294 HISTOGENESIS 



The Histogenesis of the Ectodermal Derivatives 



Besides forming the enamel of the teeth and the sahvary glands (cf. p. 161), 

 the ectoderm gives rise: (1) to the epidermis and its derivatives (subcutaneous 

 glands, nails, hair, and the lens and conjunctiva of the eye) ; (2) to the nervous sys- 

 tem and sensory epithelia; (3) to parts of certain glands producing internal secre- 

 tions such as the pituitary body, suprarenal glands, and chromaffin bodies. We 

 shall describe here the histogenesis of the epidermis, the development of its 

 derivatives, and the histogenesis of the nervous tissues, reserving for final chap- 

 ters 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 division of its cells 

 becomes differentiated into a two-layered epidermis composed of an iimer 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 by division gradually give rise to new layers above 

 it until the epidermis becomes a many-layered or stratified epithelium. The 

 periderm is always the outermost layer of the epidermis. In embryos of 25 to 121 

 mm. (C R) 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 stratum germinativum (Fig. 299 B). After 

 the fourth month the epidermis becomes many layered. The inner layers of ceUs 

 now form the stratum germinativum and are actively dividing cells united with 

 each other by cytoplasmic bridges. The outer layers of cells become cornified, 

 the cornification of the cells proceeding from the stratum germinativum toward the 

 surface. Thus, next the germinal layer are cells containing keratohyalin, which 

 constitute the double-layered stratum granulosum. A thicker layer above the 

 stratum granulosum shows cells in which drops of a substance called eleidin are 

 formed. These droplets, which are supposed to represent softened keratohyahn, 

 give these cells a clear appearance when examined unstained. Hence the layer is 

 termed the stratum lucidum. In the outer layers of the epidermis the tMckened 

 walls of the cells become cornified and in the cells themselves a fatty substance 

 collects. These layers of cells constitute the stratum corneum. The ceUs of this 

 layer are also greatly flattened, especially at the surface. 



When the hairs develop they do not penetrate the outer periderm layer of 

 the epidermis, but, as they grow out, hft it off (sixth month). Hence tliis laver 



