OF THE CELL THEORY u6i 



surrounding matrix or blastema. In 1845 how- 

 ever Goodsir first promulgated the doctrine that 

 cells never originate without pre-existing cells, from 

 which they arise by fission ; a view strongly 

 supported by Remak, Kolliker, and Virchow, and 

 soon universally accepted. The next great step 

 was von Baer's discovery of the mammalian egg or 

 ovum, and his recognition of the fact that it was a 

 single nucleated cell ; a discovery that threw an 

 entirely new light on Harvey's dictum, omne vivum ex 

 ovo. 



The cell theory was now firmly established. 

 Histology, or the microscopical study of the 

 animal body, showed that all its parts and tissues 

 were really built up of cells, as a wall is of bricks ; 

 while embryology furnished even more cogent proof, 

 showing that the ovum or egg of all animals is one 

 single cell, and that from this single cell by 

 repeated division all the component cells of the 

 adult animal are derived. Thus expressed the 

 theory is the grandest generalisation and the 

 most firmly established fact in all morphology ; 

 and the division of the animal kingdom into Pro- 

 tozoa and Metazoa i.e., into animals which on the 

 one hand remain single cells all their lives, and 

 on the other hand commence as single cells or 

 ova but speedily become multicellular, takes rank 

 as the most fundamental and most natural of all 

 zoological distinctions. 



The cell theory early gained acceptance as 

 regards its main conclusions, but concerning the 

 detailed structure of the unit or cell discussions 



