FORMATION OF GEEMINAL LAYERS IN CHELONIA. 233 



tiens on the formation of the mesoblast and of the notochord : 



In the embryo represented in figs. 1 a and Z>, the mesoblast was 

 found only in the region behind the blastoporic passage, radiating in 

 the shape of an open fan from the posterior wall of the passage, as well 

 as from the ectoblast along the primitive steak, and constituting the 

 structure called the "'sickle." In further course of development the 

 mesoblast becomes extended into the region in front of the blastoporic 

 passage. Here it arises as a paired mass, and its point of origin 

 is invariably at the junction of the chorda-en toblast 

 with the darm-entoblast. In other words, one part of 

 the mesoblast is always continuous with the chorda- 

 entoblast, while the other part passes into the darm- 

 entoblast. 



Besides this source the mesoblast receives large contributions of 

 cells from the germinal wall, and even from the outermost part of 

 the darm-entoblast contiguous with the germinal wall. 



The notochord is formed out of the chorda-entoblast. It is 

 completed first in the middle, and then extends both backward and 

 forward. Its mode of formation is the same, both in front and be- 

 hind. First, at the point of the origin of the mesoblast the connec- 

 tion of the three structures that meet there, viz. the mesoblast, the 

 chorda-, and the darm-entoblast, is loosened. The mesoblast is then 

 found as two separate masses, one on each side of the median line. 

 The darm-entoblast rests with its free edges against the sides of the 

 chorda-entoblast ; it, however, passes gradually under the chorda- 

 entoblast, until finally the darm-entoblast of two sides fuses in the 

 median line, and forms a continuous sheet over the digestive cavity. 

 In the meantime the chorda-entoblast has arranged itself into the 

 finished chorda dorsalis. 



