THE ALIMENTARY AND EXCRETORY SYSTEMS 



119 



tr 



The adipose cells are yery numerous, forming almost the whole 

 bulk of the fat-body. They are of the usual type, closely packed 

 together, so as to assume an irregular polygonal form, but appearing 

 round and distended wherever their surfaces are free. The nucleus 

 is large and rounded, and the cell itself filled to overflowing with 

 a store of fatty matter, held between the meshes of a fine network 

 of cytoplasm. In all sections previously treated with soap-alcohol, 

 this fat is completely extracted by the reagent, leaving a very 

 characteristic structure, the "skeleton" of the fat-cells (fig. 53, fb). 



The pericardial cells (fig. 53, pc) are large oval or rounded 

 cells of a brownish or yellowish 

 colour, and frequently having 

 two large nuclei. They stain 

 darkly, and are not affected by 

 soap-alcohol. They are collected 

 in two small bands along each 

 abdominal segment, close to- 

 gether and just below the heart. 

 Each band contains only a few 

 cells, and not more than three 

 or four are cut in any given 

 section. They appear to be 

 supported by two loops of con- 

 nective tissue, which partly close them off into two parallel 

 channels beneath the heart. They frequently contain brown or 

 yellow excrement received from the blood, which collects in small 

 crystals, usually towards the ends of the cells. 



The fat-body has been shewn to arise from the mesoderm, by 

 differentiation of cells in the walls of the coelomic sacs. When 

 the haemocoele develops, the fat-body forms a lining to it. 

 By proliferation of the cells in certain regions, this lining may 

 come to be several layers of cells thick. Thus the fat-body is 

 everywhere in direct communication with the blood stream, 

 from which it receives the nutritive products of digestion for 

 storage, as well as any excess of impurities which the excretory 

 system may not be able to deal with for the time being. It is 

 therefore an exceedingly important adjunct, both to the alimentary 

 and to the excretory system. 



Fig. 53. T.S. through region of heart in 

 fifth abdominal segment of half -grown 

 larva of Synlestes weyersi Selys ( x 160). 

 fb fat-body; h heart; pc pericardial 

 cells; tr trachea. Original. 



