DEVELOPMENT ~. 151 



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; Ig, prothoracic leg; li, left fundament of labium; In, lingua; m, 

 mandible; mx, maxilla; p, maxillary palpus; si, superlingua. After FOLSOM. 



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 froki cells (Fig. 



