CHAPTER X 

 HISTOGENESIS 



The primitive cells of the embryo are alike in structure. The protoplasm of 

 each exhibits the fundamental properties of irritability, contractility, conductivity 

 and metabolism (the absorption, digestion, and assimilation of nutritive sub- 

 stances and the excretion of waste products, processes through which growth 

 and reproduction are made possible). As development proceeds, there is a 

 gradual differentiation of the cells into tissues, each tissue being composed of like 

 cells, the structure of which has been adapted to the performance of a certain 

 special function. In other words, there is division of labor and adaptation of cell 

 structure to the function which each cell performs. The differentiation of tissue 

 cells from the primitive cells of the embryo is known as his to genesis. On page 64 

 the derivatives of the germ layers are given. We shall take up briefly the histo- 

 genesis of the tissues derived from the endoderm, mesoderm, and ectoderm in the 

 order named. 



THE HISTOGENESIS OF THE ENTODERMAL EPITHELIUM 



The cells of the entoderm are little modified from their primitive structure. 

 From the first they are concerned with the processes of absorption, digestion, 

 assimilation and excretion. They form always epithelial layers lining the di- 

 gestive and respiratory canals and the glandular derivatives of these. In the 

 pharynx, esophagus and trachea the cells are early of columnar form and ciliated. 

 The epithelium of the pharynx and esophagus becomes stratified and the surface 

 layers flatten to form squamous cells. The stratified epithelium is developed 

 from a basal germinal layer like the epidermis of the integument (see p. 304). 

 Throughout the rest of the digestive canal the simple columnar epithelium of the 

 embryo persists. At the free ends of the majority of the cells is developed a cutic- 

 ular plate. Other cells are converted into unicellular mucous glands or goblet 

 cells. As outgrowths of the intestinal epithelium, are developed the simple tu- 

 bular glands of the stomach and intestine and the liver and pancreas. 



In the respiratory tract the entoderm forms at first a simple columnar epi- 

 thelium. Later, in the trachea and bronchi this is differentiated into a pseudo- 



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