424 



EMBRYOLOGY OF THE BRAIN AND SPINAL CORD. 



fibers on the lateral surface ventral to the solitary tract. They 

 constitute the spinal tract of the trigeminal nerve. Soon after the 

 appearance of the solitary tract and the spinal tract of the fifth 

 nerve, the posterior margin of the dorsal lamina is folded out- 

 ward and forward until it rests upon the external surface. That 

 fold, which is called the rhombic lip, covers both the above bundles 

 of fibers and places them in the position they occupy in the mature 

 brain. Between the rhombic lips the roof epithelium stretches 

 across the ventricle and pushes a transverse fold into it. The 

 mesoblast which dips into that fold develops the chorioid plexus 

 of the fourth ventricle (Fig. 91). 



TABLE X. 



DERIVATIVES OF MYELENCEPHALON. 



(Modified from McMurrich.) 



Roof-plate 



Dorsal Zone 



Ventral Zone 



Surrounding Mesoblast 



Roof epithelium 



Nuclei funiculi gracilis, and cuneati, 

 and nucleus tractus spinalis n. tri- 

 gemini. 



Olivary nuclei (inferior) 

 -] Arcuate nuclei 



Terminal nuclei of sensory roots of cere- 

 bral nerves (intermediate, eighth, ninth 

 and tenth) 



Sensory tracts grow up through it. 



Nucleus lateralis inferior 



Genetic nuclei of motor roots and nerves, 



ninth, tenth and eleventh (its bulbar 



root) and twelfth 

 Motor tract (pyramid) grows down 



through it. 



Meninges 



Chorioid plexus of fourth ventricle. 



The substance of the myelencephalon, like other divisions 

 of the neural tube, presents under the microscope three distinct 

 layers at the fourth week of embryonic life (Fig. 125). The 



