SEGMENTATION. 



lower layer cells, to distinguish them from the true hypoblast 

 which is one of their products. 



A cavity very soon appears in the lower layer cells, near the 

 non-embryonic end of the blastoderm, but the cells afterwards 



cp 



A 



FlG. 16. TWO LONGITUDINAL SECTIONS OF THE BLASTODERM OF A PRISTIURUS 

 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 ; '. nuclei of yolk ; er. embryonic 

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



disappear from the floor of this cavity, which then lies between 

 the yolk and the lower layer cells (fig. 16 A,sc). This cavity is 

 the segmentation cavity equivalent to that present in Amphi- 

 oxus, Amphibia, etc. The chief peculiarity about it is the 

 relatively late period at which it makes its appearance, and the 

 fact that its roof is formed both by the epiblast and by the 



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



EMBRYO OF THE SAME AGE AS FIG. l8 B. 

 ep. epiblast ; er. embryonic rim ; m. mesoblast ; al. mesenteron. 



lower layer cells. Owing to the large size of the segmentation 

 cavity the blastoderm forms a thin layer above the cavity and a 

 thickened ridge round its edge. 



The epiblast in the next stage is inflected for a small arc at 

 the embryonic end of the blastoderm, where it becomes con- 

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

 lower layer cells of the embryonic end of the blastoderm assume 



