HISTOLYSIS OF FAT-BODY OF APIS 571 



embryo (Nelson, '12) this tissue develops chiefly from the 

 splanchnic, instead of the parietal mesoderm as is usual in insects. 

 This gives rise to two groups of cells on each side of the nerve 

 chain, to which are added third groups from the parietal layer 

 near the heart. In the larva it comes to extend dorsalward in 

 two layers — splanchnic, enveloping the intestine, and parietal, 

 hning the body wall. At metamorphosis, this tissue is dis- 

 integrated with the other larval tissues, and is replaced by an 

 imaginal tissue, generally of much smaller bulk and more periph- 

 eral location. 



Differentiation. Starting as a tissue of homogeneous compo- 

 sition in the bee, the fat-body attains complexity by reason of 

 two processes. Certain cells become difTerentiated from the 

 fat-tissue itself, and other elements invade it from the outside. 

 During larval life, scattering cells have been described that either 

 lose their fat-globule content or never acquire any, and, as larval 

 life proceeds, develop characteristic granules of sodium urate. 

 These have been termed excretory cells. From outside the fat- 

 body altogether the tissue is invaded by large amoeboid cells 

 originating segmentally from the margins of the spiracles, which 

 from their wine-red color in certain insects are termed oenocytes, 

 and to which are ascribed a secretory, and by some, a respiratory 

 function. Leucocytes are also found wandering through the 

 fat-tissue, presumably from the blood. After metamorphosis, 

 pericardial cells grow down in strings from the heart region and 

 become associated with the fat-body, as do also the imaginal 

 oenocytes. The fat-cells themselves acquire during imaginal 

 life granules of sodium urate. The functions of these various 

 elements are even more obscure in the imaginal than in the larval 

 economy. 



Anatomy and function. Ignoring for the time being the con- 

 flicting interpretations that have been placed upon the fat- 

 body function, we note that the mass of this tissue in the bee, 

 aside from any other consideration, suggests a food reservoir. 

 In larvae a few days old the fat-body envelops most of the other 

 organs. It extends from the head region through the extreme 

 caudal segments, and, except for blood spaces, fills all the region 



