OLIGOENTOMATA AND APTILOTA 



181 



gonads. On the dorsal edge of the developing somite (Fig. 97, cbl) are 

 the minute cardioblasts. 



Of special interest are labial glandular structures, not found in ptery- 

 gote insects unless they correspond to the embryonic esophageal gland. 

 In Isotoma some time before the differentiation of the mesodermic 

 somites, in the labial segment at the level of the nerve ganglion, a little 

 glandular vesicle (Fig. 87, hg) is in evidence. Later during the course of 

 development the vesicle becomes flask-shaped with a slender duct leading 

 toward but not communicating with the exterior (Fig. 98, hg). This 



Fig. 97. — Isotoma. Cross section 

 of thorax, {am. ser) Amnioserosa. 

 (cbl) Cardioblasts. (do) Dorsal organ. 

 (ggl) Ganglion, (mge) Mid-gut ep- 

 ithelium, {mus) Muscle, (neurp) 

 Neuropile. {yc) Yolk cell. 



Fig. 98. — Isotoma. 

 tion through head. 

 Ocelli. (,ggl) Ganglion. 



Parasagittal sec- 



(6r) Brain, (oc) 



{hg) Head gland. 



structure is therefore not of ectodermic origin, though later an ectodermic 

 connection with the exterior is established. According to the evidence 

 presented by Philiptschenko (1912) these glandular structures appear 

 to be homologous with nephrideal head glands found in certain other 

 Arthropoda. 



As has already been stated, the germ cells of Isotoma were segregated 

 in the 16- or 32-cell stage. By the time the proctodaeum is well formed, 

 the germ cells have divided into two groups and pass from the yolk into 

 the tissue of the visceral wall of the mesodermic somites (Fig. 99, gc) of the 

 third and fourth abdominal segments. Later the germ cells migrate 

 forward into the first to the third segments where the gonads form ventrad 

 of the aUmentary canal. The gonads and their ducts develop from the 

 visceral mesoderm, the latter in the fourth abdominal segment. By 

 reason of their ventral position, the gonads do not develop a terminal 

 filament. 



