218 POSTANAL SECTION OF ALIMENTARY CANAL. 



columnar than the remainder, and between stages K and L it 

 loses its cylindrical form and becomes much more flattened. 

 By stage L the external layer of columnar cells is more defin- 

 itely established, and the central rounded cells are no longer so 

 numerous (PL xvii. fig. 4, s oes.). 



In the succeeding stages the solid part of the oesophagus 

 immediately adjoining the stomach is carried farther back 

 relatively to the heart and overlies the front end of the liver. 

 A lumen is not however formed in it by the close of stage Q, 

 and beyond that period I have not carried my investigations, 

 and cannot therefore state the exact period at which the lumen 

 reappears. The limits of the solid part of the oesophagus are 

 very satisfactorily shewn in longitudinal and vertical sections. 



The solidification of the oesophagus belongs to a class of 

 embryological phenomena which are curious rather than in- 

 teresting, and are mainly worth recording from the possibility 

 of their turning out to have some unsuspected morphological 

 bearings. 



Up to stage Q there are no signs of a rudimentary air- 

 bladder. 



TJie postanal section of the alimentary tract. 



An account has already been given (p. 91) of the posterior 

 continuity of the neural and alimentary canals, and it was there 

 stated that Kowalevsky was the discoverer of this peculiar ar- 

 rangement. Since that account was published, Kowalevsky has 

 given further details of his investigations on this point, and 

 more especially describes the later history of the hindermost 

 section of the alimentary tract. He says^ : 



The two germinal layers, ej)iblast and hypoblast, are continuous 

 with each other at the border of the germinal disc. The primitive 

 groove or furrow appears at the border of the germinal disc and 

 is continued from the upper to the lower side. By the closing of 

 the groove there is formed the medullary canal above, while the 

 pai-t of the groove on the under surface directed below is chiefly 

 converted into the hmd end of the alimentary tract. The connection 

 of the two tubes in Acanthias persists till the formation of the anus, 

 and the part of the nervous tube which lies under the chorda passes 



1 Archivf. Mic. Anat. Vol. xiii. pp. 104, 195. 



