CHAPTER X 



HISTOGENESIS 



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

 each exhibits the fundamental properties of irritability, contractihty, reproduc- 

 tion, 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 histogenesis. On page 54 

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

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

 order named. 



The Histogenesis of the Entodermal Epithelium 



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

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

 assimilation, and excretion. They form always epitheHal layers, Kning 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 cihated. 

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

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

 from a basal germinal layer like the epidermis of the integument (see p. 294). 

 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 a cuticular mem- 

 brane develops. 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- 

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

 stratified, ciliated epithehum. The columnar epithehum of the alveoli and alveo- 



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