AMCEBA. 67 



Now, in the development of higher animals, while all the 

 cells which result from the original cell (egg) are at first 

 essentially alike, not only in structure but in properties, 

 they grow unlike in this latter respect as well; that is, 

 while all the cells have at first in equal measure the proper- 

 ties of motion, sensation, digestion, respiration, assimi- 

 lation, and reproduction, each set (tissue) soon develops 

 in a special degree some one of these properties. Take, for 

 instance, the nervous tissue ; the cells composing this tissue 

 are regarded as having had originally not only irritability 

 but contractility, and all the other characters enumerated 

 above in describing the amoeba ; but this kind of tissue has 

 developed, in a high degree, the property of irritability, and 

 has lost, in a large measure, the other properties ; so the 

 muscular tissue, while it has not wholly lost its irritability, 

 has it feebly developed when compared with nervous tissue, 

 but has the power of contraction in a very high degree. 

 Thus each tissue has some one of the general properties in 

 a very marked degree, while the other properties are less 

 apparent, though seldom entirely wanting. All the tissues 

 work together for the common good of the whole animal ; 

 all the tissues grow by the increase of the number of their 

 cells, and this is accomplished by the division of the cells ; 

 but that division of cells which gives rise to new individ- 

 uals is limited to the reproductive tissues. 



Although one set of tissues (the organs of digestion) 

 has the chief work of preparing the food to be built into 

 the tissues of the body, yet each cell must take for itself 

 the food thus prepared for it, and, really, each cell leads an 

 independent life bathed in the liquid part of the blood, 

 which soaks through the walls of the blood-vessels and 

 surrounds every cell ; from this liquid nourishment is 



