38 ^- MITSUKUEI; 



level of this ledge, and that the uprising of the medullary 

 folds has therefore carried the yolk-plug upwards along 

 with them. This perhaps accounts for the great diminu- 

 tion in the breadth of the yolk-plug, since we must 

 consider the same mass of cells spread over a greater extent. 



d is two sections behind c. The posterior w^all of the 

 neurenteric canal (earlier the floor of the blastoporic 

 passage) which is formed of a mass of cells without any 

 regular arrangement is nearly reached, and a part of it 

 is seen on the right (apparent left) side standing up from 

 the bottom of the canal. The yolk-plug is in contact, 

 but is not yet fused, with it. The wall of the medullary 

 canal is now inseparably fused with the mesoblast along 

 the larger part of its outer side. 



e is the section next behind d. The yolk-plug has now 

 united with the posterior wall of the neurenteric canal 

 and is seen as a tall mass standing up from the bottom 

 of the canal. Its most superficial pnrt is somewhat 

 marked as a cap sitting on the underlying part. This 

 probably is the first indication of the epiblast which, as a 

 comparison with Woodcut VI. shows, encroaches on the 

 dorsal surface of the yolk-plug before the posterior points 

 of the lateral limbs of the horse-shoe shaped blastopore 

 are reached, 



f is the next section behind e. The groove on each side of 

 the yolk-plug is becoming shallower, as we are nearing 

 the ends of the lateral limbs of the horse-shoe shaped 

 blastopore. 



g is two sections behind /. The epiblast of the right (ap- 

 parent left) side has become conti nu<Kis with the epiblast 

 over the dorsal surface of the yolk-plug. In the left 

 (apparent right), there is still a groove present. The 

 columnar-epithelium-like arrangement of cells marking 

 the posterior ends of the medullary chord which has been 

 Sfetting- less and less in the last few sections has almost 



