32 BORDER LINES OF KNOWLEDGE 



temporary boundaries of our knowledge. I will begin 

 with the largest fact and with the most absolute and 

 universally encountered limitation. 



The '' largest truth in Physiology " Mr. Paget con- 

 siders to be " the development of ova through multi- 

 phcation and division of their cells." I would state 

 it more broadly as the agency of the cell in all Hving 

 processes. It seems at present necessary to abandon 

 the original idea of Schwann, that we can observe the 

 building up of a cell from the simple granules of a 

 blastema, or formative fluid. The evidence points 

 rather towards the axiom, Omnis cellula e eellula ; 

 that is, the germ of a new cell is always derived from 

 a pre-existing cell. The doctrine of Schwann, as I re- 

 marked long ago (1844), runs parallel with the nebu- 

 lar theory in astronomy, and they may yet stand or fall 

 together. 



As we have seen Nature anticipating the plasterer 

 in fibro-cartilage, so we see her beforehand with the 

 glass-blower in her dealings with the cell. The artisan 

 blows his vitreous bubbles, large or small, to be used 

 afterwards as may be wanted. So Nature shapes her 

 hyaline vesicles and modifies them to serve the needs 

 of the part where they are found. The artisan whirls 

 his rod, and his glass bubble becomes a flattened disk, 

 with its bull's-eye for a nucleus. These lips of ours 

 are all glazed with microscopic tiles formed of flattened 

 cells, each one of them with its nucleus still as plain 

 and relatively as prominent, to the eye of the micro- 



