DEVELOPMENT OF THE MEDULLA OBLOXGATA. 65 



reduced to a thin layer of flattened epithelium, the substance of this part of the 

 medulla oblongata is wholly formed by a thickening of the shifted lateral boundaries. 

 In these, the bend marking the distinction between the ventral and dorsal laminae 

 now by change of position mesial and external continues to be evident, and is in 

 fact recognizable even in sections of the fully -developed brain. 



Of the longitudinal columns of the medulla oblongata the restiform bodies first become 

 prominent (third month in the human embryo). The (anterior) pyramids are obvious in the fifth 

 month, and the olivary tubercle about the sixth. But before any of these, and indeed with 



Fig. 71. SECTIONS ACROSS THE REGION OP THE CALAMUS SCRIPTORIUS OF THE BRAIN REPRESENTED 



IN FIG. 70, A. (His.) 



A, region of the glossopharyngeal ganglion. 



B, of the auditory-facial ganglion. 



Fig. 72. SECTIONS ACROSS THE FOURTH VENTRICLE OF A SOMEWHAT OLDER EMBRYO (His.) 



A, section taken through the lower part. 



B, across the widest part (trigeminus region). 



C, through upper part (cerebellar region). 



r, roof of neural canal : al, alar lamina ; bl, basal lamina ; v, ventral border. 



Fig. 73. SECTIONS ACROSS THE LOWER HALF OF THE FOURTH VENTRICLE OF A STILL OLDKR EMBRYO, 



SHOWING GRADUAL OPENING OUT OF THE NEURAL CANAL AND THE COMMENCING FOLDING OVER OF 

 THE ALAR LAMINA (at/). 



r, ventral border ; t, tsenia ; ot, otic vesicle ; rl, recessus labyrinth i. 



In the succeeding stage (not here represented) the angle at v has almost disappeared, the fold/ has 

 extended over the alar lamina, and the two thickened halves are in the same horizontal plane, covered 

 by a greatly expanded and thinned out roof. 



VOL. I. F 



