CHAPTER III 



TISSUES, ORGANS, AND PHYSIOLOGICAL SYSTEMS 



Development. Every Human Body commences its individual 

 existence as a single nucleated cell. This cell, known as the ovum, 

 divides or segments and gives rise to a mass consisting of a number 

 A B 



FIG. 11. A, an ovum; B to E, successive stages in its segmentation until the 

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



of similar units and called the mulberry mass or the morula. At 

 this stage, long before birth, there 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 processes occur which ulti- 

 mately give rise to the complex adult body with its many tissues 

 and organs. Groups of cells ceasing to grow and multiply like 

 their parents begin to grow in ways peculiar to themselves, and 

 so come to differ both from the original cells of the morula and 

 from the cells of other groups, and this unlikeness becoming more 

 and more marked, a varied whole is finally built up from one 

 originally alike in all its parts. Peculiar growth of this kind, form- 

 ing a complex from a simple whole, is called development; and the 



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