36 K. MTTSUKURI AND C. ISHIKAWA. 



passage has been changed to the neurenteric canal. Figs. 35 

 a — d, will show the relations of the germinal layers round the 

 passage. In a, the most anterior section given, the darm- 

 entoblast, the notochord, the mesoblast, and the medullary- 

 canal are all separate. In h the chorda has fused above with 

 the walls of the medullary canal, appears for a little space in 

 the median line on the roof of the digestive cavity, and divides 

 the darm-entoblast of the two sides which seem to rest against 

 it. In c the canal opens below into the digestive cavity. 

 The mesoblast is now continuous with the darm-entoblast and 

 the walls of the neurenteric canal at the junction of the two. 

 In d the posterior part of the neurenteric canal has been cut. 

 In the next section (not figured), the cells in the axial region 

 are only more compact than elsewhere, and show that the 

 posterior wall of the canal is reached. Thus from the mass 

 behind the blastoporic passage (i. e. the " Endwulst '^), the 

 posterior wall of the neurenteric canal seems to have been 

 developed in situ. From this mass the mesoblast is extend- 

 ing laterally on each side. It is not possible for us to state 

 exactly how the yolk-plug disappears. A part of it which 

 formed the posterior wall of the blastoporic passage is no 

 doubt changed into the posterior wall of the neurenteric canal. 

 A part placed more dorsally is perhaps changed directly to the 

 ectoblast of the general surface of the embryo. 



In an embryo six days old, i. e. one day older than that of 

 fig. 35, the neurenteric canal still persists. In an embryo 

 seven days old it is no longer found. We are not in a position 

 to state how its disappearance is brought about. 



To state briefly the principal facts brought out by our observa- 

 tions on the formation of the mesoblast and of the notochord : 



In the embryo represented in figs. 1 a and h, the mesoblast 

 was found only in the region behind the blastoporic passage, 

 radiating in the shape of an open fan from the posterior wall 

 of the passage, as well as from the ectoblast along the primi- 

 tive steak, and constituting the structure called the " sickle." 

 In further course of development the mesoblast becomes ex- 

 tended into the region in front of the blastoporic passage. 



