FORMATION OF GERMINAL LAYERS IN CHELONIA. 35 



Fig. 30 is the most posterior section given. It is slightly in 

 front of the ventral opening of the blastoporic passage^ which 

 is still visible as a groove in the median line. The darm- 

 entoblast {end.), which is distinct laterally, does not reach the 

 chorda-entoblast, but passes into a zone from which the meso- 

 blastic sheet spreads away laterally, and which in its turn 

 becomes continuous with the chorda-entoblast. This corre- 

 sponds to fig. 25 or fig. 29 of the anterior region, or to fig. 20 

 of the previous stage. 



In fig. 31 the chorda-entoblast is beginning to be marked 

 off from the mesoblast, which is, however, still united with the 

 darm-entoblast, at least on the left side. This corresponds to 

 fig. 21 of the previous stage. 



In fig. 32 the mesoblast has become entirely separated from 

 both the chorda- and the darm-entoblast (excepting a little 

 spot on the left). The chorda-entoblast is now a compact 

 mass by itself, against the sides of which the darm-entoblast is 

 applied. This is more clearly shown on the right side than on 

 the left. This corresponds to fig. 26 of the anterior region, and 

 to fig. 22 of the previous stage. 



In fig. 33 the darm-entoblast has passed some way under 

 the chorda which has almost the appearance of the finished 

 structure. This corresponds to fig. 27. 



In fig. 34 the darm-entoblast has passed completely under 

 the chorda and forms a continuous sheet across, and the for- 

 mation of the notochord is finished. This corresponds to 

 fig. 28. 



The formation of the chorda at the anterior region comes to 

 an end much earlier than in the posterior region, where it is 

 continued on until considerably later, and where the growth in 

 length of the embryo seems mainly to take place. 



It remains now to state the fate of the blastoporic passage. 

 In an embryo taken out two days later than that given in 

 figs. 4 a and b, from the same deposit of eggs, in an 

 embryo, therefore, five days old with five or six mesoblastic 

 somites, the passage is no longer dorsally open. The medul- 

 lary canal has completely closed over it and the blastoporic 



