CHAPTER III. 



THE DIFFERENTIATION OF THE TISSUES AND THE 

 PHYSIOLOGICAL DIVISION OF EMPLOYMENTS. 



Development. Every Human Body commences its indi- 

 idual existence as a single nucleated cell. This cell, known 

 s the ovum, divides or segments and gives rise to a mass con- 



FtG. llA.-^l, an ovum; B to E, successive stages in its segmentation until the 

 orula, F, is produced; a, cell-sac; 6, cell contents; c, nucleus. 



sting of a number of similar units and called the mulberry 

 mss or the morula. At this stage, long before birth, there 

 re no distinguishable tissues entering into the structure of 



Body, nor are any organs recognizable. 



For a short time the morula increases in size by the 

 rowth and division of its cells, but very soon new processes 

 ccur which ultimately give rise to the complex-adult body 

 ith its many tissues and organs. Groups of cells ceasing to 

 row and multiply like their parents begin to grow in ways 

 eculiar to themselves, and so come to differ both from the 

 riginal cells of the morula and from the cells of other groups, 

 ad this unlikeness becoming more and more marked, a 

 iried whole is finally built up from one originally alike in 



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