4Q K. MITSUKURI ; 



b is nine sections iDehind a, and is the second in which the 

 medullary chord shows signs of fusion with the noto- 

 chord. The mesoblast and hypoblast are independent. 



C is five sections behind h. The notochord can no longer be 

 recognised as such. The chorda-hypohhat is fused above 

 with the medullary chord, laterally with the mesoblast, 

 and is continuous below with the gut hypoblast. 



d is four sections further back than c and is the first in 

 which the neurenteric canal is open below\ The canal 

 is now almost vertical. The medullary chord is now fused 

 with the mesobhxst along the entire height of its sides. 

 The epiblast, the medullary chord, the mesoblast, the 

 c/(0/Y?rt-hypoblast and the gut-hypoblast are all united in 

 this section. 



e is three sections behind d and is the first in which the 

 neurenteric canal is closed ventrally. From the floor of 

 the canal stands up a small club-shaped mass. Its con- 

 tinuation is also seen in the next section /'. This is pro- 

 bably the same thing as what is called the " Achsenfaden " 

 by DuiiSY. It seems most reasonable to suppose that such 

 a yolk-plug as that described in Series II (c-, d, e,) has left 

 a few cells here in moving backwards. 



f is the section next behind e. There we reach the posterior 

 wall of the neurenteric canal \vhicli can be recognised by 

 the columnar-epithelium arrangement. 



g is two sections behind /'. Here we have already left the 

 region of the medullary groove and we see the section of 

 the groove which appeared like the posterior prolongation 

 of that canal in the surface view — we called it the primitive 

 groove. Note that the epiblast which was continuous with 

 the wall of the medullary canal in all the preceding sec- 

 tions bends down in this section on each side of the 

 aforesaid groove to become directly continuous with the 

 pei-istomal mesoblast. The cells arranged to form the pos- 

 terior wall of the neurenteric canal are still recognisable. 



