ELASMOBRANCHII. 



53 



FIG. 16 SECTION THROUGH 

 THE ANTERIOR PART OF A PRIS- 

 TIURUS EMBRYO TO SHEW THE 

 FORMATION OF THE ALIMENTARY 



TRACT. 



Ch. notochord ; hy. hypoblast; 

 al. alimentary tract ; na. cells 

 passing in from the yolk to form 

 the ventral wall of the alimentary 

 tract. 





/? c 



opening behind by a widish aperture, the blastopore or anus of 

 Rusconi (vide fig. 19 B). 



The conversion of this irregular cavity into a closed canal 

 commences first of all at the anterior 

 extremity. In this conversion two 

 distinct processes are concerned. 

 One of these is a process of folding 

 off of the embryo from the blasto- 

 derm. The other is a simple growth 

 of cells independent of any fold. 

 To the first of these processes the 

 depth and narrowness of the ali- 

 mentary cavity is due ; the second 

 is concerned in forming its ventral 

 wall. The process of the folding off 

 of the embryo from the blastoderm 

 resembles exactly the similar process 

 in the embryo bird. The fold is a perfectly continuous one round 

 the front end of the embryo, but may be 

 conveniently spoken of as composed of 

 a head-fold and two lateral folds. 



Of far greater interest than the 

 nature of these folds is the formation 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). 

 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, no). The ventral 

 wall of the mesenteron is in fact, to a 

 large extent at any rate, formed as a dif- 

 ferentiation 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. 



FIG. 27. LONGITUDINAL 

 VERTICAL SECTION OF AN 

 EMBRYO SLIGHTLY YOUNGER 

 THAN THAT IN FIG. 26 D. 



The section shews the 

 communication which exists 

 between the neural and ali- 

 mentary canals. 



nc. neural canal ; al. ali- 

 mentary tract ; Ch. noto- 

 chord ; Ts. tail swelling. 



