EMBRYOLOGY 491 



mid-dorsal line, but presently it separates here also, leaving 

 in the middle a cord of cells attached to the dorsal wall of 

 the gut. This cord soon separates as the notochord. The 

 mesoblast forms a sheet on each side of the gut, below 

 which the two sheets soon meet, enclosing it completely 

 except in the mid-dorsal line, where the notochord lies 

 (Fig. 371). On each side of this line the mesoblast is 

 thicker than elsewhere, forming the segmental plate. A 

 split, the rudiment of the ccelom, appears and separates an 

 outer or somatic , -layer from an inner or splanchnic layer. 



end. -_, 



blc- 



-ond 



A B 



FlG. 371. — Sections of an embryo at about the stage of Fig.1370. 



A , Vertical longitudinal ; B, transverse. 

 die, Blastocoele ; end., yolky cells of endoderm ; end'., small cells of endoderm ; 

 ent., gut ; mes., mesoderm ; n.f., neural fold ; n.p., neural plate ; nek., noto- 

 chord ; y.p.y yolk plug. 



This split does not extend far into the segmental plate, 

 from which it soon disappears. The segmental plates now 

 divide into a series of blocks or mesoblastic somites, 

 separating from the lateral plates, which do not segment. 

 The somites form from before backwards, but the head 

 region is not segmented in the frog, though it is so in the 

 embryo dogfish. 



Meanwhile external changes have been taking place. 



The embryo at the end of gastrulation was 

 External s till roughly spherical, the blastopore mark- 



Embryo. 50 ing the future hind end. In front of this 



the future dorsal surface flattens to form the 

 neural plate. The edges of this thicken to form the neural 



