POSTANAL SECTION OF ALIMENTARY CANAL. 447 



tract, but at a stage slightly younger than K its lumen becomes 

 smaller, and finally vanishes, and the original tube is replaced 

 by a solid rod of uniform and somewhat polygonal cells. A 

 section of it in this condition is represented in PL n, fig. 8 a. 



At a slightly later stage its outermost cells become more 

 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 definitely 

 established, and the central rounded cells are no longer so 

 numerous (PI. 18, fig. 4, sees.). 



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. 



The postanal section of tJie alimentary tract. 



An account has already been given (p. 307) of the posterior 

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

 stated that Kowalevsky was the discoverer of this peculiar 

 arrangement. 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 1 : 



The two germinal layers, epiblast and hypoblast, are continuous with 

 each other at the border of the germinal disc. The primitive groove or 



1 Archiv f. Mic. Anat. Vol. XIII. pp. 194, 195. 



