CHAPTER II. 



ORIGIN OF TISSUES. EPITHELIAL TISSUES : STRATIFIED, TRAN- 

 SITIONAL, SIMPLE. 



Origin of tissues. In the first chapter we stated that the body 

 is made up of different parts or organs, each having some 

 special work to do; each part is made up of tissues, of which 

 there are four distinctive kinds ; each tissue is made up of 

 structural elements, cells and fibres ; and finally, each fibre 

 being a modified cell, the cell is the basis of all the bodily 

 structures. 



Thus, in the early embryo, the whole body is an agglomera- 

 tion of cells. These have all been formed from the ovum or 

 egg-cell, which divides into two cells; these again into two, 

 and so on until numbers of cells are produced. Eventually 

 these cells arrange themselves in the form of a skin or mem- 

 brane which is composed of three layers. These layers are 

 known respectively as the epiblast, or upper layer; the meso- 

 blast, or middle layer; the hypollast, or under layer. The 

 epiblast is supposed to give rise to the nervous tissue and most 

 of the epithelial tissue ; the mesoblast to the connective and 

 muscular tissues; the hypoblast to the rest of the epithelial 

 tissue. Of these tissues, the epithelial is the simplest, and most 

 nearly allied to the primitive tissue, and will first engage our 

 attention. 



Epithelial tissue. Epithelial tissue is composed entirely of 

 cells united together by cohesive matter. The cells are gener- 

 ally so arranged as to form a skin or membrane, covering the 

 external and internal surfaces of the body. This membrane is 

 seen when the skin is blistered, the thin and nearly transparent 

 membrane raised from the surface being epithelial tissue in 

 this situation called epidermis, because it lies upon the surface 



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