184 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



Numerous scattered entoderm cells (ent) that are derived from the ento- 

 derm cells of the ring wall (Fig. 102, ent) and that were pushed toward the 

 yolk may now be observed on the inside of the thin blastoderm wall 

 (Fig. 103, ent). As development proceeds, the upper, nonembryonic part 

 of the blastoderm becomes thin, while in the lower or embryonic part the 

 outer layer of ectodermal cells becomes columnar and those of the inner 

 layer flat (Fig. 104). The germ band is broadest at the head end where 

 the head lobes are prominent, narrower in the middle section, and again 

 somewhat wider at the posterior end (Fig. 105). At this stage the thin 

 lateral parts of the original blastoderm may be designated as the "amnio- 



do 



mmm'^ 



J'-> 





-^?. 



■.V® 



Fig. 105. — Campodea. Germ band. 



Fig. 106. — Campodea. Embryo 

 with dorsal organ (do). 



serosa." Meanwhile, the inner-layer mesoderm in the abdomen has 

 divided longitudinally. Segmentation of head and thorax occurs with the 

 appendages (labrum, antennae, mouth parts, and legs) becoming evident. 

 With the segmentation of the anterior part of the abdomen the proctodeal 

 invagination appears. When segmentation of the abdomen is completed, 

 the appendages of the intercalary (premandibular) segment become 

 distinct, and later rudimentary appendages form on the intermediate 

 abdominal segments. The last (tenth) abdominal segment later will be 

 provided with cerci. 



The entoderm cells (Fig. 104, ent), which at an early stage are found 

 rather uniformly dispersed on the inner surface, later migrate into the yolk 

 where they become distributed. Of their further history Uzel (1898) has 

 given no information. 



Before the segmentation of the abdomen has been completed, a dorsal 

 migration of some cells from the amnioserosa gives rise to a primary 

 dorsal organ (Fig. 106, do). Meanwhile an extremely thin cuticula has 

 been shed from the surface of the embryo and envelopes and lies close 



