THE CELL THEORY 105 



vertebrates. If we are inclined to hint that Baer, having- 

 gfone so far, might well have gone a little farther, it 

 is only fair to recollect that every leader in science is 

 more or less open to the same reproach. 



The Cell Theory. 

 Any one of the higher animals or plants admits of 

 analysis into organs^ each adapted to one or more 

 functions. Bichat (.1801) showed that the body of one 

 of the higher animals is not only a collection of organs, 

 but also a collection of tissues, and the same is true of 

 the higher plants. Analysis of the organism was carried 

 a step further when in 1838-9 Schleiden and Schwann 

 announced that all the higher animals and plants are 

 made up of cells, which were at first supposed to consist 

 in every case of a cell-wall, fluid contents, and a nucleus.^ 

 It was soon discovered that the cell-wall is as often 

 absent as present, and that the cell-contents are not 

 simply fluid; the nucleus is still believed to be universal. 

 Schwann proved that nails, feathers, and tooth-enamel, 

 though not obviously cellular, consist of nothing but 

 cells, and it was afterwards shown that bone, cartilage, 

 fatty tissue, and fibrous tissue arise by the activity of 

 cells which disappear from view in the abundance of 

 their formed products. The individual cells of a com- 

 plex organism are usually themselves alive ; sometimes, 

 as in ciliated epithelium, they give indications of life 



» Hooke fig-ured a thin section of dry cork in his Micrographia 

 (1665), remarking;- that it was divided into " litth^ boxes or cells." 

 The word cell was sugg-ested by the resemblance of the tissue to 

 a honeycomb ; since 1838 it has been thoughtlessly extended 

 from the skeleton to the particle of living matter enclosed within 

 ii. Robert Brown (1831) showed that a nucleus is usual in 

 plant cells ; it had been figured by P'ontana and others long- 

 before. Down to 1838 no results of biological interest followed 

 from the discovery. 



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