1751. Linnseus publishes the Sy sterna Naturae. 



1759 Wolff in his Theoria Generationis discards the 



preformation theory of Malpighi and Bonnet 

 and proposes the developmental theory of the 

 embryo. 



1760. Kolreuter produces the first plant hybrid in a 



scientific experiment. 



1786. Spallanzani shows that sperms are essential to 



fertilization. 



1793. Sprengel discovers insect fertilization of plants 



and dichogamy. 



1809. Lamarck in La Philosophie Zoologigue expounds 



the theory of Use and Disuse and the Inheri- 

 tance of Acquired Characters. 



1817. Pander discovers the three primary germ-layers 



of the embryo. 



18 7-8 Von Baer discovers the mammalian ovum and 

 lays the foundation of comparative embry- 

 ology. 



1831. Brown describes the nucleus in plant cells. 



1835. Dujardin discovers and describes sarcode (pro- 



toplasm). 



1838-9. Schwann and Schleiden formulate their cell- 

 theory. 



18*41. Kolliker demonstrates the cellular origin of 



sperms in the testes. 

 Remak proposes the theory that every cell 

 comes from a pre-existing cell by a process of 

 division. 



1843. Martin Barry observes the union of sperm and 



ovum in the rabbit. 



1846. Von Mohl brings the term protoplasm into gen- 



eral use. 



1859. Charles Darwin expounds the Theory of Natural 



Selection in the Origin of Species. 



1861. Max Schultze recognizes the cell as composed of 



protoplasm and a nucleus. 



1863. Fritz Muller formulates the Recapitulation 



Theory, first stated by Von Baer. 



1864. Herbert Spencer in the Principles of Biology 



suggests the existence of "physiological units' 

 in the reproductive elements. 



172 



