CHAPTER IV. 



NERVOUS TISSUE. 



Nervous tissue develops from the ectoderm. At an early stage in the 

 development of the embryo, after it has elongated, a dorsal longitudinal 

 groove, the medullary groove (or neural groove) [Fig. 26], forms, 

 and the edges of this groove soon come together, forming the medullary 

 tube (neural tube) [Fig. 27]. The walls of the anterior part of this 

 tube become later very thick, forming the brain [Fig. 28] ; with relatively 

 small cavities, the ventricles, representing the original cavity of this part 

 of the tube. The walls of the posterior part of the tube thicken to a less 



Fig. 26. Transverse section of ferret embryo, showing medullary (neural) groove. 

 (Cunningham, Anatomy.) EC, ectoderm. EN, endoderm. GC, germinal cell. N, 

 notochord. NG, neural groove. PM, paraxial mesoderm. SpM, splanchnic meso- 

 derm. SoM, somatic mesoderm. SB, spongioblast. SG, spinal ganglion. 



SoP 



8»P 



SpP 



IMC 



Fig. 27. Transverse section of ferret embryo of greater age than shown in Fig. 26, 

 showing medullary canal. (Cunningham, Anatomy.) NC, neural crest. CC, cen- 

 tral canal. SG, spinal ganglia. 



