1H \ I I'M' 



represented by three pan- <>i fundament >. and in Thy~am;i 



Cnllembola a fourth pair i> present to form the -nperlin 



already referre. I to. Next in order are the three pairs of thor.nic legs 



and then, in many CaSCS, paired abdominal appendage- 

 . indicating an ancestral myn'opod-likc condition ; t hesc 



abdominal limhs disappear in t lie embryo hut ot her- develop int.. abdom - 

 inal prolei^s ( Lcpidoptcra and Tent tin- 

 dinid.i rnal genital organs (Orthop- 



Hymenoptera, etc.) or other structure.^. 

 The study of these embryonic fundaments 

 sheds much liijht 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 

 rovm 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. 

 t he 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 



,1 pair of parallel cords from cells (Fig. 203, // 1 which 



n derived by direct proliferation from those of the germ band. 



FIG. 199. Embryo of CEcan- 

 thus, ventral aspect, a, antenna; 

 a^a' 3 , abdominal appendages; e, 

 end of abdomen; /, labrum; li, 

 left fundament of labium; Ip, 

 labial palpus; /M 3 , thoracic legs; 

 m, mandible; tnp, maxillary 

 palpus; mx, maxilla; p, pro- 

 cephalic lobe; pr, proctodaeum. 

 After AYERS. 



