FORMATION OF GERMINAL LAYERS IN CHELONIA. 



223 



Fig. 18, the most posterior section represented, goes through 

 the lateral limhs of the horseshoe-shaped blastopore and the yolk-plug 

 occupying its concavity. The ectohlast, which consists of only a 

 single layer of cells at the sides, becomes gradually thicker towards 

 the median line, which it does not, however, reach. At a short 

 distance from the latter, and at the lips of the blastopore, the ectoblast 

 turns ventrahvard, and becomes lost in the mass of cells found in the 

 axial line. It retains, however, its columnar character for some 

 distance downwards. The considerable space between the two lateral 

 Jips of the blastopore is filled almost entirely by a plug (>ß- p-) of 

 considerable size, which projects upwards from the axial mass of cells 

 as far as the level of the general surface of the embryo. The dif- 

 ference between the ectoblast and this plug is at once unmistakeable 

 and striking. A\^hile the cells in the ectoblast are columnar and 

 always arranged perpendicularly to the surface, the cells in the plug 

 are polygonal and without any definite arrangement. We shall 

 return to the discussion of this structure directly. 



As just stated, the ectoblast turns downwards near the median 

 line, and loses itself in the axial mass. All the germinal layers are, 

 in fact, fused here, for the entoblast, although it has some appearances 

 of being ditferentiated, is not entirely distinct, and the mesoblast also 

 stretches away from this mass on each side. Toward the sides 

 the entoblast is yet undifterentiated ; it consists of an abundant 

 protoplasmic network with numerous nuclei, and is full of yolk- 

 spheres and granules. There is no question whatever that laterally 

 the mesoblast receives cells from the entoblast or yolk. Especially 

 along one line (a, figs. 18 and 20) nuclei are heaped in a special mass, 

 from which cells are Ijeing given off to the mesoblast. This contribu- 

 tion to the mesoblast from the germinal wall is only in the posterior 

 part, as it is no longer observable in fig. 23, and as the germinal wall 



