FROM EGG TO INSECT — 3N0DGRASS 



409 



The mesodermal wings then become split into outer and inner 

 layers, and the spaces between them, joined with that just noted 

 beneath the alimentary canal, constitute the body cavity of the 

 insect (fig. 24, BC). The outer mesoderm layers {t\Meso) become 

 applied to the body wall; the inner ones {IMeso) to the wall of 

 the alimentary canal {End). From the first are formed principally 

 the muscles of the body and appendages, and the fat tissue; the 

 second forms the muscles of the stomach, and the reproductive 



organs. 



A muscle consists of fibers, and each fiber appears to be a highly 

 specialized cell, except for the fact that it contains many nuclei. 

 Investigators differ as to 

 whether the fiber is a prod- DpK 



uct of many cells that have 

 united, or whether it is a 

 single cell, the nucleus of 

 Avhich has many times di- 

 vided without accompany- 

 ing division of the cell itself. 

 In either case the building 

 of the complex muscular 

 system of the adult, per- 

 fectly adjusted to all the 

 mechanical needs of the in- 

 sect, from the layers of 

 simple mesodermal cells is 

 one of the most remarkable 

 things in insect development, 



THE FAT TISSUE 



IMesoN 



Fig. 24. — Diagrammatic cross section tlirougli 

 abdomen of an embryo when internal organs 

 are mostly deflned 



Apn, segmental appendage (if present) ; BC, 

 body cavity ; Dph, dorsal diaphragm ; D», dorsal 

 sinus ; Ect, ectoderm ; EMeso, external mesoderm 

 layer (somatic layer) ; End, stomach (endo- 

 derm) ; GCls, germ cells; Ht, heart; IMcso, 

 inner mesoderm layer (splanchnic layer) ; Rep, 

 reproduclive organ ; Tia. outer end of trachea 

 and opening to exterior (spiracle) ; T'-VC, ventral 

 nerve cord. 



In the body cavity of 

 insects there is a widely 

 scattered tissue consisting 

 of large cells, sometimes compactly united, sometimes loosely held to- 

 gether, that constitutes what is ordinarily called the " fat body." The 

 name is onlv partly appropriate, for, though the cells always store up 

 within them a large amount of fatty oil (fig. 25, Ft), they also form 

 and hold in storage proteid materials in the form of small granules 

 (AJh) and, in some insects, animal starch or glycogen. The fat body 

 is really an organ for elaborating reserve food materials, and is of 

 greatest importance in those insects that change from a wormlike 

 larva to the adult through an intervening pupal stage. The ma- 

 terials in the fat cells then serve as nutriment during the recon- 

 structive period, when the insect takes no food. 



