CHAPTER XV. 



THE BRAIN AND SPINAL CORD. 



Formation of the Central Canal. — The medullary plates of 

 epiblast, which form the spinal cord and brain, rise up, meet, and 

 enclose a canal — the central canal of the spinal cord and brain 

 (Figs. 158 and 69, p. 90). The lips of the medullary plates meet 



yolk, sac \ 



amnion rent 

 open — 



ceph. end 



j-med. groove 



med. fold 



blastopore 

 primitive groove 

 body stalk 



-chorion 



Fig. 158.— Medullary Folds uniting to form the Neural Tube in a Human Embryo 

 of about 14 days. (After Graf Spee.) 



and fuse together in the cervical region first (Fig. 1 5 8), the process 

 of union spreading forwards and backwards, the last parts to be 

 enclosed being the cephalic and caudal extremities. The optic 

 vesicles start to grow out from the medullary plates before they 

 have united to enclose the cavity of the fore-brain. The canal 

 is completely closed by the middle of the 3rd week. 



