THE CCELOM AND DORSAL VESSEL. 85 



The superior metathoracic disc is placed behind and below 

 the superior mesothoracic disc. It is exceedingly like the latter, 

 but much smaller. It is attached by bands of mesoblast to the 

 wing disc, and to the metathoracic leg disc. One or more of 

 the trachese which are in relation with the wing disc are 

 usually entirely surrounded by minute embryonic cells, which 

 are probably the rudiments from which the wing tracheas of the 

 nymph are developed (Fig. 16, tr). Similarly modified tracheae 

 are also found in relation with the other discs in the resting stage 

 of the larva. 



10. THE C(ELOM, DORSAL VESSEL, AND SPLANCHNIC 

 SYSTEM OF NERVES. 



The Coelom. — By the term coelom, I include all the tissue 

 interspaces between the hypodermis and the viscera. It con- 

 tains a reticulum of cells, which divides it into larger and smaller 

 blood sinuses. The cells may be classed in the following 

 groups: endothelioid, stellate, connective, adenoid, and fat 

 cells ; but numerous transitional forms occur, and the whole 

 are probably modifications of the primary stellate mesoblast. 



Under the term endothelioid, I include the cells of the sub- 

 hypodermic layer, those of the peritoneal coats of the tracheae 

 and viscera, and true endothelial plates which bound the larger 

 blood sinuses. They pass by numerous transitional forms into 

 the stellate connective cells, in which the tracheal capillaries 

 are developed. 



Under the term adenoid, I include certain strings of cuboid 

 or spheroidal cells, which often attain a large size, and are 

 frequently multi-nucleated ; sometimes as many as four or five 

 nuclei are present in each cell. They form Weismann's cell- 

 chaplet (see page 61), the pericardial septum, and the fringes of 

 the dorsal vessel. 



The fat cells are also united in strings, and form a large 

 reticular sheet, the fat body or omentum. This appears to the 

 naked eye as a glistening, opaque, white sheet of tissue, which 

 floats out of the body cavity when the integument is slit up 



