290 ANIMAL BIOLOGY. [Part II. 



somewhat resembling the mesoblastic somites of the vertebrate, 

 which become hollowed, and by apposition give rise to the 

 body segments. 



The nervous system arises by two distinct thickenings of the 

 epiblast : (1) in the supra-oesophageal region, (2) in the region 

 of the ventral cord. The junction between the two takes place 

 comparatively late. 



