224 



K. MITSUKUEI AND C. ISHIKAWA. 



itself, even in a more advanced stage, is found only round 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 {tjh. p.) which 

 sticks out to the external surfoce between the lateral lips of the 

 blastopore. When we compare our figure 18 with the frontal section 

 through the yolk-plug of a Triton embryo, 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 Rusconi found in the Ampliibian eggs. Allowing for the 

 diff'erences between a holoblastic and a meroblastic egg^ the relations 

 in the two figures are almost exactly alike, part for part. If the slits 

 between the plug and the lateral lips of the blastopore extended in our 

 figure a little more into the midst of each mesoblastic mass the 

 resemblance would be complete; but even for the Amphibian eggs the 

 slits do not always extend as far as represented in fig. 9, as Hertwig 

 himself mentions (No. 6, p. 14). At any rate, in each case there is an 

 axial mass of cells, (1) into which the ectoblast turns down at the 

 lateral lips of the blastopore, (2) in which the entoblast is not to be 

 distinguished, (3) from which the mesoblastic masses start away 

 toward each side, and (4) which sends a plug upwards between the 

 lips of the blastopore. If we compare the longitudinal section of the 

 plug in ïrionyx (fig. 24, yh. p.) with the sagittal section of the 

 Amphibian yolk-plug (Hertwig, No. 6, Taf. ii, fig. 4), we see again 

 that the relations of difterent parts are alike. It is true that the plug 

 in Trionyx is not bounded posteriorly by a groove, and passes directly 

 into the ectoblast of the primitive streak, but when we consider that 

 the l)lug in Trionyx is only rudimentary, this is not to be wondered 

 at, and is of little significance. 



We think we arc justified, on these grounds, in concluding that 



