8 



MORPHOLOGY OF THE 



The dorsal zones become widely separated from each other, 

 and as a result of this the roof plate is very much widened. 

 (Fig. 4.) The central canal presents a five-sided figure. 

 Tlie same process continues, the dorsal zones grow farther 



Fig. 4. 



(Minot, 374.) Section of the 

 medulla oblongata of a five Aveeks' 

 human embryo. (His's Ko.) D, 

 dorsal zone of His. F, ven- 

 tral zone. XI, accessorius. XII, 

 hypoglossus. After W. His. 



Fig. 5. 



(Minot, 379.) Transverse section of the 

 medulla oblongata of His's embryo. Ru 

 (Nackenliinge 9.1 mm.), after W. His. RL^ 

 Rautenlippe. Ts, tractus solitarius. X, vagus 

 nerve. .17/, hypoglossal nerve. 



and farther apart, until the ventral zones come to lie in 

 nearly the same plane. 



The oval bundle (which is now known as the tractus 

 solitarius, as identified by His) can be seen in the Rand- 

 schleier of the dorsal zone. 



It is here formed by the posterior nerve roots of the 

 IX and X nerves, which are homologous to the sensitive 

 roots of the spinal nerves. (Fig. 5.) 



At the junction of the roof with the dorsal zone, the 

 rhomboidal lip is developed. These two lips grow down- 



