MESOBLAST 



form from before backwards, but the head region is not segmented 

 m the embryo of the frog, though it is so in the embryo dogfish. 



fhr. m.br ^*''- ^Act. 



nch. 



' — »i«.«. 



-an. 



OUgg. 



Fig. 495. — Sections of an embryo frog at about the stage of Fig. 494. 



A, Transverse ; B, longitudinal. 



an., Anus ; coel., coelom ; ed., ectoderm or epiblast ; end., endoderm or hypoblast ; f.br., fore-brain ; h.br., 

 hind-brain ; ht., rudiment of heart ; int., intestine ; l.p., lateral plate of mesoblast ; Ir., rudiment of 

 liver ; m.br., mid-brain ; m.s., mesoblastic somite ; mes., mesoblast ; mes'., mesoblast continuous with 

 epiblast of neural canal and hypoblast of notochord in region of primitive streak ; m.c, neurenteric 

 canal (closing) ; nch., notochord ; ph., pharynx ; pit., rudiment of pituitar>' body ; so.m., somatic 

 mesoblast ; sp.c, spinad cord ; sp.tn., splanchnic mesoblast ; stm., stomodsum. 



EXTERNAL FEATURES OF EMBRYO 



Meanwhile external changes have been taking place. The 

 embryo at the end of gastrulation was still roughly spherical, 

 the blastopore marking the future hind end. In front of this 

 the future dorsal ectoderm flattens to form the pear-shaped 

 neural plate. The edges of this rise up owing to the incipient 

 formation of folds — the neural folds, which are continuous in 

 front, and behind join the side lips of the blastopore. On either 

 flank of the anterior end of the neural plate appears a thickening 

 which becomes divided by a furrow into a gill plate and a sense 

 plate which joins with its fellow of the opposite side. A neural 

 groove appears along the middle of the neural plate, while the 



