53 K- MITSUKURI; 



of fusion, as evinced by spindle-shaped cells lying at the 

 boundary of the two. 



b is eight sections behind a. The mednllary chord, the 

 notochord, and the hypoblast are united. The notochord 

 shows a division into two halves. 



C is two sections l^ehind b and is the first in which the 

 neurenteric canal opens below. The two halves of the 

 notochord, although separated, are recognisable as such. 

 The mesoblast has united on each side with the chorda 

 dor salis. 



d is seven sections behind c and is the last in which the 

 neurenteric canal is open below. The mesoblast has now 

 united with the Avail of the neurenteric canal along its 

 whole height. 



e is two sections behind d. We are now in the " Endwulst" 

 region. The posterior part of the neiu-enteric canal is 

 still present. 



f is two sections behind e. The last vestio;es of the neuren- 

 teric canal is visible in the upper median part of the 

 " Endwulst." 



g is three sections behind/. The "Eudwulst" is simply a 

 compact mass of indifferent cells. It is united with the 

 lateral mesoblast hij its wliole lieight, and with the hypo- 

 blast below ; it is independent of the epiblast. 



h is five sections behind g. The " Endwulst " mass is 

 smaller. It also projects upwards above the level of the 

 lateral mesoblast. We must reçrard this as the first sis^n 

 of the upheaval. 



i is three sections behind //. The median " Endwulst " 

 mass is still less than in /;. 



j is next to i. The epiblast which has hitherto been in- 

 dependent, is now continuous with the last part of the 

 " Endwulst " and the mesoblast. That is, the primitive 

 streak which began in Series III from the neurenteric 

 canal, begins here in this series. 



