ELASMOBRANCIIII. 



the ovum. Yolk spherules are present in this disc as elsewhere, 

 but are much smaller and of a different character. The segmen- 

 tation has the normal meroblastic character (fig. 15) and is 

 confined to the germinal disc. Before it commences the ger- 

 minal disc exhibits amoeboid movements. During the segmen- 

 tation 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 seg- 

 mentation numerous nuclei are present in the granular matter 

 below the blastoderm (fig. 16 A, //) ; and around these cells 



nx 



'ivif s&- s ^ 



'?.-. ::' '...:-:; ' , '-'; 



ret - -P^\ 

 :: : "o' '-'X.70 ^ ^a 1 

 Jooo* > \. 



*/ 



FIG. 15. SECTION THROUGH GERMINAL DISC OK A PRISTIURUS EMBRYO DURING 



THE SEGMENTATION. 



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

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



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 some- 

 what 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 



