DIFFERENTIATION OF THE CELLS. 301 



layer arises solely the outer skin (epidermis), together 

 with the central parts of the nervous system (spinal marrow 

 and brain) ; out of the lower or inner layer arises only 

 the inner delicate skin (epithelium) which lines the whole 

 intestinal tube from the mouth to the anus, together with 

 all tlie glands connected with it (lung, liver, salivary 

 glands, etc) ; out of the middle germ-layer lying between 

 the two others arise all the other organs, muscles, bones, 

 blood-vessels. Xow, the processes by which the various and 

 exceedingly complicated parts of the fully-formed body of 

 vertebrate animals arise out of such simple material — out of 

 the three germ -layers composed only of cells — are, in the 

 first place, the repeated division, and consequently the 

 increase of cells ; in the second place, the division of labour 

 or differentiation of these cells; and thirdly, the union of 

 the variously developed or differentiated cells, for the 

 formation of the different organs. Thus arises the gradual 

 progress or perfecting which can be traced step by step 

 in the development of the embryonic body. The simple 

 embryonic cells, which are to constitute the body of the 

 vertebrate animal, stand in the same relation to each other 

 as citizens who v/ish to found a state. Some take to one 

 occupation, others to another, and work together for the 

 good of the whole. By this division of labour, or differen- 

 tiation, and the perfecting (the organic progress) which is 

 connected with it, it becomes possible for the whole state to 

 accomplish undertakings which would have been impossible 

 to the single individual. The whole body of the vertebrate 

 animal, like every other many-celled organism, is a republi- 

 can state of cells, and consequently it can accomplish organic 

 functions which the individual cell, as a solitary individual 



