FATE OF BLASTOPORE, ETC., IX CHELOXIA. 



61 



is four sections behind n. The tail-swellino- is now 

 circumscribed by the epiblast all around, except on the 

 ventral surface. The " Endwulst " mass within is now 

 quite homogeneous and is continuous with the layers 

 below only by a stalk wliich is, however, still rather 

 broad. Note that, in this as irell as in the preceding and fol- 

 lowimj sections, the lateral plates of the mesohlast do not take 

 part in the tail-su-elling and are left helow. 



is two sections behind //. The stalk of the "Endwulst'' is 

 now rather narrow. 



is three sections behind i and is nearing the posterior termi- 

 nation of the tail-swelling. It is becoming difficult to 

 distinguish the mass within from the epiblast — not l)e- 

 cause there is not really a limit, but because the section 

 cuts these parts somewhat obliquely near the rounded 

 termination of the tail. A comparison with Series IX 

 shows that, unlike Chelonia, the primitive streak 

 and groove are early obliterated from the dorsal surface 

 and the tip of the tail-swelling in Clemmijs. The 

 primitive streak below the tail-swellinof seems to beo-in 

 at about the level of this section. 



is two sections behind /. It has just scraped the 

 tip of the tail-swelling. There is a distinct primitive 

 streak in the epiblast below. The nuclei are arranged in 

 two rows in the stalk that connects the epiblast and meso- 

 blast, and there is a distinct cavity between the two rows. 

 I think, we may regard this as a very deep primitive 

 groove which is closed above near the surface by the 

 coalescence of the lips. 



is two sections behind /.■. We are now behind the tail- 

 swelling, and in the posterior tube of the amnion. The 

 primitive streak is distinct. Although the median part 

 of the floor of the amniotic tube is elevated, I do not 

 think it can be regarded as the yolk-plug which, I believe, 

 is found further behind. 



