422 



EMBRYOLOGY OF THE BRAIN AND SPINAL CORD. 



end in the nuclei pontis. The pyramidal fibers to motor nuclei of 

 spinal nerves grow down through the pons, intersecting its trans- 

 verse fibers. 



MYELENCEPHALON. 



The myelencephalon forms the medulla oblongata (Figs. 118, 

 I, and 122). It is constricted off from the metencephalon at the 

 twenty- eighth day; but, later, that constriction largely disappears 

 and the common cavity of the two vesicles, broad in the middle 



V 



Fig. 125. Transverse section of medulla from an embryo of 91. mm. (McMur- 



rich after His.} 



dz. Dorsal zone. fp. Floor-plate, fs. Tractus solitarius. I. Lip. rp. Roof -plate, vz. Ven- 

 tral zone. X and XII. Tenth and twelfth nerves. 



and contracted to a slender canal at each end, persists as the 

 fourth ventricle of the mature brain. Like the metencephalon, 

 the fifth brain vesicle is divided at the third week into a ventral 

 and a dorsal zone by a deep furrow on the ventricular surface of each 

 lateral wall (Figs. 125 and 126). That lateral furrow actually per- 

 sists in the sulcus limitans and inferior fovea of the fourth ventricle. 

 The roof -plate of the superior half of the myelencephalon stretches 

 out widely and remains a single layer of epithelial cells. It forms 

 no nerve tissue. Other portions of the myelencephalon develop 

 quite uniformly. But by the expansion of the roof, just mentioned, 

 the dorsal extremities of the lateral walls are pushed outward and 



