308 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



The mid-gut ribbons apparently develop from the ends of the stomo- 

 daeum and proctodaeum. They extend beneath the yolk and grow- 

 posteriorly and anteriorly until they meet and fuse. As they lengthen 

 they also widen out, eventually closing in under the yolk to form a trough. 

 On the sixth day they are already well developed but have not met along 



stom 



Fig. 257. — Brachyrhinus. Sagittal section of ten-day embryo. i]br) Brain, {h) Heart, 

 (rreffg) Mid-gut epithelium, (nc) Nerve cord. (procO Proctodaeum. (s«om) Stomodaeum- 

 (x) Loop of mid-gut. 



the mid-ventral longitudinal line (Fig. 251). Two days later the trough 

 of the mid-gut is completed ventrally, and the embryo has grown more 

 than halfway around the yolk toward the dorsal side (Fig. 256). 



The dorsal wall of the mid-gut and the dorsal wall of the ectoderm are 

 completed at the same time. For several days the mid-gut walls appar- 

 ently continue to grow, for in an embryo shortly before hatching there 

 appears a fold on the ventral side of the mid-gut that extends deeply into 

 the yolk (Fig. 257). 



