OLIGOENTOMATA AND APTILOTA 



173 



merited, three divisions posterior to the tubus ventralis usually being 

 visible, while at the posterior end the beginning of the proctodaeum is 



Fig. 



abd4— 



abd3- 



mx I ma 



abd' 



79. — Isotoma. Longitudinal section, {abd) Abdominal rudiments, {do) Dorsal 

 organ, {hi) Head lobe, {md) Mandible, (mx) Maxillae. 



evident. The stomodaeum, which made its appearance a short time 

 before, has lengthened, the apex of the invagination reaching the yolk 

 (Fig. 80), the inner layer being absent at this point. Both in front of and 

 behind the stomodaeum is a cell mass which will give rise to the anterior 



"=-?S 



sfom 



Fig. 80. — Isotoma. Sagittal 

 section of anterior end. {am. 

 ser) Amnioserosa. {hi) Head 

 lobe. {Ir) Labrum. {nc) Nerve 

 {stom) Stomodaeum. 



■md 



Fig. 81. — Isotoma. Cross section of 

 mandibular segment, {coel) Coelome. 

 {ect) Ectoderm, {md) Mandible, (mds) 

 Middle strand of inner layer, {n) Nerve 

 tissue, {si) Superlingua. 



end of the future digestive epithelium of the ventriculus. Meanwhile, the 

 cells of the nervous system (Figs. 81, 82) become more sharply marked, 



