ANATOMY OF A 17.8 MM. HUMAN EMBRYO 53 



plate, caiidad of the isthmus. It will form the corresponding 

 lateral portion of the cerebellmn. The vermis is believed to be 

 developed from the slightly thickened roof-plate intervening 

 between these thickenings. 



The ventral zones (Z.vent.) of the metencephalon, which form 

 the pons, are deep longitudinal bands separated by a thick median 

 raphe which represents the floor-plate. They extend caudad 

 to the angle made by the pontal flexure where they blend with 

 the corresponding zones of the medulla. 



Myelencephalon. The myelencephalon (Myelen.) is the re- 

 maining portion of the brain, which arches over the cervical 

 flexure {Flexxerv.) and joins the spinal cord. The roof-plate is 

 exceedingly thin. It is widest in the region of the plica choroidea 

 posterior where it forms the roof and the dorsal half of the lateral 

 wall. Its caudal extension gradually tapers out to pass into the 

 narrow roof-plate of the spinal cord. It becomes the caudal part 

 of the posterior medullary velum. 



The dorsal zones are thick. In the region near the spinal cord 

 they are nearly vertical, but, by becoming progressively oblique, 

 their internal surfaces form the lateral region of the ventricular 

 floor at the cephalic end of the myelencephalon. The ventral 

 zones, as in the metencephalon, are thick longitudinal bands on 

 either side of the median line. In the floor of the ventricle a 

 median longitudinal groove extends between them. Ventral 

 to the groove a thickened raphe, floor-plate, unites the ventral 

 zones. As the raphe approaches the region of the spinal cord it 

 gradually becomes thinner. 



Spinal cord 



The spinal cord (Md.sp.) is represented in median sagittal 

 section in plates 1 and 4. The surface is shown, in part, in plates 

 2 and 3. It has a narrow slit-like cavity, somewhat expanded 

 dorsally. The lateral walls of the cord have clearly marked ven- 

 tral and dorsal zones which are continued into the corresponding 

 zones of the myelencephalon. 



