ON THE CELLULAE THEORY OF DEVELOPMENT. 89 



form and extent of it ? What is the point of view which it 

 compels its votaries to take ? 



It is not easy to answer this question ; it is, in fact, as diffi- 

 cult to answer as that other question so often asked of the 

 teacher by his pupil — what is a cell ? The source of the diffi- 

 culty is that we are dealing with a kind of phantom which 

 takes different forms in different men's eyes. There is a want 

 of precision about the cell-phantom, as there is also about the 

 layer-phantom, which makes it very difficult to lay either of 

 them. Neither of these theories can be stated in so many 

 words in a manner satisfactory to every one. The result is 

 that it is not easy to bring either of them to book. 



To answer the question — what is the cellular theory of deve- 

 lopment? — the best plan will be to consider for a moment the 

 ideas which are taught to the student of biology, and which 

 influence him in his future work. We tell him that the cell is 

 the unit of structure, that an organism may consist of a single 

 cell, or of several cells in association with one another: we 

 draw the most fundamental distinction between the two kinds 

 of organism, and we divide the animal kingdom into two great 

 groups to receive them. As a proof of the importance which 

 we attach to this feature of organisation we assert that a man 

 is nearer, morphologically, to a tapeworm, than a tapeworm is 

 to a paramoecium. We tell him that the various structures 

 present in a protozoon are all parts of one cell, whereas in a 

 metazoon the various parts are composed of groups of cells 

 which differ from one another in structure. Finally, when we 

 ask him in the examination to tell us the principal differences 

 between hydra and vorticella, we consider that he is very 

 inadequately prepared if he does not sum them up by saying 

 that hydra has tissues composed of definite cells and is multi- 

 cellular, while vorticella is without definite cellular tissues 

 and is unicellular. Carrying on the idea thus implanted in 

 his mind as to the fundamental importance of the cell, we tell 

 him about the neuro-epithelial cell and the myo-epithelial 

 cell, and we point out their primitive distinctness, — an idea 

 which is still further impressed upon him when he studies the 



