DEVELOPMENT OF THE SPINAL COLUMN. 671 



hind part of the medullary groove remains unclosed consider- 

 ably later than the fore part. It, however, gradually becomes 

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

 ward it obliterates the primitive streak and groove, which are 

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



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 dorsalu 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 



FIG. 262. 



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

 lary canal is closed. (KOlliker.) wr. Medullary canal, h. Epiblast. uwh. Cavity of 

 protovertebrautr. ung. Wolffian duct. mp. Mesoblast dividing into hpl. Somatopleure. 

 df. Splanchnopleure. p. Pleuroperitoneal cavity, dd. Hypoblast. cA. Notochord. 



externally are other thinner masses of mesoblast called the 

 lateral plates. 



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 pleuroperitoneal cavity. The ver- 

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

 gitudinal partition, so that on either side of the neural canal is 

 a mass of undivided mesoblast extending laterally toward the 

 divided mesoblast. 



In each vertebral plate there appear transverse vertical inter- 

 ruptions at definite intervals which split the plate up into a 

 number of quadrangular blocks of mesoblast, known as the pro- 



