THE FAT BODY 131 



the larger flies, and, without other dissection, making a small nick 

 in the membrane connecting two of the abdominal plates. A pipette 

 with a fine bore is then inserted, and a considerable 

 quantity of a yellowish fluid obtained. Blood may also 

 be obtained in small drops, but with more risk of contamination, by 

 pulling off the legs and picking up the fluid as it escapes on a cover- 

 slip ; or in the case of flies with a well-developed pseudotracheal 

 membrane, by compressing the head until the blood is pressed into the 

 labella, from which it can be withdrawn in a pipette, or smeared out by 

 rupturing the labella on a slide. 



Blood so obtained is a rather viscid yellow fluid, containing an 

 extremely small number of cells ; so poor is the fluid in cellular contents 

 that several large smears may have to be examined before finding a 

 single cell, and those which are seen may be so fragmentary and indis- 

 tinct as to be hardly recognizable as such. Many varieties of cells 

 have been described from insect blood, but when special precautions 

 are taken to avoid touching any of the internal organs one frequently 

 finds no cells at all, or at most a few which might be the small hypodermal 

 cells internal to the integument. The blood corpuscles, sometimes called 

 leucocytes (the amebocetti of Berlese), are very small rounded or oval 

 bodies, each containing a single nucleus and a reticulated cytoplasm. 

 In many a definite nucleus is absent. 



On opening the abdomen of a recently-hatched insect all that is 

 seen at first is a sheet of whitish tissue, with a greenish or bluish 

 tinge, which envelopes all the organs. When this is split 

 up by dissection it is found to consist of a number of 

 minute lobules connected together by thinner portions, which are drawn 

 into threads as they are pulled on by the needle. This is the so-called fat 

 body, really a mass of only slightly differentiated mesoblastic tissue, the 

 walls, if such they may be called, of the haematocoele. It envelopes all the 

 internal organs, and is always found in considerable quantity around 

 the digestive part of the alimentary tract ; a thinner sheet of it forms 

 an inner coating to the internal surface of the body wall ; between 

 these two layers, and in all the interstices between the lobules and 

 the cells of which the tissue is composed, the blood of the fly passes 

 as it circulates to-and-fro in the body. The cells of the fat body 

 are large, and when closely packed together are moulded to one another 

 so as to present a mosaic pattern on section ; in newly-hatched flies 

 they contain a nucleus and a finely-reticulated protoplasm, and .are 

 -loaded with globules of fat (Plate XXVII, fig, 3). 



