CHAPTER V 



DEVELOPMENT OF TISSUES AND ORGANS 



25. WE have seen that the first organized 

 matter that forms the physical basis of life 

 is protoplasm. Then appears the more 

 specialized form, the cell, and from the 

 primitive cells the layers of the embryo are 

 developed. From these layers the various 

 organs and tissues of the future being are 

 produced. All embryonic tissues are very 

 similar to each other, consisting of protoplasm, 

 in which appear nuclei and cells in a more or 

 less rapid state of transition. From these the 

 various tissues are formed. The cells cover- 

 ing surfaces, either external, or those lining the 

 alimentary canal and the ducts and pouches 

 of the various glands, form a layer termed 

 epithelium. Such epithelial cells may be 

 close together to form a single layer, or the 



layer may consist of cells three or four deep, 

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