FORMATION OF THE GERM-LAYERS 



559 



formation a more median section of the ventral plate, the so-called 

 middle plate (ra), becomes separated from the side plates (s). As 

 the middle plate curves in and becomes overgrown by the folds 

 forming the edges of the blastopore, the gastrula-tube (Fig. 539, 

 A, r) is formed, and furnishes the rudiments of the lower (inner) 

 layer. The ectoderm, then, according to Heider, arises from the 



V _ . 



FIG. 585. Two embryonic stages of a saw-fly (Hylotoma berberiditt) in schematic median sec- 

 tion : a l - 10 , 1st to 10th abdominal segments; bff, ventral nervous cord; off, brain; ol, germ of 

 labrum ; up, salivary gland; ed, hind-gut ; a?, *', inner folds of amnion : other letters as before. 

 After Graber, from Korschelt and Heider. 



lateral plates of the primitive band. The growth of the edges 

 of the blastopore, by which the closure of the gastrula-tube 

 is effected, takes place latest in the region of the most anterior 

 part of the furrow (Fig. 515, B and (7), corresponding to that 

 place in the primitive band in which the stomodeeum afterwards 

 develops. 



