150 ANIMAL INDIVIDUALITY [OH. 



normal development: "the plastic forces heed no 

 cell-boundaries, but mould the germ-mass regardless 

 of the way it is cut up into cells." Such considera- 

 tions have led him and several others to throw up 

 the cell-theory altogether, saying that the cells of 

 a metazoan are not homologous with free-living 

 protozoan individuals, but are merely convenient 

 bricks, so to speak, or centres of local government, 

 produced by the forces of life after the form of the 

 creature had been established. But such a conclusion 

 cannot be justified. We must carefully distinguish 

 between what exists to-day, whether in adult body 

 or developing embryo of a metazoan, and what we 

 believe to have happened in the past. 



Yolvox and Haplozoon, whose cells we can with 

 no shadow of doubt affirm to be homologous with 

 free-living Protozoa, show that it is possible for a 

 higher individual to be evolved from a collection of 

 lower ones. If we refuse to the Metazoa an ancestor 

 formed thus by aggregate differentiation, we are 

 landed in far more and far worse difficulties than 

 any we escape from. Whitman is right in drawing 

 attention to the remarkable fact that the so-called 

 Kupffer's vesicle of embryonic Teleost fish is non- 

 cellular, a mere thin sheet of protoplasm which is not 

 even nucleated, whereas it is certainly homologous 

 with a structure of other vertebrates which is 

 composed of very definite cells, but to reject the 



