OLIGOENTOMATA AND APTILOTA 



175 



of the fourth abdominal segment (the fiirca), as well as abdominal seg- 

 ments five and six, being directed outwardly. The embryo retains this 

 position until emergence. It is obvious that frontal sections made of the 

 head of the embryo at this time 



,,-^^^y 



if carried on through the body, would 

 do 



<■.' 



T\ 



^^ 







'% 



Fig. 84. — Isotoma. Late embryo, (ant) 



fk 



Antenna. 



•^' 



arrt- 



(do) Dorsal organ. 



(fk) Furca. 



furnish frontal sections of the abdomen but cross sections of the thorax. 

 The proctodaeal invagination is located in the sixth segment; the telsonis 

 fused with it and represented by a small supra-anal and two subanal lobes. 



As development progresses, the 

 edges of the ectoderm of the body 

 wall, by proliferation, gradually 

 replace the amnioserosa which 

 forms the temporary lateral and 

 dorsal wall of the embryo. Mean- 

 while, the lower layer has become 

 segmented to form the somites or 

 coelomic mesodermic masses. In 

 the gnathal region of the head, the 

 inner-layer bridge above the neural 

 ridge, which connects the right and 

 left sides, has disappeared (Fig. 85) 

 but is still present in thorax (Fig. 

 82) and abdomen. The lateral 

 mesodermic somites are apparently 

 lacking in the sixth abdominal segment, but the posterior inner-layer cell 

 mass covering the tip of the proctodaeal invagination is present (Fig. 86) , 

 which, hke the anterior cell mass, takes no part in the formation of the 13 

 pairs of somites. 



coel 



^proi 



palp 



mx 



Fig. 85. — Isotoma. Cross section of 

 maxillary segment, {am. ser) Amnio- 

 serosa. (.coel) Coelome. (mx) Maxilla. 

 (rO Nerve cord, (palp) Maxillary palpus. 

 (prot) Protocerebrum. 



