224 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



which meets the germ disk some distance inward from the edge of the 

 latter. By the thirty-fourth hour the membrane has degenerated, and no 

 trace of it can be seen, thus being much more ephem- 

 eral than in Carausius where it has a share in the 

 formation of the mid-gut epithelium. 



At about the thirty-fourth hour the germ disk 

 is more markedly multilayered than before, but this 

 condition is only temporary and disappears before 

 the differentiation of the inner layer, a process that 

 takes place in a unilayered germ disk. About the 

 forty-second hour the inner layer {il) is seen to arise 

 as a proliferation of cells from the roof of a deep 

 median-ventral groove (second ventral groove, or 

 gastral groove). This inner layer first is seen near 

 the anterior end but extends rapidly caudad and 

 finally extends almost the entire length of the embryo 

 with the exception of the extreme cephalic end (Figs. 

 141, 145, il). The second ventral groove lasts but 

 three or four hours and is not visible after about the 

 forty-sixth hour. 



The inner-layer formation in the Acrididae, 

 according to Roonwal, occurs only by means of cell 

 proliferation from the roof of the mid-ventral, 

 longitudinal invagination. In Locusta migratoria the 

 differentiation of the inner layer occurs from a single area on an elongated 

 median longitudinal line proceeding from the cephalic to the caudal end 



FiQ. 139.— Lo- 

 custa. Embryonic re- 

 gion. 28-hour stage, 

 (yc) Secondary yolk 

 cell. 



Fig. 140. — Locusta. 30-hour stage, (c) Cells proliferating from roof of ventral groove 

 {vg). iy) Yolk, (yc) Secondary yolk cells, (ycm) Yolk-cell membrane. 



(Figs. 142, 143, 144, il). The ectoderm, or outer layer, now becomes more 

 than one-layered in places, the lateral halves of the germ band becoming 

 thicker in the middle than on the sides. 



