554 GENERAL CONSIDERATIONS 



those of human and comparative anatomy. For that reason he is usually- 

 credited with having founded modern physiology. 



629. Cell Theory. — This has been reviewed in one of the early 

 chapters (Sec. 42) in which was described the work of Hooke, Dujardin 

 (1801-1860), Von Mohl (1805-1872), Schleiden (1804-1881), Schwann 

 (1810-1882), and Schultze (1825-1874). 



630. Embryology. — It has already been stated that Aristotle studied 

 the embryology of the chick and its gradual development. Harvey 

 analyzed development in a critical manner, and Malpighi gave a very 

 complete description of the development of the hen's egg. Just before 

 the time of Caspar Wolff (1733-1794), however, a theory of preformation 

 had become dominant; this theory was to the effect that existent in the 

 egg was a miniature adult which needed only the stimulation of the sperm 

 cell to develop. Others thought that this homunculus existed in the 

 sperm cell and that the egg cell was simply a bed of nourishment for it. 

 The proponents of this theory were driven logically to the assumption 

 that succeeding generations were also represented in some way one within 

 the other. Wolff successfully championed the antagonistic theory of 

 epigenesis, which was that the structures of the adult were gradually 

 developed from an egg cell and that each generation began anew with 

 another such cell. The modern science of embryology, however, dates 

 from the time of Von Baer (1792-1876), who established the facts in 

 regard to the germ layers and put the subject on a comparative basis. 

 He was the author of the biogenetic law, which has also been called Von 

 Baer's law. 



631. Taxonomy. — The names of those most prominently connected 

 with the development of taxonomy have been given in the preceding 

 chapter and will not be repeated here. 



632. Evolution and Genetics. — Theories of spontaneous generation 

 have been treated earlier in this text, where the work of Redi (1626-1697), 

 Pasteur (1822-1895), and Tyndall (1820-1893) was discussed. The 

 steps in the development of the evolutionary conception have been taken 

 up in a separate chapter (Chap. LXXII), and statements made there 

 need not be repeated. The greatest name, and the one which marks the 

 beginning of precise knowledge in the field of genetics, is that of Mendel. 



633. Pasteur. — Louis Pasteur (Fig. 335) was a chemist who made 

 noteworthy contributions in the fields of microbiology and preventive 

 and curative medicine. He pursued extensive investigations on fermen- 

 tation and discovered that the bacteria in milk could be killed by raising 

 it to a temperature much below boiling point and keeping it there for a 

 time, a process now known as pasteurization. He discovered the micro- 

 organism causing a disease in silkworms known as pebrine and thus 

 saved the silk industry in France when the silkworm was threatened 

 with extinction by this disease. He also discovered a method of inocula- 



