FORMATION OF GERMINAL LA.YERS IN CHELONIA. 37 



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

 is invariably at the junction oftheehorda-entoblast 

 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 behind. First, at the point of the origin of the mesoblast 

 the connection 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, how- 

 ever, 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. 



The formation of the Blastoporic Passage. 

 There are differences of opinion among previous writers on 

 the subject in regard to the formation of the blastoporic 

 passage in Reptilia. Balfour (No. 2, p. 424-5) says: ''After 

 the segmentation and the formation of the embryonic shield 

 (area pellucida) the blastoderm becomes distinctly divided into 

 epiblast and hypoblast. At the hind end of the shield a some- 

 what triangular primitive streak is formed by the fusion of 

 the epiblast and hypoblast, with a number of cells between 

 them, which are probably derived from the lower rows of the 

 segmentation cells. At the front end of the streak a passage 

 arises, open at both extremities, leading obliquely forwards 



