252 AN INTRODUCTION TO ZOOLOGY 



lined by a delicate vitettine membrane. As in the crayfish, cleav- 

 age proceeds without the intervention of cell walls. Most of the 

 cleavage nuclei migrate to the periphery, where they form a single 

 layer of cells, the blastoderm; the rest remain in the yolk, which it 

 is their duty to change into protoplasm as development continues. 

 The blastoderm soon becomes thicker on one side of the egg, 

 forming the germ band or primitive streak. This later becomes 



FlG. 144. Embryo of honeybee within eggshell, ab., hind-intestine; c, 

 cesophageal connectives ; ch., chorion or eggshell ; fb., fore-intestine ; 

 ga., ganglia ; mb., mid-intestine ; s. ga., brain. (From Packard after 

 Cheshire.) 



the ventral side of the young bee. Next a median groove appears 

 in the germ band and two layers of cells arise in this region; the 

 outer layer is the ectoderm, the inner, the entomesoderm. The 

 latter, as its name implies, gives rise to both the entoderm and 

 mesoderm. The germ band now grows around the egg until it 

 covers the entire surface. The antennae and four pairs of append- 

 ages appear near the anterior end of the embryo. One pair of 

 the latter disappear; the others become the mouth parts. Three 

 pairs of appendages also develop on the thorax, but disappear 

 before the embryo hatches (200). Some of the organs of the 

 embryo are shown in Figure 144. The ganglia of the brain 

 (s. ga.) and ventral nerve cord (ga.) are quite distinct at this time, 

 and the three principal parts of the alimentary canal, the fore- 

 intestine (fb.), mid-intestine (mb.), and hind-intestine (ab.) occupy 

 the longitudinal axis of the body. 



Metamorphosis. The life history of an individual may be 

 divided into four periods, egg, larva, pupa, and adult or imago. 



