THE STUDY OF STRUCTURE. 3^3 



similarity of process and result in spite of many 

 peculiarities in individual cases, (6) the occurrence 

 of complex tensions, strains, and stresses in the proc- 

 ess, and (c) the impossibility (at present) of any 

 mechanical interpretation. 



(d) Various facts, such as the multiplication of 

 nuclei in embryos without corresponding cell-delimi- 

 nation, and the influence that the growth of the mass 

 has upon the forms of cell-division which follow, 

 have led many to add saving-clauses to the cell- 

 theory, as Sachs did when he said " cell-formation is 

 merely one of the numerous expressions of the for- 

 mative forces which reside in all matter, in the high- 

 est degree, however, in organic substance '' ; or as De 

 Bary did when he said, " That the plant forms cells 

 is more accurate than the statement that cells form 

 plants." '^ Die Pflanze hildet Zellen, nicht die Zelle 

 hildet Pflanzen" In short, the conception of the cell 

 has to change with increasing knoAvledge of its na- 

 ture and origin ; though it may be still defined as a 

 protoplasmic area in which nucleoplasm and cyto- 

 plasm are combined in a unified life. 



(e) Though it is not exactly relevant in this chap- 

 ter, we must note the gradually increasing body of 

 facts which inform us as to the physiological rela- 

 tions of the individual cell to its environment (of 

 physical and chemical influences, and of its fellows). 

 The bulk of Davenport's Physiological Morphol- 

 ogy is occupied with a discussion of this problem. 



(/) Finally, the progress of cytology has had its 

 influence on that study of Bacteria and other micro- 

 organisms which has been one of the features of the 

 latter part of the nineteenth century. The door 

 which Leeuwenhoek opened in the seventeenth cen- 



