ELASMOBRANCHIL 



35 



the lower layer cells; at the same time some of the lower layer cells 

 of the embryonic end of the blastoderm assume a columnar form, 



A 



e-s 



sc 



FlG. 16. TWO LONGITUDINAL SECTIONS OF THE BLASTODEKM OF A PEISTIUKUS EMBRYO 

 DURING STAGES PRIOR TO THE FORMATION OF THE MEDULLARY GROOVE. 



ep. epiblast; //. lower layer cells or primitive hypoblast ; m. mesoblast; hy. hypo- 

 blast; sc. segmentation cavity; es. embryo swelling; n'. nuclei of yolk; er. embryonic 

 rim. r. lower layer cells at the non-embryonic end of the blastoderm. 



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 



FIG. 17. LONGITUDINAL SECTION THROUGH THE BLASTODERM OF A PRISTIURFS 

 EMBRYO OF THE SAME AGE AS FIG. 28 B. 



ep. epiblast; cr. embryonic rim; ?H. mesoblast; al. mesenteron. 



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 network 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 



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