214 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



ceph 



to form the serosa. The inner layer, or, as Cholodkovvsky (1891) terms 

 it, the primary ''entoderm," is formed by a feeble invagination followed 

 by delamination, proliferation, and migration. At the cephalic end the 

 inner layer divides to underlie the cephalic lobes which have been formed 

 by lateral proliferation of the germ band. The amnion forms in a normal 

 way (Fig. 129), the tail fold appearing first (Fig. 130). 



Segmentation is not evident until the rudiments of the appendages are 

 well developed. Riley (1904) recognized six head segments: the ocular 

 (preoral), antennal, second antennal (premandibular), 

 mandibular, maxillary, and second maxillary (labial). 

 The leg rudiments lie folded beneath the thorax and 

 the anterior part of the abdomen. The rudimentary 

 appendages of the abdomen disappear about six days 

 after fertilization, except the pair on the first segment 

 which persists until the envelopes rupture. The 

 segments, as also the appendages, develop from the 

 cephalic toward the posterior end. 



With the lengthening of the embryo the caudal end 

 folds over ventrally (Fig. 131), and then the embryo 

 shifts to a more posterior position (Fig. 132). Mean- 

 while, further lengthening pushes the head forward 

 until it reaches the cephalic end of the egg. With 

 this increase in length the amnion and serosa tear 

 longitudinally along the mid-ventral line, the amnion 

 reflecting back although maintaining its connection 

 with the margin of the embryo, the serosa contracting 

 until both amnion and serosa are drawn into the yolk 

 on the dorsal side in the prothoracic region. While 

 the remains of the envelopes are gathering on the 

 dorsal side, the lateral parts of the body wall grow upward to enclose the 

 yolk. The dorsal growth of the body wall is accompanied by the dorsal 

 growth of the mesoderm and the mid-gut epithelium. 



When the ectodermic evaginations for the appendages of head, thorax, 

 and abdomen appear, the mesoderm layer having become segmented 

 acquires a lumen for each segment except the first and last. These sacs 

 are approximately triangular in shape in cross section and more or less 

 trilobed in the abdominal segments, the dorsal (splanchnic) part in contact 

 with the yolk, the remaining (somatic) part largely lining the ectoderm 

 of the appendages. Between the right and left rows of sacs there is 

 formed a free space, the epineural sinus, just above the nerve cord. 

 When the somatic walls of the coelomic sacs break down to form the 

 muscles, the epineural sinus and coelomic cavities together will constitute 

 the schizocoele (pseudocode), or definitive body cavity, of the insect. 



Fig. 130. — Blat- 

 tella. Frontal aspect 

 of embryo. Amnio- 

 serosal folds {am. ser) 

 extending over cephal- 

 ic and caudal ends. 

 {ceph) Cephalic lobes. 



