3°4 



HISTOGENESIS 



here the histogenesis of the epidermis and the development of its derivatives and 

 the histogenesis of the nervous tissues, reserving for final chapters the develop- 

 ment 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 inner layer 

 of cuboidal or columnar cells, the stratum germinativum, and an outer layer of 

 flattened cells, the epitrichium or periderm (Fig. 292 A). 



Epitrichium 



Stratum . s 

 germinativum -3L_ 



Conum 



Epitrichium 



intermed- 

 iate layer~ 



Stratum 



qerminaT- ^ 

 iyum 



Corium 



f^Z^-- Ko^K«.rrr*C Mill 



Fig. 292. — Sections of the integument from a 65 mm. embryo. A, section through the integument 

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

 from the integument of the chin in which three layers are well developed in the epidermis. X 440. 



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 30 to 

 88 mm. 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 being the stratum germinativum (Fig. 292 B). This con- 

 dition may persist until the end of the fourth month. After the fourth month 

 the epidermis becomes many layered. The inner layers of cells now form the 

 stratum germinativum and are actively dividing cells united with each other by 



