180 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



stomodaeum from the ventriculus, however, is not broken down until the 

 .yolk is wholly absorbed. It may be observed that late in the develop- 

 mental period a blue pigment forms in the ectoderm, a characteristic 

 common to the adult also. This pigment is also present in both stomo- 

 daeum and proctodaeum but absent in the cells of the ventriculus, an 

 indication, perhaps, that the mid-section and the end sections of the 

 alimentary canal have a different origin. There is no indication in 

 Isotoma that the yolk cells play any part in the formation of the digestive 

 epithelium. 



After 10 days' incubation the neuropile is first in evidence in the dorsal 

 part of each ganglion with nerve cells surrounding it. The three-seg- 



am . ser 



Fig. 95. — Isotoma. Cross section 

 through first abdominal segment. 

 {am. ser) Amnioserosa. {ggl) Gan- 

 glion, (mge) Mid-gut epithelium. 

 (neurp) Neuropile. 



Fig. 96. — Isotoma. Cross sec- 

 tion of third abdominal segment. 

 {am. ser) Amnioserosa. {go) 

 Germ cells, (ggl) Ganglion. 

 (mge) Mid-gut epithelium. 

 (neurp) Neuropile. (ret) Ret- 

 inaculum. 



mented brain is fully formed during the latter half of the developmental 

 period, when also the ganglia of three gnathal segments fuse into the 

 definitive subesophageal ganglia. Later also the cephalization and fusion 

 of the abdominal ganglia take place (Fig. 94, ggl). In general, the devel- 

 opment of the nervous system appears to follow the course of pterygote 

 insects. 



The differentiation of the mesodermic somites in each segment begins 

 about 10 days after egg deposition. Whether the absence of an epineural 

 sinus is characteristic (Figs. 95, 96) or due to an artifact is not apparent. 

 The laterodorsal muscles are derived from the somatic walls of the 

 somites; mesad of this is the fat body (/) ; and still nearer the median fine, 

 the ventral longitudinal muscles, both derived from the median somatic 

 walls of the somites. The splanchnic wall of the somites furnishes the 

 muscles of the ventriculus and later also takes part in the formation of the 



