CHAPTER X 

 HISTOGENESIS 



THE primitive cells of the embryo are alike in structure. The pro- 

 toplasm of each exhibits the fundamental properties of irritability, con- 

 tractility, reproduction, and metabolism (the absorption, digestion, and 

 assimilation of nutritive substances 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 histogenesis. 

 On page 56 the derivatives of the germ layers were given. We shall now 

 take up briefly the histogenesis of the tissues derived from the entoderm, 

 mesoderm, and ectoderm. 



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 absorp- 

 tion, digestion, assimilation, and excretion. They form always epithelial 

 layers, lining the digestive 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. 294). Through- 

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

 embryo persists. At the free ends of the majority of the cells a cuticular 

 membrane develops. Other cells are converted into unicellular mucous 

 glands, or goblet cells. As outgrowths of the intestinal epithelium, are 

 developed the simple tubular glands of the stomach and intestine, and the 

 liver and pancreas. 



In the respiratory tract, the entoderm forms at first a simple columnar 

 epithelium. Later, in the trachea and bronchi this is differentiated into a 

 pseudostratified, ciliated epithelium. The columnar epithelium of the 



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