CHAPTER III. 



THE DIFFEKENTIATION OF THE TISSUES AND 

 THE PHYSIOLOGICAL DIVISION OF EMPLOY- 

 MENTS. 



Development. Every Human Body commences its indi- 

 vidual existence as a single nucleated cell. This cell, 

 known as the ovum, divides or segments and gives rise to 



FIG. 8. A, an ovum ; B to E, successive stages in ite segmentation until the 

 morula, F, is produced. 



a mass consisting of a number of similar units and called 

 the mulberry mass or the morula. At this stage, long 

 before birth, ihere are no distinguishable tissues entering 

 into the structure of the Body, nor are any organs recog- 

 nizable. 



For a short time the morula increases in size by the 

 growth and division of its cells, but very soon new pro- 

 cesses occur which ultimately give rise to the complex 



