172 THE CELL-THEORY 



life, the other a dependent life in so far as the cell has 

 become an integral part of the plant" (p. 138). 



So long as the definition of the plant cell embraced little 

 more than the hardened cell-wall it was little wonder that 

 " cells " in this sense were not recognised in animal tissues, 

 except in a few exceptional cases as in the notochord by 

 Johannes Muller. 1 Careful observation of animal tissues 

 discovered in some cases the existence of discontinuous units 

 of structure, but these were not, as a rule, recognised before 

 1838 as analogous to plant cells. Von Baer, for example, 

 observed that the young chick embryo was composed partly 

 of an albuminous mass and partly of Kugelchen or little 

 globules suspended in it (Entwickelungsgeschichte^ i., pp. 19, 

 144). Since such KiigclcJicn disposed in a row formed the 

 notochord (i., p. 145) it seems probable that his Kilgelchen 

 were really cells. Similarly A. de Ouatrefages' 2 in 1834 

 saw and figured segmentation spheres in the developing 

 egg of Liuimca, but he called them globules and did not 

 recognise their analogy with the cells of plants. According 

 to M'Kendrick, ;J Fontana, so far back as i/Si, 4 described 

 cells with nuclei in various tissues, and used acids and alkalis 

 to bring out their structure more clearly. But it was not till 

 1836-7-8 that a fairly widespread occurrence of cells in 

 animal tissues was recognised. The pioneer in this seems to 

 have been Purkinje, who described cells in the choroidal 

 plexus in i836, 5 and compared gland cells with the cells 

 of plants in 1837.'' Henle in 1837 ~ and 1838 s described 

 various kinds of epithelial tissue, distinguishing them 

 according to the kind of cell composing them ; he also 

 discovered the mode of growth of stratified epithelium. 



1 Myxinoiden, i. Theil., p. 89, 1835. 

 - Ann. Sci. nat. (2) (Zool.} ii., pp. 107-18, pi. u, 1834. 

 3 Proc. Phil. Soc. G/astfw, xix., pp. 71-125, 1887-8. 

 1 Traitc sur le venin de la vipire, 1781. 

 1 Miiller's Archiv, 1836. 



' ; J. Muller, Jithrcsbcricht ii. d. Fortschritte deranat.-physiol. Wisscn- 

 schaftcn ii jahre 1838. Miiller's Arc/iiv, 1838. 



~ Symbolce ad anatomiam villorum imprimis eorum cpitJiclii, Berlin, 



1837- 



< U. d. Ausbreitung dcs Epitheliums im menschlichen Kvrper, 

 Miiller's Arc/ii7', 1838. 



