176 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



At this time, i.e., about 10 days after egg deposition, there are 13 pairs 

 of nerve gangha, those of the abdomen as yet less pronounced, none being 



8G. — Isotoma. Sagittal section of posterior end. (abd) Abdominal appendages. 

 igc) Germ cells, (il) Inner layer, (prod) Proctodaeum. 



found in the sixth segment, and indicated only by the apparent presence 

 of ephemeral neuroblasts in the fifth segment. The head (Figs. 85, 87, 

 n.ggl) and thoracic ganglia are well developed; of the three brain segments 

 the tritocerebrum is smallest and postoral in position. The germ cells, 



am.ser 



stem 



Fig. 87.~Isotoma. Half of 

 cross section through labial seg- 

 ment, (ggl) Ganglion, (hg) Tubu- 

 lar head gland, (lb) Labium. 

 {palp) Labial palp. 



Fig. 88. — Isotoma. Oblique section 

 through mandibles, (am. ser) Amnioserosa. 

 (67-) Brain, (md) Mandible, (si) 

 (stem) Stomodaeum. 



though increased in number at this stage, are still in the yolk near the 

 posterior part of the embryo (Fig. 86). 



