FATE OF BLASTOPORE, ETC., IX CHELOXIA. 53 



ceral layers. /' and i are through the region of the yolk- 

 pkig and that part inckided between the stars must be 

 considered as a rudimentary form of the yolk-plug, like 

 that in Series VI, i or Series YII, a. In g, there is a 

 very slight groove on each side. 



On tracing the series still further backwards, the trian- 

 gular downward projection of the epiblast on each side 

 becomes less and less, until the epiblast is of an uniform 

 thickness across. The connection of the epiblast with the 

 mesoblast is given up some sections before that. 



Series IX. 



(PI. YII.) 



Cheloiua caouana. 



A Glielonia emhnjo with about 16 mesohlastic somites; talen out 



10^1:1 days after deposition ; represented in Figs. 8 (Fl. I.) & Sa (FL IL) 



(B12 ^) 

 This is one of the most important series. The tail-swelling is 

 fully arisen, and the yolk-plug is very distinctly visible behind. it, 

 as a white mass. 



a is from the posterior dorsal region and shows the first sign 

 of the fusion of the notochord and the medullary chord 

 by the presence of some spindle-shaped cells lying trans- 

 versely at the boundary between the two chords. 



b is eight sections behind a. The notochord and the medul- 

 lary chord are fused together. 



C is next behind h. The notochord shows division into two 

 halves, and also is continuous with the gut-hypoblast. 



d is next Ijehind c and is the first section in which the neu- 

 renteric canal is open below. Xote that the chorda- 



