10 INTRODUCTION 



Schwann's ininicdiato followers throw doubts ujion it,^ and as early 

 as 1855 X'irchow positively maintained the universality of cell-divi- 

 sion, contending- that every cell is the offspriuii; of a preexisting 

 parent-cell, and summing' up in the since famous aphorism, '^ oninis 

 Ctllula e ccllulay- 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 

 their common ancestor. Such is, in fact, the case in every plant and 

 animal whose development is accurately known. The first step in 

 development consists in the division of the o.^^^ into two parts, each 

 of which is a cell, like the egg itself. The two then divide in turn to 

 form four, eight, sixteen, and so on in more or less regular progres- 

 sion (Fig. 4.) until step by step the o.^^ has split up into the multitude 

 of cells which build the body of the embryo, and finally of the adult. 

 This process, known as the cleavage or segvicntatiou of the Q.%^, 

 was observed long before its meaning was understood. It seems to 

 have been first definitely described in the case of the frog's egg, by 

 Prevost and Dumas ( 1824), though earlier observers had seen it; but 

 at this time neither the egg nor its descendants were known to be 

 cells, and its true meaning was first clearly perceived by Bergmann, 

 Kolliker, Reichert, Von Baer, and Remak, some twenty years later. 

 The interpretation of cleavage as a process of cell-division was fol- 

 lowed by the demonstration that cell-division does not begin with 

 cleavage, but ca7i be traced back into tJic foregoing generation ; for the 

 egg-cell, as well as the sperm-cell, arises by the division of a cell pre- 

 existing in the parent-body. // is therefore derived by direct descent 

 from an egg-cell of the foregoing generation, and so on ad infinitnm. 

 Embryologists thus arrived at the conception so vividly set forth by 

 Virchow in 1858'^ of an uninterrupted series of cell-divisions extend- 

 ing backward from existing plants and animals to that remote and 

 unknown period when vital organization assumed its present form. 

 Life is a continuous stream. The death of the individual involves no 

 breach of continuity in the series of cell-divisions by which the life 

 of the race flows onwards. The individual body dies, it is true, but 

 the germ-cells live on, carrying with them, as it were, the traditions 

 of the race from which they have sprung, and handing them on to 

 their descendants. 



1 Among these may l^e especially mentioned Mohl, Unger, Nageli, Martin Barry, Goodsir, 

 and Remak. 



2 Arch, fur Path. A}iat., VITI., p. 23, 1855. 



3 See the quotation from the original edition of the Cellularpathologie at the head of 

 Chapter II., p. 63. 



