DEVELOPMENT OF THE SPINAL COLUMN. 673 



canal is the earliest representative of the nervous centres, and 

 eventually becomes the brain and spinal cord. The front part 

 of the canal, when completely closed in, becomes dilated into a 

 bulb, thus forming the earliest indication of the brain. The 

 hind part of the medullary groove remains unclosed consider- 

 ably later than the forepart. It, however, gradually becomes 

 converted into a canal at the tail end, and as it extends back- 

 wards it obliterates the primitive streak and groove, which are 

 lost, and take no permanent part in the formation of the embryo. 



FIG. 262. 



Transverse section through the embryo of a chick on the second day where 

 the medullary canal is closed. (Kolliker.) mr. Medullary canal. A. Epi- 

 blast. uwh. Cavity of protovertebra uw. ung. Wolffian duct. mp. Meso- 

 blast dividing into hpl. Somatopleure. df. Splanchnopleure. sp. Pleuro- 

 peritoneal cavity, dd. Hypoblast. ch. Notochord. 



Beneath the medullary canal the cells of the mesoblast are 

 altered to form a rod-shaped cellular body, which following the 

 line of the canal lies in the axis of the embryo ; this is the chorda 

 dorsalis or notochord. 



Supporting the medullary canal on either side of the chorda 

 dorsalis are masses of mesoblast, somewhat quadrangular in sec- 

 tion, which are termed the vertebral plates; continuous with these 

 externally are other thinner masses of mesoblast called the lateral 



The lateral plates become divided into an upper part or somato- 

 pleure, which is in close relationship to the epiblast, and a lower 

 part, the splanchnopleure, which is next to the hypoblast; the 

 space between these being the pleuro-peritoneal cavity. The ver- 

 tebral plates become separated from the lateral plates by a longi- 



