ELASMOBRANCHII. 



43 



a columnar form, and constitute the true hypoblast. The 

 portion of the blastoderm, where epiblast and hypoblast are 

 continuous, forms a projecting structure which will be called the 

 embryonic rim (fig. 16 B, er). 



This rim is a very important structure, since it represents the 

 dorsal portion of the lip of the blastopore of Amphioxus. The 

 space between it and the yolk represents the commencing 

 mesenteron, of which the hypoblast on the under side of the lip 

 is the dorsal wall. The ventral wall of the mesenteron is at 

 first formed solely of yolk held together by a protoplasmic net- 

 work with numerous nuclei. The cavity under the lip becomes 

 rapidly larger (fig. 17, al}, owing to the continuous conversion of 

 lower layer cells into columnar hypoblast along an axial line 

 passing from the middle of the embryonic rim towards the 

 centre of the blastoderm. The continuous differentiation of the 

 hypoblast towards the centre of the blastoderm corresponds with 

 the invagination in Amphioxus. During the formation of the 

 embryonic rim the blastoderm grows considerably larger, but, 

 with the exception of the formation of the embryonic rim, retains 

 its primitive constitution. 



The segmentation cavity undergoes however important 

 changes. There is formed below it a floor of lower layer cells, 

 derived partly from an ingrowth from the two sides, but mainly 

 from the formation of cells around the nuclei of the yolk (fig. 

 1 6). Shortly after the floor of cells has appeared, the whole 

 segmentation cavity becomes obliterated (fig. 17). 



The disappearance of the segmentation cavity corresponds 

 in point of time with the formation of the hypoblast by the 

 pseudo-invagination above described ; and is probably due to 

 this pseudo-invagination, in the same way that the disappear- 

 ance of the segmentation cavity in Amphioxus is due to the true 

 invagination of the hypoblast. 



When the embryonic rim first appears there are no external 

 indications of the embryo as distinguished from the blastoderm, 

 but when it has attained to some importance the position of the 

 embryo becomes marked out by the appearance of a shield-like 

 area extending inwards from the edge of the embryonic rim, 

 and formed of two folds with a groove between them (fig. 28 B, 

 mg), which is deepest at the edge of the blastoderm, and 



