THE BUILDING-STONES OF THE ORGANIC WORLD 79 



needed the laborious research of many decades before science 

 gained an accurate conception of the multiplication of the cell, 

 and even to-day we are still far from an accurate understanding 

 of all the details of this process which is of the utmost 

 importance for the preservation of life. 



Formerly it was usual to compare cells with crystals. It is 

 probably due to this wrong conception that Schleiden and 

 Schwann, the founders of the cell-theory, formed such faulty 

 ideas regarding the genesis of new cells : they thought that cells 

 crystallized, as it were, out of a mother-liquor, the germinal 

 matter. By his observations Schleiden had been driven to the 

 conclusion that the light-coloured fluid which fills the adult 

 plant-cell forms this germinal matter ; according to him this 

 liquid first forms by condensation a minute granule the 

 nucleolus, around which afterwards the nuclear substance aggre- 

 gates. The nucleus then becomes a formative centre for the 

 protoplasm. Schwann went yet a step further in his error and 

 taught that free cell-formation also permanently takes place 

 outside the cells, in the so-called intercellular substance of the 

 animal body. The first scientist who opposed this doctrine of 

 the spontaneous generation (Urzeugung) of the cell was the botanist 

 v. Mohl. He demonstrated that at least in the plant kingdom 

 the new formation of cells can only take place in correlation 

 with, and by the division of, already existing cells. It took, 

 however, many years before this erroneous conception was 

 finally disposed of by Kemack, Virchow, and others who 

 demonstrated that the law enunciated by Mohl applies to the 

 whole organic world, and that therefore no free cell-formation 

 can take place in the body of animals. Virchow coined the 

 famous sentence, " Omnis cellula e cellula, " a pronouncement 

 which has been brilliantly justified by all subsequent researches. 

 But not only do the cells not form themselves anew, their most 

 important organ, the nucleus, too, owes its origin to a pre-existing 

 nucleus, and we may therefore further say " Omnis nucleus e nucleo." 



We have already had an opportunity of dealing with the white 

 blood-corpuscles, the leucocytes. In addition to this ' sanitary 

 police ' which represents only a very small part of the blood we 

 find in human blood enormous numbers of small round discs, the 

 red blood-corpuscles. Though they are non-nucleated in the 



