28 K. MITSUKURI AND C. ISHIKAWA. 



the lips of the blastopore, the ectoblast turns ventralward, 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 

 lips of the blastopore is filled almost entirely by a plug {yk.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 difference between the ectoblast and this plug is at once 

 unmistakeable and striking. While the cells in the ectoblast 

 are columnar and always arranged perpendicularly to the sur- 

 face, the cells in the plug are polygonal and without any defi- 

 nite 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 differentiated, is not entirely distinct, and 

 the mesoblast also stretches away from this mass on each side. 

 Toward the sides the entoblast is yet undifferentiated; it con- 

 sists 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 

 being given off to the mesoblast. This contribution 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 itself, even in a more advanced stage, is found only rouud 

 the posterior part as a horseshoe-shaped ridge (fig. 5). 



Having gone over the description of the various parts of this 

 section, let us return to the discussion of the plug {yk. p.) 

 which sticks out to the external surface between the lateral 

 lips of the blastopore. When we compare our figure 18 with 

 the frontal section through the yolk-plug of a Triton em- 

 bryo, which Hertwig (No. 6) gives in his fig. 9, Taf. ii, we 

 think nobody will hesitate long before concluding that the 

 plug in our figure is homologous with the yolk -plug of 



