XIV.] LIVING MATTER AND ITS EFFECTS. 225 



and one of the products of this oxidation, carbonic acid, 

 is returned to the atmosphere. After death, the process of 

 decay is accompanied by slow oxidation. The carbon goes off 

 chiefly in the form of gaseous carbonic acid ; the nitrogen, 

 in the shape of animoniacal salts ; and the mineral salts are 

 dissolved away by rain and carried to the general store of 

 the waters. But if, by the overflow of a river, the plant 

 should become silted up in mud, or carried away by floods 

 and buried in the sea-bottom, the process of decay may be 

 so slow and so imperfect, that its carbonized remains, often 

 infiltrated with mineral matter, may be preserved as a 

 "fossil" ^ when the mud is hardened into a stone, and 

 thus permanently contribute to the sohd land. 



So much for the plant ; let us now turn to the animal. The 

 laid egg of a pigeon answers to the ripe pea. Within the 

 shell, and suspended in the white of the egg, is the rounded 

 yellow mass of the yolk, and on one side of the yolk is a 

 small round patch — the cicatricula? Though apparently 

 homogeneous, the microscope shows that the cicatricula is 

 made up of minute nucleated cells ; and this cell-aggregate 

 is an embryo pigeon, just in the same sense as the little 

 plant within the coat of the pea is an embryo pea-plant ; 

 though it is far less like a pigeon than the latter is like a 

 pea-plant. 



The embryo pigeon, like the embryo plant, contains pro- 

 tein compounds, fats, mineral salts and water. The yolk 

 in which it lies is composed of similar materials; but no 

 starch or cellulose enters into its composition. 



The cicatricula exhibits no more signs of life than the 



^ Fossil, IjsX. fossilis, from/odio, to dig ; a term applied by old writers 

 to anything dng out of the earth, and therefore including minerals, 

 but now conventionally restricted to organic remains. 



- Cicatricula, Lat. diminutive of cicatrix, a scar. 



Q 



