FROM TWELVE TO THIRTY-SIX SOMITES 



157 



appear to form mesenchyme. The formation of mesenchyme 

 from certain parts of the neural crest is most marked in the 

 region of the brain. 



The primordia of the ganglia contain the cells (neuroblasts) 

 which form the dorsal root fibers of the spinal nerves and parts 

 of certain cranial nerves. They also appear to contain the cells 

 from which the sheaths of the nerve fibers are formed; thus 

 three kinds of cells at least are found in the neural crest, viz., 

 mesenchyme forming cells, neuroblasts, and sheath cells. 



The Cranial Neural Crest and its Derivatives. The neural 

 crest in the head may be divided into pre- and post-otic divisions, 

 and these arise at different times. 



Su/. cer. 



n.Cr. 



Mch. 



FIG. 90. Transverse section of , the fore-brain, and optic 

 vesicles at the stage of 7 s. ^ 



M'ch., Mesenchyme. n. Cr., Neural crest. Ph., Phar- 

 ynx. Sut. cer., Anterior cerebral suture. X., Mass of cells 

 in which the anterior end of the intestine, the neural tube 

 and the notochord fuse. 



(1) The pre-otic division, which extends from the extreme 

 anterior end of the neural tube to about the center of the audi- 

 tory pit, is well developed at a stage of 7-8 somites, but it is not 

 found at the 5 s stage. The origin is everywhere the same, viz., 

 from the dorsalmost cells of the medullary plate and the ecto- 

 derm immediately adjacent; it arises at the time of contact of 

 the medullary folds and is thus thickest in the region of the 

 suture. Fig. 90 is a section through the developing optic vesicles, 

 and shows the neural crest continuous with the tube and ectoderm 



