32 THE LEECHES OF MINNESOTA 



mounting - . The third was cleared in cedar oil and examined as a ' 

 transparent object. Both external and internal features, as far as 

 they could be made out, were drawn with the camera lucida. This 

 specimen was then imbedded in paraffin, cut in transverse series 

 5/1000 mm. thick and stained with Ehrlich-Biondi stain. The ner- 

 vous system, (Figs. 17 to 22.), reproductive system, (Figs. 11 to 

 14), and the oesophageal gland, were worked but by the Born Re- 

 construction Method from series three, and the wax models thus 

 made were checked up with similar models made from sagittal 

 series two. Wax models made from these two series were found 

 to differ only in minor details, due to slight anatomical variations 

 and distortions which one would expect to find in any two individu- 

 als of the same species. 



While the method which I have used is essentially the Born 

 method, I have modified it in several ways. My drawings for this 

 purpose were all made with a soft Faber pencil, on unsized paper, 

 from the third series noted above. But instead of drawing every 

 section, only alternate sections were drawn, and in some cases only 

 ever) - fourth section was drawn, the sections between being used 

 to show the relations of parts which had changed too much to have 

 their relations perfectly clear in the drawn sections. The result 

 attained is the same as though the series had been cut two or four 

 times as thick, with the added advantage that one is able to trace 

 out minute changes which would not appear in the thicker series. 

 The thickness of the wax plate was made to correspond with the 

 magnification and the distance between the sections as by Born. 

 For the models of the central nervous system, I took alternate sec- 

 tions and drew them with a magnification of 400 diameters, so the 

 wax plates required were 4 mm. thick. 



In making the wax plates I used a method which is quite dif- 

 ferent from that of Born, and one which I think is in many ways 

 superior. I first found by mathematical calculation the amount of 

 melted wax which it would take to produce a plate 1 mm. thick in 

 pans of a convenient size, and then procured a ladle which, when 

 filled, would hold exactly that amount. I then got ready a large dish 

 of melted wax and a quantity of hot water. The pans were, one 

 by one, filled nearly full of hot water and, according to the thickness 

 of the plate wanted, one, two, three or four of the ladles full of 

 hot wax were poured upon the surface of the water and spread over 

 it evenly with a hot spatula. A little extra wax was always added 



