A TYPE OF EMBRYONIC DEVELOPMENT IN INSECTS 



15 



Gradually appendages appear on the segments, the antennae arising pos- 

 torally but later migrating forward. In front of the stomodaeal opening 

 which has formed in the first segment is the labrum. The hypopharynx 

 which develops from the fusion of two small elevations near the median 

 line behind the stomodaeum belongs to the intercalary segment; the basal 



coel 



neur neurg 



Fig. 16. — Cross section of embryo, (am) Amnion, (coel) Coelomic cavities, (ect) 

 Ectoderm, (mes) Mesoderm, (mst) Median nerve strand, (neur) Neuroblasts, (neurg) 

 Neural groove. 



part of the hypopharynx, however, is formed from the sternal regions of 

 the jaw segments. The jaws and the legs at first appear as short, broad 

 elevations (Fig. 15). The second maxillae later fuse along the median 

 line to form the labium. As development proceeds, the appendages 

 become segmented. On the first abdominal segment they are glandular 

 embrvonic structures (pleuropodia). 



mds 



'm^w 



ect f iicui f-jc 



Fig. 17. — Cross section of embryo, (am) Amnion, (coel) Coelomic cavity, (ect) 

 Ectoderm, (eps) Beginning of epineural sinus. (/) Fat body, (gr) Genital ridge, (mds) 

 Median entoderm strand, (neur) Neuroblast, (nc) Developing nerve cord, (yc) 

 Yolk cell. 



Differentiation of the Mesoderm. — The first indication of mesoderm 

 segmentation occurs when the intersegmental parts of the lateral bands 

 become thinner or actually rupture. The segmental parts (somites) 

 acquire in each lateral half a lumen (Fig. 16, coel), which in section is 

 more or less triangular or circular in the thoracic and in at least some of 



