III.] THE MEDULLARY CANAL. 67 



FIG. 23. 



DORSAL VIEW OF THE HARDENED AREA PELLUCIDA OF A CHICK 

 WITH FIVE MESOBLASTIG SOMITES. THE MEDULLARY 

 FOLDS HAVE MET FOR PART OF THEIR EXTENT, BUT HAVE 

 NOT UNITED. 



a.pr. anterior part of the primitive streak ; p.pr. posterior part 

 of the primitive streak. 



Me.). It is not completely closed in till a period con- 

 siderably later than the one we are considering. 



Meanwhile important changes are taking place in 

 the axial portions of the mesoblast, which lie on each 

 side of the notochord beneath the medullary folds. 



In an embryo of the middle period of this day, 

 examined with transmitted light, the notochord is 

 seen at the bottom of the medullary groove between 

 the medullary folds, as a transparent line shining 

 through the floor of the groove when the embryo is 

 viewed from above. On either side of the notochord 

 the body of the embryo appears somewhat opaque, 



52 



