OLIGOENTOMATA AND APTILOTA 



179 



The part played by cells from the middle strand has already been men- 

 tioned. The muscle layer of the ventriculus, as in other insects, arises 

 from the splanchnic wall of the mesoderm. 



lim.m 



Fig. 93. — Isotoma. Frontal section through posterior end of body, (g) Gonad and 

 duct. {lim. m) Limiting membrane, {mge) Mid-gut epithelium, (mws) Muscle, {proct) 

 Proctodaeum. 



guf. 



Fig. 94. — Isotoma. Sagittal section of late embryo, (ant) Antenna, (br) Brain. 

 iggl) Ganglion, (gut 1) Esophagus, {gut 2) Mid-gut. {gut 3) Hind-gut. (suboesg) 

 Subesophageal ganglion, {tv) Tubus ventralis. 



Before the yolk in the ventriculus has wholly disappeared, the thin 

 wall (Fig. 93, lim.m) that separates the proctodaeum from the ventriculus 

 is absorbed, thus establishing continuity. The membrane that separates 



