34 SEGMENTATION^. 



disc exhibits amoeboid movements. During the segmentation nuclei 

 make their appearance spontaneously (?) in the yolk adjoining the 

 germinal disc (fig. 15, nx), and around them portions of the yolk°with 

 its protoplasmic network become segmented off. Cells are thus 

 formed which are added to those resulting from the segmentation 

 proper. Even after the segmentation numerous nuclei are present in 

 the granular matter below the blastoderm (fig. 16 A, n')-, and around 





J' 



Fig. 15. Section through geeminai disc of a Pristiurus embryo during the 



segmentation. 

 n. nucleus; nx. nucleus modified prior to division; nx'. modified nucleus in the 

 yolk; /. furrow appearing in the yolk adjacent to the germinal disc. 



these cells are being continually formed, which enter the blastoderm, 

 and are more especially destined to give rise to the hypoblast. The 

 special destination of many of these cells is spoken of in detail below. 



At the close of segmentation the blastoderm forms a somewhat 

 lens-shaped disc, thicker at one end than at the other ; the thicker 

 end being the embryonic end. It is divided into two strata — an 

 upper one, the epiblast — formed of a single row of columnar cells ; 

 and a lower one, the primitive hypoblast, consisting of the remaining 

 cells of the blastoderm, and forming a mass several strata deep. 

 These cells will be spoken of as the 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 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 Amphioxus, 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 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 continuous with 



