THE EPIDERMIS 



293 



HISTOGENESIS OF THE ECTODERMAL DERIVATIVES 



Besides forming the enamel of the teeth and the salivary glands 

 (p. 1531!), the ectoderm gives rise : (i) to the epidermis and its derivatives 

 (subcutaneous glands, nails, hair, and the lens and conjunctiva of the 

 eye); (2) to the nervous system and sensory epithelia; (3) to parts of 

 certain glands producing internal secretions, such as the pituitary body, 

 suprarenal glands, and chromaffin bodies. 



We shall describe here the histogenesis of the epidermis, the develop- 

 ment of its derivatives, and the histogenesis of the nervous tissues, re- 

 serving for final chapters the development of the nervous organs and the 

 glands formed in part from them. 



THE EPIDERMIS 



The single-layered ectoderm of the early embryo, by the multiplica- 

 tion of its cells, becomes differentiated into a two-layered epidermis, 



Epitrichium 



Intermediate layer 

 Stratum germinativum 



FIG. 299. Sections of the integument from a 65 mm. human fetus. X 440. A, From 

 the neck, showing a two-layered epidermis and the beginning of a third intermediate layer; 

 B, from the chin, with three well developed epidermal layers. 



composed of an inner layer of cuboidal or columnar cells, the stratum 

 germinativum, and an outer layer of flattened cells, the epitrichium, or 

 periderm (Fig. 299 A}. 



The stratum germinativum is the reproducing layer of the epidermis. 

 As development proceeds, its cells divide and gradually give rise to new 

 layers above, until the epidermis becomes a many-layered, or stratified 

 epithelium. The periderm is always the outermost layer of the epidermis. 

 During the third and fourth months the epidermis is typically three- 

 layered, the outer, flattened layer forming the periderm, a middle layer of 

 polygonal cells, the intermediate layer, and the inner, columnar layer, the 



