OLIGOENTOMATA AND APT I LOT A 



171 



beyond the cellular layer and spread out mushroom-like. The outer end 

 in most cases shows a connection with the second crenated membrane 

 (Figs. 72, 74) but in some cases is still connected with the first crenated 

 membrane (Fig. 75). The dorsal organ appears to be glandular and may 



Fig. 75. — Isotoma. Dorsal organ, 



ser) Amnioserosa. {cut) Crenated membrane. 



possibly be concerned with the molting of the crenated membranes. 

 Buds of head and thoracic appendages are now evident as well as the 

 anterior abdominal segments (Figs. 76, 77). A section through the region 

 of an appendage now reveals that the inner layer consists of two layers 

 laterally and of one layer in the middle which indicates developing 

 coelomic sacs ("somites") without a cavity (Fig. 78). In this species 



Fig. 76. — Isotoma. Embryo before blastokinesis. (ant) Antenna, {do) Dorsal 

 organ, {gc) Germ cells, {hi) Head lobe, {lb) Labium, {md) Mandible, {mx) Maxilla. 

 {th) Thoracic appendages. 



Philiptschenko (1912) found no evidence of an intercalary segment 

 between antennae and mandibles. With the increase in size of the 

 antennae their position becomes more lateral although at this stage still 

 postoral. 



