312 PROGRESS OF SCIENCE IN THE CENTURY. 



cellular structure, i.e., are either single corpuscles 

 of living matter (the unicellular Protozoa and Proto- 

 phytes), or are built up of a large number of such 

 corpuscles and modifications of these; (2) Embryo- 

 logical, that every organism, reproduced in the ordi- 

 nary sexual way, starts in life as a fertilised ovum, 

 which divides and re-divides into a coherent em- 

 bryonic mass of cells, — the beginning of a body; 

 and (3) Physiological, that the functions of a multi- 

 cellular organism are to some extent expressible in 

 terms of the activities of its component cells. * 



The history of microscopic analysis will be 

 alluded to in the next chapter, but it may be noted 

 here that the cell-doctrine is a fine example of a 

 generalisation reached gradually by work done along 

 many different lines and by many investigators. We 

 may particularly associate its formulation with the 

 work of Schleiden (1838) and Schwann (1839), 

 Goodsir (1845) and Virchow (1858), but there were 

 many others who contributed to the result. As to 

 the different paths pursued, we should notice (a) 

 the analysis of the body into tissues (Bichat), (h) 

 the discovery and study of unicellular organisms 

 (e.g., the investigation of Bacteria and Infuso- 

 rians by Leeuwenhoek, of the Amoeba by Roesel von 

 Rosenhof, of Foraminifera by Dujardin), (c) the 

 recognition of the unicellular nature of ovum and 

 spermatozoon and of the cleavage that follows fertil- 

 isation, and {d) the gradual disclosure of the cel- 

 lular structure of organisms, — first in plants, and 

 then in animals. 



Cellular Physiology. — This is a distinctively 

 modern study and is still embryonic. Its central 

 idea is that of expressing vital processes in terms of 



* See The Science of Life, p. 103. 



