NERVOl'S SYSTEM. 



43 



ECTODERMAL STRUCTURES. 



THE derivatives of the ectoderm may be subdivided into 

 epidermal, nervous, and sensory organs, the differentiation of 

 the nervous from the epidermal structures beginning with or 

 even before the completion of the process of gastrulation. 



THE CENTRAL NERVOUS SYSTEM. 



The central nervous system begins its development as a 

 structure distinct from the rest of the ectoderm by the forma- 

 tion of a neural or medullary plate on the dorsal surface of the 

 embryo. On either side of the primitive groove (fused blasto- 

 poral lips, p. /) the ectodermal cells become elongated (cylin- 

 drical or fusiform^, while in those regions destined to give rise 

 to the epidermis they retain their more flattened character, the 

 line between the two regions being sharply drawn. Soon after 

 being outlined the lateral edges of the medullary plate begin to 

 bend upwards and 

 i n w a r d s, the 

 whole thus form- 

 ing a medullary 

 groove bounded 

 by the medullary 

 folds, the outer 

 portion of each 



FlG. 45. Section through embryo Acanthias before 

 the closure of the medullary groove, g. f, eoMom: <, 

 entoilorm : rv. ectoderm : at, mesotheliumj ;/, notochord. 



fold being formed 



by unaltered ectoderm (Fig. 45"). This inward bending of the 

 medullary folds continues until the edges meet, the medullary 

 plate being converted by this process into the walls of a tube, 

 which later develops into brain and spinal cord. The edges of 

 the fold now fuse. neural parts with neural, epidermal with 

 epidermal, so that the nervous portion becomes internal, and 



