NATURAL SCIENCE IN THE NINETEENTH CENTURY 375 



world of the infinitely little, similar to that reaped after the intro- 

 duction of the telescope by Galileo, and his exploration of the 

 stars, in the world of the infinitely great. The cell theory of 

 Schleiden and Schwann appeared in 1839 and yeast was redis- 

 covered (see p. 378) in 1837. The first contagious disease (Mus- 

 cardine) traced to a fungus parasite was worked out by Bassi 

 in 1837, the first contagious disease (Favus) of man due to a 

 fungus, by Schoenlein in 1839. Protoplasm was first described in 

 1846. Ehrenberg, in 1838, made numerous and important studies 

 on microscopic plants and animals. 



EMBRYOLOGY. If in 1828 one sharp boundary which had 

 always been supposed to stand between the organic and the in- 

 organic world was broken down by Wohler's discovery that urea, 

 a substance hitherto exclusively of animal origin, could be ob- 

 tained in the laboratory by heating an inorganic substance, 

 ammonium cyanate, another well-defined boundary believed to 

 exist between the higher and the lower animals had been broken 

 down a year earlier, when a Russian zoologist, Karl Ernst von 

 Baer (1792-1876), announced that mammals, including man 

 himself, reproduce by eggs, precisely as do the lower animals. 

 In 1828 von Baer published our first important work on compara- 

 tive embryology, of which science he thus became the founder. 



The discovery by von Baer of the human ovum overthrew 

 completely the " animalculists " who for centuries had contended 

 that within the earliest embryo of man the future offspring existed 

 completely formed, but only in miniature. This theory, because 

 it assumed for development a mere unfolding, was known as em- 

 bryologic evolution. Harvey, on the other hand, had propounded 

 a theory of epigenesis, i.e. development comprising growth and 

 differentiation out of an originally minute, simple, and undiffer- 

 entiated body. This "body" the human ovum was now 

 described by von Baer as giro- inch in diameter and nowise different 

 in appearance from other animal eggs in their earliest stages. 

 Comparative anatomy had already shown that Linnaeus was 

 right in placing man among the animals, and now embryology con- 

 firmed and strengthened this view of man's place in nature. 



