340 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



a striking difference, the cells of the former being in active mitotic 

 division, whereas the latter, binucleate and quadrinucleate, have ceased 

 dividing (Fig. 295 A). The germ band, when thus first formed, lies 

 wrapped around the equator of the yolk, in length 

 nearly equal to the circumference of the egg, the 

 head and tail ends being separated by a narrow 

 isthmus of serosa cells. The band is about one- 

 third as wide as it is long (Fig. 296). Though 

 most of the cells of the germ band are actively 

 dividing, the divisions are not synchronous. The 

 serosa cells and those of the germ band are still 

 about equal in thickness. At this stage the serosa 

 is dumbbell-shaped and occupies rather less area 

 With its edges now lapping over the margin of the 

 germ band the serosa gradually spreads, until by the sixteenth hour the 

 germ band and yolk are entirely covered. The increase in the area of 



Fig. 296. — Diacrisia. 

 Germ band {gh) 10 hours 

 old. (ser) Serosa. 



than the germ band. 



Fig. 297. — Diacrisia. Embryos free from yolk and envelopes, as seen from the yolk side. 

 A, 30-hour; B, 35-hour; C, 42-hour; Z), 54-hour embryo. 



the serosa is accomplished by a spreading and thinning of the membrane 

 and not by cell proliferation. So far there is no trace of an amnion. 

 The completed serosa, at this stage thinner than the germ band, exhibits 



