INTRODUCTION 



formation " was absolutely proved to be a myth, though many of 

 Schwann's immediate followers threw doubts upon it, and as early 

 as 1855 Virchow positively maintained the universality of cell-divis* 

 ion, contending that every cell is the offspring of a pre-existing 

 parent-cell, and summing up in the since famous aphorism, " omuis 



Fig. 3. Cleavage of the ovum of the sea-urchin Toxopneusles, X 330, from life. The suc- 

 cessive divisions up to the i6-cell stage (H) occupy about two hours. / is a section of the embryo 

 (blastula) of three hours, consisting of approximately 128 cells surrounding a central cavity or 

 blastocoel. 



cellula e cellula" 1 At the present day this conclusion rests upon a 

 foundation so firm that we are justified in regarding it as a universal 

 law of development. 



Now, if the cells of the body always arise by the division of pre- 

 existing cells, all must be traceable back to the fertilized egg-cell as 



1 Arch, fiir Path. Anal., VIII., p. 23, 1855. 



