DEVELOPMENT I5I 
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 posi- 
tion and the amnion and serosa arise by a process of over- 
growth (Figs. 190, 191), as distinguished from the invaginated 
type, illustrated in Odonata, in which the germ band inva- 
ginates into the egg, as in Fig. 197, until the ventral surface 
Anterior aspect of embryonal mouth parts of a collembolan, Anurida maritima. a, 
antenna; /, labrum; /g, prothoracic leg; li, left fundament of labium; /n, lingua; m, 
mandible; mx, maxilla; p, maxillary palpus; s/, superlingua.—After Forsom. 
of the embryo becomes turned around and faces the dorsal side 
of the egg. In this event, a subsequent process of revolution 
occurs, by means of which the ventral surface of the embryo 
resumes its original position (Fig. 198). 
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 outward and 
upward (Fig. 199) until they finally close over to form the 
dorsal wall of the insect. Besides this simple method, how- 
ever, there are several other ways in which the dorsal closure 
may be effected. 
Nervous System.—Soon after gastrulation, the ventral ner- 
vous system arises as a pair of parallel cords from cells (Fig. 
