198 EMBRYOLOGY. 



medullary groove. Before the closure of this groove to form the 

 medullary canal an offshoot from the epiblast may be observed, 

 which is the source of the posterior nerve-roots ; and they become 

 attached to the cord as it develops. The anterior roots spring from 

 the cord after it has developed fibres. The two roots then join and 

 the nerve grows out into the mesoblast. 



Are the cranial nerves similarly derived? 



In much the same way the cranial nerves arise primarily, except 

 the nerves of special sense. In function the motor nerves seem to 

 form a sort of anterior root for the sensory, so that they may be 

 arranged in pairs corresponding to the anterior and posterior roots 

 of the spinal nerves ; and it does not seem entirely fanciful to re- 

 gard their development as somewhat similar, thus : 



Third, fourth, sixth, and seventh motor, fifth sensory. Twelfth 

 motor, ninth sensory. Eleventh motor, tenth sensory. 



How does the earliest rudimentary brain appear? 



The medullary canal is widened from the very beginning at its 



anterior end, and very early develops three ves- 



FIG. 69. icles, which are shown diagrammatically in Fig. 



a _ i 69. These vesicles, with their lining of epiblast, 



f ~ J ai> e the primitive brain. 



^ 2 5 How does the anterior cerebral vesicle become 

 f *\ changed? 



\ J This vesicle is destined to become the third ven- 



tricle of the brain, and from its anterior surface 

 are developed two protrusions which expand to 

 form the hemispheres of the brain ; thus, this ves- 

 icle is early divided into two parts, the fore-brain 

 or prosencephalon, and the inter-brain or thalam- 

 encephalon. 



What is the middle primary vesicle called ? 



The mesencephalon. It corresponds to the 

 aqueduct of Sylvius. 



How is the Posterior vesicle altered? 



i, vesicle of the This is destined to become the fourth ventricle, 



hemisphere : 2. ves- j i -i -^11 



icie of the tubercu- and by a similar protrusion it develops a second 

 I a & dri f^ ina;3> portion, which becomes the cerebellum. These 



VcSlCic OI tQC 1116- j /i i 11 i 



duiia obiongata. divisions oi the posterior cerebral vesicle are 



