THE GERM-BAND AXD THE GERM-EXVELOPE. f 269 



A 



and an inner layer which comprises n 



the entoderm and the mesoderm, 

 and as long as these two germ- 

 layers are not sharply distinguished 

 one from the other, this is known 

 as the inner or lower layer (n). . 



It is characteristic of the Insecta, 

 but only rarely occurs in other 

 Arthropoda (e.g., in the Scorpiones, 

 Fig. 3, p. 5), that the germ-band 

 does not remain freely exposed on 

 the surface of the egg, but is grown 

 over by an amniotic fold rising 

 from its edges (Figs. 132 A, af, 

 and 133 B, af), so that the former 

 appears somewhat sunk below the 

 ventral surface. As the amniotic 

 fold extends from all sides over 

 the germ-band, a cavity is enclosed 

 between the two. This is the 

 amniotic cavity (ah), which, when 

 the amniotic folds have completely 

 covered the <?erm-band and have 

 united over it, appears as an en- 

 tirely closed cavity (Figs. 132 5 and 

 133 C). (See footnote t, p. 268.) 



The germ-band, after its develop- 

 ment, thus appears covered by a 

 double cellular envelope derived 

 from the amniotic fold. The outer 

 of these two envelopes is distin- 

 guished as the serosa (s). This 

 passes without any break of con- 

 tinuity into that undifferentiated 

 portion of the blastoderm which 

 takes no part in the formation of 

 the germ-band (Fig. 132, s), and 



Fig. 133.— Transverse sections through three consecutive stages in the formation of the 

 germ-band and the embryonic envelopes of an insect-embryo. A, formation of the ventral 

 plate (bp) and the gastrula-invagination (p). B, rise of the amniotic fold ("/). C, complete 

 overgrowth of the genn-band by the amniotic folds, o, ventral, d, dorsal surface ; af, amniotic 

 fold; ah, amniotic cavity; am, amnion; W, blastoderm; '»/), ventral plate; do, food-yolk; 

 ec, ectoderm ; <j, gastrula-invagination ; s, serosa ; u, lower layer. 



