

DEVELOPMENT 



133 



represented by three pairs of fundaments, and in Thysanura and 

 Collembola a fourth pair is present to form the superlinguae (Fig. 198, si), 

 already referred to. Next in order are the three pairs of thoracic legs 

 (Fig. 197) and then, in many eases, paired abdominal appendages (Figs. 

 197, 199), indicating an ancestral myriopod-like condition; some of these 

 abdominal limbs disappear in the embryo but others develop into abdom- 

 inal prolegs (Lepidoptera and Tenthre- 

 dinidae), external genital organs (Orthop- 

 tera, Hymenoptera, etc.) or other structures. 

 The study of these embryonic fundaments 

 sheds much light upon the morphology of 

 the appendages and the subject of segmen- 

 tation. 



Two Types of Germ Bands. The germ 

 band described above belongs to the simple 

 overgrown type, exemplified in Clytra, in 

 which the germ band retains its original 

 position and the amnion and serosa arise by 

 a process of overgrowth (Figs. 193, 194), as 

 distinguished from the invaginated type, 

 illustrated in Odonata, in which the germ 

 band invaginates into the egg, as in Fig. 

 200, until the ventral surface of the embryo 

 becomes turned around and faces the dorsal 

 side of the egg. In this event, a subse- 

 quent process of revolution occurs, by 

 means of which the ventral surface of the 

 embryo resumes its original position (Fig. 

 201). 



Dorsal Closure. As was said, the germ 

 band forms the ventral part of the insect. 

 To complete the general form of the body 

 the margins of the germ band extend out- 

 ward and upward (Fig. 202) until they 

 finally close over to form the dorsal wall of the insect. Besides this 

 simple method, however, there are several other ways in which the 

 dorsal closure may be effected. 



Nervous System. Soon after gastrulation, the ventral nervous 

 system arises as a pair of parallel cords from cells (Fig. 203, n) which 

 have been derived by direct proliferation from those of the germ band, 



FIG. 199. Embryo of CEcan- 

 thus, ventral aspect, a, antenna; 

 a 1 -^ 5 , abdominal appendages; e, 

 end of abdomen; /, labrum; li, 

 left fundament of labium; Ip, 

 labial palpus; Z 1 -/ 3 , thoracic legs; 

 m, mandible; mp, maxillary 

 palpus; mx, maxilla; p, pro- 

 cephalic lobe; pr, proctodseum. 

 After AYERS. 



