22 K. MITSÜKÜRI ; 



nearly to the posterior end of the embryo nrid its free edge h;is the 

 characteristic horse-shoe shape. The hinder part of this embryo is given 

 on a larger scale in Fig. 5b. The posterior end of the medullary canal 

 is still open so far as it is visible behind the free edge of the amniotic 

 fold. The opening is in the figures at the anterior, wide, and 

 darkest, part of the median streak seen behind the free edge of the 

 amnion. The neurenteric canal also lies just under this opening, 

 and marks the posterior end of the medullary canal. The light 

 elliptical area around this end of the medullnry canal is the 

 " Endwulst." The yolk-plug, which has become somewhat pentagonal 

 in shape, has now receded considerably behind the posterior edge of the 

 "Endwulst" and is at the edge of the embryonic area (not the " Stamm- 

 zone "). A delicate median groove extends between the posterior end 

 of the medullary canal (delineated darker) and the yolk-plug, and is un- 

 usually distinct. In front this groove is straight and fine; behind 

 it is a little broader and slightly wavy. The ventral opening of 

 the neurenteric canal seen in the ventral view (Fig. 5:i, PL II) is 

 biscuit-shaped, being narrowest in the middle. This shape seems to 

 be characteristic of Chrlonia caoucnia. 



In Fig. 6 is represented the hinder portion of another embryo, 

 obtained from the same deposit of eggs at the same time as that given 

 in Fig. 5. It also shows unusually clearly the relations of different 

 structures in this part of the embryo at this stage. The medul- 

 lary canal is entirely closed, with the exce])tion of its most posterior 

 portion. This opening, showing darker in the figure than the rest of 

 the median streak, is slightly dumb-bell shaped, the medullary folds 

 coming closer together in the middle of the opening than either in 

 front or behind. This part also marks the position of the neurenteric 

 canal. Behind this opening and in direct continuation of it, there 

 stretches b:ick wards in the median line a well-defined wide and shallow 



