90 THE LARVA OF THE BLOW- FLY. 



the middle line, but I have been unable to determine their 

 number, which appears to me variable. These cells are oval in 

 transverse section and measure 'i mm. in long diameter; they 

 stain deeply and have large oval nuclei. They are connected 

 with the muscular alas of the ventricle, and I have frequently 

 traced a direct continuity of their cell substance with a muscle 

 fibre. 



The Muscular Alse of the Ventricle consist of three groups of 

 diverging muscle fibres on each side (Fig. 18, /), which are 

 inserted in part into the cells of the septum and in part into the 

 middle line of the ventral surface of the ventricle. The finest 

 of these fibres are not more than 2 M to 3^ in diameter, and are 

 very distinctly striated. They are surrounded by a myolemma, 

 which also encloses the large cells of the septum. 



The muscular alse appear to arise from the peritoneal coat of 

 the great lateral tracheal trunks, and they divide and subdivide 

 in their course towards their insertion. 



Valves. The cavity of the ventricle communicates with the 

 pericardial sinus by a series of slit-like openings, guarded by 

 valves. It is exceedingly difficult to determine the exact 

 number of these openings, as they are most readily studied in 

 transverse sections. There are certainly two lateral openings 

 to each chamber, and one or more ventral slits. 



There are also several openings (four ?) at the posterior 

 extremity of the heart. 



The ventral openings (Fig. 19, 2} are between the cells of the 

 septum, and the alar muscles pass into the wall of the ventricle 

 immediately behind them. The valves which guard the 

 openings are thin membranous flaps, which project into the 

 ventricle, and are not nucleated projections of the wall of the 

 cavity. I have been unable to distinguish valves between the 

 chambers of the ventricle. 



The second, or intermediary, part of the dorsal vessel 

 (Fig. 18) varies considerably in diameter and in the form of 

 its cross section at different points. It is circular at its origin, 

 becomes pentagonal in the middle of its course, and circular 

 again near its termination. It measures from '15 mm. to 



