304 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



disappeared. By the tenth day they have fused with the head lobes to 

 form the definitive head, their appendages now being grouped around 

 the mouth (Fig. 249). 



During the time that the appendages 

 have been developing, the neuroblasts 

 through continued divisions produce the 

 neural cells that will form the ventral nerve 

 cord (Fig. 247). Gangha appear in each 

 segment, and the neuropile is apparent by 

 the sixth day (Fig. 251). The coelomic sacs 

 disappear at this time, the mesoderm now 

 consisting of the somatic layer next to the 

 body wall and the splanchnic layer lying 

 immediately under the developing mid-gut 

 ribbons (Fig. 251, mge). As the nerve cord 

 develops, the ectodermal wall beneath it 

 becomes very thin so that from the ventral 

 side the ganglia and the two longitudinal 

 connectives of the nerve cord can be plainly 

 seen through the integument (Fig. 249). 

 On the eighth day the body wall of the 

 embryo together with the mid-gut ribbons has grown around the yolk 

 until about three-quarters of the yolk is covered. Two days later the 

 two edges of the advancing body wall have met in the anterior and 

 posterior ends leaving only a small area of the yolk still exposed (Fig. 250). 



Fig. 250. — Brachyrhinvs 

 Ten-day embryo. Dorsal as 

 pect showing dorsal closure. 



neurp 



Fig. 251.— Brachyrhinus. Cross section of abdomen of six-day embryo, (ect) Ecto- 

 derm, (mge) Mid-gut epithelial ribbon. («c) Nerve cord, (neurp) Neuropile. (splm) 

 Splanchnic mesoderm. 



A few hours later this, too, is covered. The embryo remains in the egg 

 after the dorsal wall is complete for several days before hatching occurs. 



The stomodaeum is formed at the time of segmentation and appears 

 as a shallow pit on the third day when the embryo reaches its greatest 



