360 PROGRESS OF SCIENCE IN THE CENTURY. 



fore be placed beside the evolution-theory as one of the 

 foundation-stones of modern biology." 



The progress of cellular biology or cytology since 

 the formulation of the cell-doctrine has been along 

 several different lines, connected of course by side 

 branches. 



(a) The complexity of cell-structure has become 

 more and more apparent. It includes many com- 

 ponents, the general cell-substance or cytoplasm, 

 the nucleus with its readily stainable " chromatin " 

 and illusive unstainable " achromatin," the centro- 

 somes (present in the majority of animal-cells) 

 which play an important part in division, the cell- 

 wall or the cell-margin which shows many degrees 

 of differentiation, the intercellular bridges which in 

 many cases bind one cell to another, and so on. The 

 cell is a little world of extraordinary complexity, as 

 the work of Auerbach, Biitschli, Carnoy, Memming, 

 Fol, Guignard, Hertwig, Strasburger, Van Beneden, 

 and a score of other prominent workers has shown. 



(&) The same impression of a progressive reve- 

 lation of complexity is afforded if we consider any 

 particular component of the cell, such as the nucleus, 

 or the system of radiating filaments which form a 

 halo round the centrosome, or the structure of a 

 vibratile lash or cilium, or the general cell-substance. 

 In regard to the last, some, like Frommann and Ar- 

 nold, have described an intricate network; others, 

 like Flemming, a tangled coil of fibrils ; others, like 

 Altmann, a crowd of granules in a gelatinous ma- 

 trix; and others, like Biitschli, a fine alveolar or 

 vacuolar appearance like that of an emulsion. It 

 seems probable that the minute structure of cell-sub- 



