44 



THE HYPOBLAST. 



Fig. 26. Section through the 



ANTERIOR PART OF A PrISTIURUS 

 EMBRYO TO SHEW THE FORMATION 

 OF THE ALIMENTARY TRACT. 



Gh. notochord; hy. hypoblast 

 al. alimentary tract ; 7ia. cells 

 passing in from the yolk to form 

 the ventral wall of the alimentary 

 tract. 



Of far greater interest than the nature of these folds is the forma- 

 tion of the ventral wall of the alimentary canal. This originates in 

 a growth of cells from the two sides to the middle line (fig. 26). 



I The cells for it are not however mainly 

 derived from pre-existing hypoblast cells, 

 but are formed de novo around the nuclei 

 of the yolk which have already been 

 spoken of (fig. 26, na). The ventral 

 wall of the mesenteron is in fact, to a 

 large extent at any rate, formed as a 

 differentiation of the primitive yolk 

 floor. 



The folding off and closing of the 

 alimentary canal in the anterior part of 

 the body proceeds rapidly, and not only 

 is a considerable tract of the alimentary 

 canal formed, but a great part of the 

 head is completely folded off from the 

 yolk before the medullary groove is 

 closed. 



The posterior part of the alimentary canal retains for a longer time 

 its primitive condition. Finally however it also becomes closed in, 

 by the lips of the blastopore at the hind end of the embryo meeting 

 and uniting. The peculiarity of the closing in of the posterior part 

 of the alimentary canal consists in the fact that a similar continuity 

 to that in Amphioxus obtains between the neural and alimentary canals. 

 This is due to the medullary folds being 

 continuous at the end of the tail with the 

 lips of the blastopore, which close in the 

 hind end of the alimentary canal ; so that, 

 when the medullary folds unite to form a 

 canal, this canal becomes continuous with 

 the alimentary canal, which is closed in at 

 the same time. In other words, the me- 

 dullary folds assist in enveloping the blas- 

 topore which does not therefore become ab- 

 solutely closed, but opens into the floor of 

 the neural canal. It will afterwards be 

 shewn that it is only the posterior part of the 

 blastopore that becomes closed during the 

 above process, and that the anterior and 

 ventral part long remains open. The general 

 arrangement of the parts, at the time when 

 the hind end of the mesenteron is first closed, 

 is shewn in fig. 27. The same points may be 

 seen in the diagrammatic longitudinal section fig. 19 C. 



The middle portion of the alimentary tract is the last to be closed 

 in, since it remains till late in embryonic life as the umbilical or 



Fig. 27. LoNGiTUDiNAii 



VERTICAL SECTION OF AN EM- 

 BRYO SLIGHTLY YOUNGER THAN 

 THAT IN FIG. 26 D. 



The section shews the 

 communication which exists 

 between the nem-al and ali- 

 mentary canals. 



nc. neural canal; aL ali- 

 mentary tract; Ch. noto- 

 chord; Ts. tail swelling. 



