DE VZLOPMENT. 



549 



blastoderm, or segmented area, some nuclei (so-called 

 " merocytes ") are formed in the outer part of the subjacent 

 yolk (Fig. 290, .). It seems most probable that these 

 are hypoblast elements which assist in the preparation of 

 the yolk for absorption, and eventually degenerate in the 

 empty external yolk-sac. 



At the close of segmen- 

 tation the blastoderm is a 

 lens-shaped disc with two 

 strata of cells. It is thicker 

 at one end where the em- 

 bryo begins to be formed. 

 Towards the other end, be-' 

 tween the blastoderm and 

 the yolk, lies a segmentation 

 cavity (Fig. 290, sg.c.). 



At the embryonic end 

 the outer layer or epiblast 

 undergoes a slight invagina- 

 tion (Fig. 290, x.), beginning 

 to form the roof of the 

 future gut (g.); in other 

 words, establishing the hypo- 

 blast. This inflected arc of 

 the blastoderm corresponds 

 to the blastopore or mouth 

 of the gastrula, which is 

 much disguised by the pres- 

 ence of a large quantity of 



yolk. 



FIG. 291. Embryo dogfish in egg- 

 case ("mermaid's purse") which 

 has been cut open to show con- 

 tents. 



AS the invagination e.g., "External" gills; d.f., dorsal fin 

 dPo-mpn l-i firm fold; .y.f., yolk-sac; st., stalk of yolk- 



segmentation sac . T ? tendr ii s , prolongations of egg- 



case b Y means of which it is moored to 

 seaweed, an "adaptation" in a dead 



substance. 



cavity is Obliterated. The 

 n r .LI. 



floor of the gut is formed by 

 infolding of the lateral walls. 



Along the mid-dorsal line of the epiblast a medullary 

 groove appears the beginning of the central nervous 

 system. Its sides afterwards arch towards one another, and 

 meet to form a medullary canal (Fig. 290, n.c.\ A posterior 

 communication between this dorsal nervous tube above and 

 the ventral alimentary tube persists for some time as the 

 neurenteric canal (Fig. 290, ne.c.\ 



