RECONSTRUCTIONS. 419 



into twentieths or hundredths, has been marked off by scratches in the glass; 

 the subdivisions may be drawn with the camera under the same conditions as 

 the sections, and the enlargement of the subdivisions may then be measured. 



From the camera-drawings of serial sections, wax reconstructions of 

 adult glands or embryonic organs may be made. If the sections are i.o / 

 thick and alternate sections have been drawn, magnified 50 diameters, 

 then on the scale of the drawings these alternate sections are i mm. apart. 

 Wax plates i mm. thick are therefore to be made, either by rolling the wax, 

 or by spreading a weighed amount of melted wax in a pan of hot water. 

 It floats and spreads in an even layer, solidifying as the water cools. The 

 outlines of the drawings are then indented upon the wax plates, and the 

 desired portions are cut out and piled up to make the model. In this way 

 reconstructions like those of the ear (p. 380) may be made. The details 

 of the process should be learned from demonstrations in the laboratory. 



Graphic reconstructions are usually side views of structures, made from 

 measurements of their transverse sections. Fig. 161 , p. 138, is from such a re- 

 construction. A camera drawing of the side of an embryo (or other struc- 

 ture) is made before it is sectioned. The outline of this drawing is en- 

 larged, and parallel lines equally spaced are ruled across it, corresponding 

 in number and direction with the sections into which it was cut. Often 

 only every other section or every fourth section is used for the reconstruc- 

 tion, and the number of lines to be ruled across the drawing is correspond- 

 ingly reduced. Camera drawings of a lateral half of every section used in 

 tht reconstruction are made, and across each drawing two lines are ruled. 

 The first follows the median plane of the body; and the second is at right 

 angles with it, being drawn so as to touch the dorsal or ventral surface of 

 some structure to be included in the reconstruction. Provided that the 

 camera drawings and side view have been enlarged to the same extent, the 

 perpendicular distance from the middle of the back to the junction of the 

 two lines is marked off on the side view, on the line corresponding with the 

 section in question. The perpendicular distances from the second line to 

 the dorsal and to the ventral surfaces of all structures to be reconstructed 

 are also marked off upon the line in the side view. The same is done in the 

 following section, and the points belonging with a given structure are con- 

 nected from section to section. Thus the outlines of the organs are pro- 

 jected upon the median plane; two dimensions are accurately shown but 

 the third is lost. 



Often it is undesirable to attempt to make the magnification of the 

 sections and of the side view identical; the measurements may be en- 

 larged or reduced as they are transferred for plotting, by means of the 

 draughtsman's proportional dividers, an indispensable instrument for 



