368 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



terior mesenteron ribbons that grow cephalad from the posterior enteron 

 rudiment at the end of the proctodaeum is similar to the formation of the 

 anterior pair already described. The two pairs of ribbons meet and fuse 



Fig. 324. — Sciara. Cross section of 60-hour embryo. Differentiation of somatic 

 (som. m) and splanchnic {splm) mesoderm, (ect) Ectoderm, {mge) Mid-gut epithelial 

 ribbons, {nc) Nerve cord. 



at the fiftieth hour to form two narrow strands of mid-gut epithelium 

 connecting the stomodaeum and proctodaeum. 



These strands now grow broader until they meet and join along the 

 mid-ventral line to form a shallow trough in which the yolk rests. The 





U 



<\ 



J, : 



■N. 



^^^ 



^. 



b' 



^ggi ect- 



FiG. 325. — Sciara. Cross section of 85-hour embryo, (ect) Ectoderm, (ggl) Ganglion. 

 (mge) Mid-gut epithelium, {som. m) Somatic mesoderm, {splm) Splanchnic mesoderm. 

 iv) Yolk. 



cells at the outer edges of the trough continue to divide so that the trough 

 increases in depth until at the ninetieth hour it has grown completely 

 around the yolk and the edges have fused on the dorsal side to form a 

 tube, the mid-gut. The closure of the dorsal wall of the mid-gut occurs 



