SPINAL CORD. 



471 



Spinal cord. After the formation of the brain vesicles, 

 the remainder of the medullary canal forms the spinal 

 cord. 



The canal is for a time continuous posteriorly with the 

 food canal beneath, so that a => -shaped tube results. The 

 connection between them is called the neurenteric canal 

 (Fig. 222, ne.c.\ and though it is only temporary, its frequent 

 occurrence is of much interest. 



The wall of the medullary canal becomes very much thick- 

 ened, the roof and floor grow less rapidly, and thus the cord 

 is marked by ventral and dorsal longitudinal furrows. At the 

 same time, the canal itself is constricted, and persists in the 

 fully formed structure only as a minute canal lined by ciliated 

 epithelium, and continuous with the cavity of the brain. 



