386 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



pren 



ter-4; 



-pl4 



p21 



Upon completion of the segmentation the lateral segmented mesoderm 

 bands become more widely separated (Fig. 341 A), rendering the one- 

 layered middle ectodermal strip {memhrana ventralis) more conspicuous. 

 Meanwhile the anlagen of the appendages appear. The lateral part of 

 the segment will develop into the tergite, the mesal part into the sternite 

 (Fig. S4:lA,ter, ster). 



The appendages now begin to lengthen; the antennae (ant) become 

 preoral in position; the unpaired labrum (Ir) develops as a lobe on the 



clypeal margin and overhangs the 

 mouth. A prominence in front of 

 each antenna that is pushed forward 

 by the anterior migration of the 

 antenna also elongates to become 

 the preantenna (Fig. 341A, pren). 



Bending of the Germ Band.^ — 

 When the segmentation of the germ 

 band has been completed and the 

 rudiments of the appendages are 

 well established, the right and left 

 halves of the germ band separate 

 more and more (Fig. 3-41 A) from 

 each other except at the two ex- 

 tremities. Each half bends sharply 

 at about the ninth and tenth seg- 

 ments to such an extent that the 

 anterior and posterior sternal parts 

 face each other (Fig. 342). The 

 memhrana ventralis (vm) stretches, 

 and the head and tail approach 

 each other until they meet (Fig. 3415, side view). The stretching of 

 the memhrana ventralis is accompanied by the formation of a deep 

 transverse furrow through it, cutting deeply into the yolk which finally 

 divides it, except for a dorsal bridge, into two parts. Figure 343 is a 

 diagrammatic representation of a section taken at right angles to the 

 furrow of the memhrana ventralis when the crests of the furrow from each 

 side have met inside, completely dividing the yolk. The halves of the 

 germ band, in contact only at head and tail (Fig. 34 IB), he separated from 

 each other by the yolk, connected dorsally by the memhrana dorsalis 

 (blastoderm) and ventrally by the memhrana ventralis (Figs. 343, 344). 



Development of Body Wall and Appendages. — As has already been 

 stated, the rudiments of the appendages develop in a row near the center 

 of each lateral strip, the lateral portion of the strip to form the tergites; 

 the mesal portion, the sternites. As development progresses, the anlagen 



prz- 



tel 



Fig. 342. — Scolopendra cingulata. For- 

 mation of yolk furrow, {ant) Antenna. 

 {cly) Clypeus. {dm) Membrana dorsalis. 

 {Ir) Labrum. (p) Leg. {pren) Prean- 

 tenna. {prz) Proliferation zone, {tel) Tel- 

 son, {ter) Tergite. {vm) Membrana ven- 

 tralis. {Adapted from Heymons.) 



