60 THE LIVING WORLD. 



Secondly, from the study of embryology we find 

 that all animals and all plants begin with their de- 

 velopment as single cells, and since embryology re- 

 peats past history this of course points to a former 

 unicellular condition of early animals. Here, then, 

 is our first sure starting-point. For a diffused mass 

 of protoplasm at the beginning of life we have only 

 a speculative foundation, but for a unicellular con- 

 dition of early life we have direct evidence. 



Cells. 



Before tracing the history farther, it is well there- 

 fore to say a word in regard to the meaning of the 

 term cell, as it is understood to-day by naturalists. 

 As originally used, the word cell was a good de- 

 scription of the object so named. The microscopic 

 study of plants showed long ago that they are in all 

 cases composed of a larger or smaller number of 

 little separate compartments or little boxes, like a 

 mass of honeycomb. Each box has a wall and con- 

 tains a mass of the living substance, the protoplasm. 

 These were named cells. With such objects as a 

 starting-point, for which the term cell was a good 

 description, the word has been extended in its appli- 

 cation until it has come to be applied to a very 

 different set of objects. It was soon found that all 

 plants and all animals were composed of somewhat 

 similar independent parts and the name cell was 

 naturally extended to animals as well as plants. 

 Inside of these cells there was subsequently shown 

 to be frequently present a bit of dense protoplasm 

 quite distinct from the rest, which received the name 



