FATE OF BLASTOPORE, ETC., IX CHELONIA. 65 



Willst " is now circumscribed entirely by the epiblast and 

 forms the tail. The ]<3wer allantoic mass lies almost free 

 in the lower part. The coelem goes across from side to 

 side. 



h is two sections behind (j. The tip of the tail is just 

 scraped. The lower mass, which is the continuation of 

 the allantoic wall, is now changed to the stalk that 

 connects the epiblast below the tail with the mesoblast. 

 This is comparable to Series XI, _/' or /*;. (PL IX.). 



i is two sections behind h and takes in the groove surround- 

 ing the tail ; hence tlie primitive streak is cut more or 

 less oljliquely and longitudinally. (Cf the surface view 

 Fig. 2{)). 



j is next to ?'. We recognise in the median line the mush- 

 room-like yolk-plug visible in the surface view. Unlike 

 Cleminiis (Series X and XI), the yolk-plug is recognisable 

 only in three or four sections. 

 Accompanying this is tlie longitudinal section {Scries AT/, long., 

 PL VIII.) which will make the relations of different parts clearer. 



Series XIIL 

 (PI X.) 



Chelonia caouana. 



An emhnjo witli about 18-19 niesoldastic somites ; talcen ont ll^jo days 

 after deposition ; in (fcneral appearance renj much lihe tlie Clemmys emhrijo 

 given in Fig. 15 (I'l. II.). (Bisn) 



In this embryo, the tail has much elongated and the allantois 

 has "-rown somewhat. This is the most advanced staf>-e in which I 

 have been able to recognise the remnant of the yolk-plug distinctly. 



a is from the ]wsterior dorsal region. 



b is four sections behind a. The medullarv chord and the 



