232 K. MITSUKURI AND C. ISHIKAWA. 



from the same deposit of eggs, — in an emhryo, therefore, five days 

 old with five or six mesoblastic somites — tlie passage is no longer 

 dorsally open. The medullary canal has completely closed over it 

 and the blastoporic 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 sepa- 

 rate. In h, the chorda has fused above with the walls of the medul- 

 lary 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 tlie 

 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 blasto- 

 poric passage (i. e. the "Endwulst"), the posterior wall of the neuren- 

 teric canal seems to have been developed in situ. From this mass 

 the mesoblast is extending 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- 



