CCELOM AND VASCULAR SYSTEM IN THE LEECH. 483 



they were preserved in a mixture of one volume of 4 per cent, 

 formaldehyde solution, and one volume of a saturated solution 

 of corrosive sublimate containing 5 per cent, glacial acetic 

 acid. This fixative gives excellent results for section cutting. 

 The sections were stained on the slide in a mixture of methyl 

 blue and eosin, according to a method suggested to me by Dr. 

 A. Mann which has proved most useful. When successful, 

 the combination stains the tissues blue or purple, and the 

 hsemoglobinous coagulum, the hsemolymph, brilliant scarlet. 

 This striking contrast enables one to follow out the minutest 

 capillary with comparative ease. 



The work of reconstruction had then to be undertaken. 

 For the larger vessels and sinuses (figs. 1, 2, 3, &c.) I made 

 use of a series of 600 transverse sections, 10 fx thick, from the 

 middle region of the body. Camera drawings ( x 25) were 

 made of the first and every tenth section, and these were 

 plotted out on paper ruled to scale. For this purpose arbitrary 

 fixed lines had to be adopted ; a vertical line was, therefore, 

 drawn in each case through the nerve-cord and dorsal sinus, 

 and another horizontal line at right angles to this through the 

 nerve-cord. The measurements were then taken from these 

 lines. In this way a certain element of arbitrariness is no 

 doubt introduced into the reconstruction of the curves of the 

 vessels ; but this is only very slight, and really of no import- 

 ance, since, of course, it does not in any way alter the true 

 relations and communications of the spaces. These were 

 always verified by examining the intermediate sections. 



In reconstructing the systems of smaller vessels, sinuses, 

 . and capillaries, the difficulties were much greater. Here no 

 fixed points were available to measure from, no practicable 

 arbitrary lines could be drawn, owing to the extremely com- 

 plicated character of the network visible only with a compara- 

 tively high power^ the field of which includes but a small 

 portion of the section.^ 



' Even had a fixed point been available, no purely mecbanical process of 

 reconstruction, such as the wax-plate method, could be trusted, since the 

 capillaries are so numerous and so near to each other that it would be scarcely 



