132 THE EVOLUTION OF CONTINUITY 



In connection with the dwarfing of the restored foetal arm an 

 experiment which has been performed on the developing fertilised 

 ovum of Amphioxus is very suggestive. If the first two cells into 

 which the fertilised ovum divides be separated, each may develop 

 into a complete fish, whose size, however, will only be half the size 

 of the normally developed fish. And if the first four cells be similarly 

 separated, four complete fish may result, each one-quarter of the 

 normal size. On the other hand, if the first eight cells be separated 

 from each other, no one of these can become a fish ; this showing 

 that side-path evolution has definitely begun with division into eight 

 cells, and that the two cells which each fourth becomes have different 

 destinies fixed for their descendants. This is clearly a matter of the 

 commencing evolution of cell species within the developing individual. 



The removal of one from the other of the first division-results 

 of the Amphioxus fertilised ovum is just a " cell -amputation," as 

 is also the removal of a fourth cell from its three partners ; and the 

 development potential in the removed cell is not lost, but " restored " 

 on a smaller scale. And though an eighth cell cannot when removed 

 become a fish, yet the remaining seven if left in continuity ought to 

 make good the loss suffered, at some cost to normal mature size, in 

 a given region. The fertilised ovum is the " primitive type " of the 

 body's evolution, but on the evolutionary road leading to full tissue 

 differentiation any cell is an " original type " for its " descendants," 

 and thus we might judge that according to the amount of tissue or 

 cell-loss, and the period of development during which it occurred, 

 a reduced restoration would take place in the way indicated by the 

 Amphioxus experiments. 



