44 ONTOGENESIS OF PROTOZOA. 



influence of unfavourable conditions, but it never 

 reacts so as to be converted into something of a 

 fundamentally different nature. 



No such process as absolute ' adaptation! like 

 what Haeckel describes, can thus be admitted any 

 more than ' differentiation! 



The elements in respect to shape composing the 

 various tissues of the animal body are formed out of 

 the substance of cells, their nuclei and intercellular 

 substance. And though thus similar in the mechan- 

 ism of their development, tissues differ from each 

 other, not only in shape but also in endozvments — 

 physical and vital — because they are developed each 

 tissue from a different kind of cells. In short, let 

 me reiterate : different cells, though they may 

 resemble each other in external aspect and general 

 points of structure, are potentially different in their 

 internal qualities. No transmutation, therefore, can 

 under any change of circumstances, take place of 

 one kind of cell into the tissue which is the proper 

 product of the development of another kind of cell. 



The ontogenesis of Protozoa, we have seen, is 

 literally nothing more than an example of simple cell 

 development. In the case of animals generally, the 

 individual originates from an ovum or egg. The 

 first formation of the ovum, however, as well as its 

 development into the young animal, may be viewed 

 as merely different phases of the same process of 

 cell-life. Professor^Haeckel regards the ovum itself, 



