248 



INVERTEBRATE ZOOLOGY 



form the primitive segments of the embryo. Upon these 

 primitive segments, paired outgrowths occur. These are the 

 fundaments of the legs and other appendages. At first these 

 appendages are all similar in appearance, but as development 

 proceeds some of them become suppressed while the remaining 

 ones begin to take on different forms depending upon the kind 

 of appendage each is to form in the adult animal. Lateral and 

 dorsal parts of the insect body result from the growth and 

 extension of the germ band after the ventral structures have been 

 laid down„ 



A B C D E 



Fig. 125. — Diagrammatic median sections to illustrate development of a 

 Libellulid egg. ^4, development of germ band; -B, invagination of germ band; 

 C, development of amniotic fold; D, closure of opening into amniotic cavity and 

 dtevelopment of rudiments of appendages; E, migration of embryo from amniotic 

 cavity back to surface of egg. {From. Korschelt and Heider after Brandt), 



A much more complicated condition exists when the germ band 

 undergoes invagination. In such an instance, the germ band 

 makes its appearance (Fig. 125 A) on the posterior ventral 

 surface of the egg. One end of the germ band sinks into the 

 underlying portion of the egg and, submerged within the egg, 

 begins to grow forward (B) toward the anterior pole of the egg, 

 carrying with it the blastoderm which was attached to the 

 posterior margin of the band. Practically all of the germ band 

 thus invaginates. The cavity formed within the egg by this 

 invagination is the amniotic cavity, one wall of which is composed 



