26 AN INTRODUCTION TO ZOOLOGY. 



production and growth of new individuals, called 

 reproductive organs : the feeling cells form nerves, 

 ganglia, and brains; the cells for moving form muscles. 

 Then, because these groups of cells are now each 

 cut o£E from the cells that perform other functions for 

 them, they want special structures to provide for their 

 special wants : lymph-vessels and blood-vessels, to 

 carry them food, and connective tissue, to bind them 

 together. Lastly, the whole body now requires 

 gristles and bones, to support it; and a brain, to 

 gather general information as to the feelings of all 

 parts, and to send messages to tell the muscles how 

 to move. 



All these different things are foi-med by the growth 

 and division and development of cells which were 

 originally alike, a process shortly known as diflfer- 

 entiation, i.e. the establishment of different parts, 

 with distinguishing characteristics. This term may 

 be applied to a single cell when its parts develop 

 characters which distinguish them from one another, 

 or, in like manner, to a whole organism. This process 

 takes place during the egg stage of animal life, which 

 will presently be described in full. (See Chap. V.) 

 The young animal in the egg begins as a single cell ; 

 this cell divides to form a number of cells all much 

 alike; then, when a large number of these have been 

 formed, certain cells begin to take different shapes from 

 others, and their differences increase until separate 

 tissues, or masses of similar cells, are formed : and 

 these, by progressive modifications, assume the char- 



