FATE OF BLASTOPORE, ETC., IX CHELOXIA. 45 



is from the hinder portion of the dorsal region. The 



medullary canal is closed, 

 is ten sections behind a. In passing from a to ?>, the 

 notochord and the medullar^y chord have gradually been 

 fusing with each other, until in J) the former looks merely 

 like the thick ventral wall of the latter, althono-h the 

 irregular arrangement of cells reveals its nature, 

 is four sections behind h. The noto-medullary chord is 

 now connected with the gut-hypoblast and on the right 

 (apparent left) side, Avith the mesoblast for a short extent. 

 is two sections behind c. The section in front of this 

 and therefore next to c is the first in which the neuren- 

 teric canal is open below. The wall of the canal is 

 continuous on either side with the gut-hypoblast, and with 

 the mesoblast for a short stretch directly above its junction 

 with the gut-hypoblast. The wall of the neurenteric 

 canal is also just fusing with the epiblast above. 



is the section next behind cL It is like d in every respect 

 except one, viz. : the epiblast is now distinctly con- 

 tinuous on each side with that half of the wall of the 

 neurenteric canal which is on its own side, and the canal 

 is thus open above. This corresponds of course to that 

 part in the surface view (Figs. 5 & 5b) where the 

 medullary canal is open above. 



is two sections behind c and is from about the middle 

 of the ventral opening of the neurenteric canal. It is 

 in almost every respect like e, but is introduced to 

 show that the breadth of the neurenteric canal is here 

 considerably less than either in front or behind. A 

 reference to the ventral surface-view (Fig. oa) shows that 

 the ventral opening of the neurenteric canal is dumb-bell 

 shaped. 



is seven sections behind/. The ventral opening of the 

 neurenteric canal is just beginning to close, wdiile it is 

 clearly open above. The wall of the neurenteric canal is 



