FATE OF BLASTOPORE, ETC, IX CHELOXLA. ^\ 



Series VIII. 



(PJ. VI.) 

 Trionijx jiipon icus. 



A Trioui/x euihri/o with almit 13 mesohlastic somites: takoi out 

 9^jo dai/s after deposition; represented in Figs. 25 (I'l. HI.) d- 25a 

 (PI. II.). (151 ffl) 



This is one of the embryos in wliich the tail-swelling is beginning 

 to arise, as explained on p. oO. 



a is from the region slightly iii front of the neurenteric 

 canal. Xot only is the medullary chord united ^vith the 

 notochord but the gut hypoblast also. 



b is eight sections behind a and is the sixth in which the 

 neurenteric canal is open below. It is already near the 

 posterior end of the ventral opening. 'Jlie mesoblast is 

 united on either side for a short stretch with the wall of 

 the neurenteric canal. 



C is two sections behind h. The neurenteric canal has al- 

 ready closed in the section next in front. The figure of 

 the medullary chord is, however, still recognisable in the 

 " Endwulst " mass as u median dorsal projection. The 

 epiblast is specially thinned over this projection but already 

 there is no connection between the two in Triomjx. 

 On each side of the median projection, there is a large 

 mass of cells which is sharply distinguished from the 

 lateral part of the middle layer by a shoulder-like slope. 

 This shoulder-like mass, together with the median pro- 

 jection and the mass continuous with the gut-hypoblast 

 below, form one compact central mass, which is sharply 

 distinguished from the lateral parts. This is the 

 '' Endwulst," and corresponds to the innermost of the 

 three concentric zones in the superficial view of the em- 

 bryo (Fig. 25). It is this mass already risen up some- 



