34 K. MITSUKURI; 



d is the fourth section l)ehind c, and is just at the ventral 

 opening of the Ijlastoporic passage. As the section is 

 somewhat oblique, the passage is nearly open below on 

 the left side. The dorsal and lateral walls of the passage 

 show an arrangement of cells like columnar epithelium, 

 while the bottom is formed of a mass Avithout any regular 

 arrangement. The epiblast is fused with the chorda- 

 hypoblast, as we are already in the region where the 

 epiblast is reflected and becomes continuous with the 

 hypoblast at the dorsal lip of the blastopore. 



e is the second section behind tl, and is at the dorsal opening 

 of the blastoporic passage. The epiblast is reflected on 

 each side at the lips of the blasto])ore and is proliferating 

 the peristomal mesoblast. The bottom of the passage is 

 formed of a mass of cells without any regular arrange- 

 ment. 



f is two sections behind e. The mass of cells which formed 

 in d and e the bottom of the blastoporic passage stands 

 out in this section as a prominent body between the lateral 

 lips of the blastopore which are proliferating the peristo- 

 mal mesoblast as before. This mass is the so-called yolk- 

 jylug and shows no regular arrangement of cells. The 

 gut-hypoblast, which was fused in d and e in the median 

 line with the mass above, is in this section clearly indepen- 

 dent. In the preceding section the yolk-plug is distinct, 

 but smaller. 



g is four sections behind/. The groove on each side between 

 the lateral blastoporic lip and the yolk-plug has been 

 getting shallower with each section behind /, cells on the 

 surface are arranged like columnar epithelium and although 

 as yet continuous with the peristomal mesobla;<t below, we 

 must regard this region as that of transition to the epiblast. 

 There is even an indication of the separation of the upper 

 layer from the subjacent mesoblost for a very short stretch 

 in the median line. Compare Woodcut VI. further on 



