RECONSTRUCTIONS 517 



mined by substituting a stage micrometer for the slide of sections. The 

 micrometer is a slide upon which i mm., with subdivisions into twentieths 

 or hundredths, has been marked off by scratches in the glass; the sub- 

 divisions 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 

 various embryonic organs or small structures in the adult can be built 

 up. If the sections are 10 f* 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 beeswax, 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. 466) may be made. This method was first employed by Born. 

 Further details of the process should be learned from demonstrations in 

 the laboratory. 



Graphic reconstructions (first used by His) are generally side views of 

 structures, made from measurements of their transverse sections. Fig. 

 176, p. 185, is from such a reconstruction. A camera drawing of the side 

 of an embryo (or other structure) is made before it is sectioned. The 

 outline of this drawing is enlarged, 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 reconstruction, and the number of lines to be ruled 

 across the drawing is correspondingly reduced. Camera drawings of a 

 lateral half of every section to be used in the reconstruction are then 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 

 in 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 on the side view. The same is done in the following sec- 

 tion, and the points belonging with a given structure are connected from 

 section to section. Thus the outlines of the organs are projected 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 



