III.] THE MEDULLARY CANAL. 53 



dissolved and give place to vacuoles ; where this occurs to a large extent an 

 opaque ring is formed. In the other case a solution of the protoplasm of the 

 spheres takes place, and the spherules are let loose in large numbers ; where 

 this occurs a transparent ring is formed. 



The chief events then of the second part of the first day, 

 are the appearance of the medullary folds and groove, the 

 formation of the notochord, the beginning of the head-fold 

 and amnion, and the histological changes taking place in the 

 several layers. 



7. From the 20th to the 24<th hour. The head-fold en- 

 larges rapidly, the crescentic groove becoming deeper, while 

 at the same time the overhanging margin of the groove (the 

 upper limb of the 8, Chap. II. 5), rises up above the level of 

 the blastoderm ; in fact, the formation of the head of the 

 embryo may now be said to have definitely begun. 



S. The medullary folds, increasing in size in every di- 

 mension, but especially in height, lean over from either side 

 towards the middle line, and thus tend more and more to 

 roof in the medullary canal, especially near the head. 

 About the end of the first day they come into direct contact 

 and completely coalesce with each other at a point which 

 lies at some little distance behind the head-fold, in the 

 region which will afterwards become the neck. Union, having 

 begun at this spot, rapidly runs forward till (early in the 

 second day) the head-part is completely closed in ; and then 

 passes more slowly backwards. The whole of the anterior 

 portion of the groove is closed in before the union has ad- 

 vanced more than a very short distance towards the tail. In 

 this way a tubular canal is formed, ending blindly in front, 

 but as yet open behind. This is the medullary or neural 

 canal (Fig. 13, M, Fig. 20, Me.). It is not completely 

 closed in at the tail till a period considerably later than 

 the one we are considering. 



9. 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 



