60 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



of the envelopes become invaginated into the 3^olk; forming the secondary 

 dorsal organ. 



Heymons (1895a) found that in Gryllus, after rotation, the serosa at 

 first covers the yolk at the head end, then sinks into the yolk to be 

 absorbed. Meanwhile the amnion forms the provisional dorsal closure 

 in the usual manner. As the body wall grows dorsad, the amnion 

 invaginates into the yolk in tubular form and is soon covered over by 

 the ectoderm, later to degenerate. The secondary dorsal organ in 

 Gryllus, therefore, owes its origin to the amnion instead of the serosa. 



ind.o 

 ind.i 



Fig. 36. — Xiphidium ensiferum. Development of the indusium {ind). Blastokinesis. 

 {am) Amnion, (c/i) Chorion, {gb) Germ band, {ind) Indusium: (t) inner, (o) outer. 

 {ser) Serosa, {y) Yolk. {From Wheeler.) 



THE INDUSIUM 



The organ named the "indusium" by Wheeler (1893) is found in two 

 species of meadow grasshoppers belonging to the genera Xiphidium and 

 Orchelimum. It arises as a simple circular thickening of the blastoderm 

 between and a little in front of the procephalic lobes (Figs. 36, 37, itid). 

 It divides into an outer and an inner layer and afterward spreads over 

 nearly the whole surface of the egg, leaving the poles uncovered. The 

 outer indusial envelope becomes closely applied to the serosa except at 

 the two poles and remains intact until hatching of the nymph. The 

 inner indusial envelope fuses with the amnion near the head of the embryo 



