TISSUES, ORGANS, AND PHYSIOLOGICAL SYSTEMS 31 



process itself in this case is known as the differentiation of the 

 tissues, since by it they are, so to speak, separated or specialized 

 from the general mass of mother-cells forming the morula. 



As the differences in the form and structure of the constituent 

 cells of the morula become marked, differences in property arise, 

 and it becomes obvious that the whole cell-aggregate is not 

 destined to give rise to a collection of independent living things, 

 but to form a single human being, in whom each part, while main- 

 taining its own life, shall have duties to perform for the good of 

 the whole. In other words, a single compound individual is to be 

 built up by the union and cooperation of a number of simple ones 

 represented by the various cells, each of which thenceforth, while 

 primarily looking after its own interests and having its own 

 peculiar faculties, has at the same time its activities subordinated 

 to the good of the entire community. 



The Physiological Division of Labor. As the differentiation 

 of tissues proceeds the fundamental physiological properties, orig- 

 inally exhibited in equal degree by all the cells, become distrib- 

 uted among the various tissues. Thus we find certain tissues 

 adapted to execute contractions and in these the property of con- 

 tractility is developed to an especial degree. Other tissues, on the 

 other hand, show little or no contractility but exhibit a marked 

 degree of conductivity. The higher we look in the animal scale 

 the more marked becomes this division of physiological duties 

 among the tissues. In man it attains its highest development. 



Classification of the Tissues. As we might separate the in- 

 habitants of the United States into groups, such as lawyers, doc- 

 tors, clergymen, merchants, farmers, and so forth, so we may clas- 

 sify the tissues by selecting the most distinctive properties of each 

 of those entering into the construction of the adult Body and 

 arranging them into physiological groups; 'those of each group 

 being characterized by some one prominent employment. No 

 such classification, however, can be more than approximately 

 accurate, since the same tissue has often more than one well- 

 marked physiological property. The following arrangement, how- 

 ever, is practically convenient. 



1. UNDIFFERENTIATED TISSUES. These are composed of cells 

 which have developed along no one special line, but retain very 

 much the form and properties of the cells forming the very young 



