adult, — they drew figures of it, — which increased in size 





