FORMATION OF THE GERM-LAYERS 



550 



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

 middle plate (w), 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, ?') is formed, and furnishes the rudiments of the lower (inner) 

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



Fir,. 535. Two embryonic stages of a saw-fly (Hylotoma berlierii1ix\ in schematic median sec- 

 tion : <i l -tt l , 1st to luth abdominal segments; Jiff, ventral nervous cord; 0/7, brain ; ol, germ of 

 labruin ; */>, >alivary irland ; e<l, hind-gut ; a-, 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 baud in which the stomoclseum afterwards 

 develops. 



