8 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



been fought, and with which, perhaps, no few super- 

 stitions are or have been connected. 



We will, therefore, follow those who have agreed 

 to the suggestion of Prof. Haeckel, and will use for 

 the elementary organism, whether or no provided with 

 a nucleus, the useful and suggestive term of plastid. 

 This plastid, or unit of organic structure, is com- 

 posed of protoplasm ; it may be without a nucleus, 

 when it is a cytod (or cell-like body), or it may have 

 within it a denser mass, which is very feebly, if at all, 

 contractile, the nucleus; in which case it is a cell. 

 This nucleus is ordinarily provided with one or more 

 smaller nucleoli, and, possibly, always has a distinct 

 investing membrane. It would appear to have a 

 special chemical composition, inasmuch as while a cell 

 when treated with a ten per cent, salt solution leaves 

 a precipitate, no such precipitate is stated to be found 

 when a cytod is subjected to the same reagent. The 

 body so precipitated has been called nuclein. 



Protoplasm, then, is presented to us in the form 

 of plastids, and these plastids may either be without 

 (cytods) or have (cells) distinct nuclei. All organisms 

 are composed of one or more cells, or, in other words, 

 are either unicellular or multicellular. The former, 

 as much as the latter, are capable of exhibiting all 

 the .essential phenomena of life. 



TISSUES AND ORGANS. 



"When we examine the different stages in the 

 history of a developing animal, or compare a series 

 which commences with low and passes through more 

 highly developed forms, we find a gradual increase in 

 the complexity of the parts ; of this we have already 

 had an example in comparing the cytod with the cell, 

 and we shall observe it in every chapter of this work. 

 This increase in complexity is termed the process of 

 differentiation. 



