386 METHODS. 



can be further ascertained through which dorsal segments these two sections pass. 

 By these data the plane of the two sections can be accurately fixed. Over the 

 outline of the embryo is now drawn a series of lines which represent the position 

 of the sections. It is generally sufficient to put in lines which represent only every 

 second, third, or even fourth section. If at any point where the structure is com- 

 plicated more details are needed, lines for the additional sections can be interpolated. 

 In our supposed case, our lines representing every fourth section, there would be 

 225 parallel lines, and these should be numbered to correspond to the sections 

 which they represent. 



The outlines of the actual sections corresponding to the numbered lines in 

 the diagram must now be made with the camera lucida. In regard to these great 

 care is necessary, especially if, as is likely to be the case, the sections are 

 from embryos imbedded in paraffin, because when an embryo is so imbedded it 

 always shrinks, and after imbedding is smaller than before. The shrinkage seems 

 to be uniform throughout and not to disturb the topographical relations even of 

 the finest structures. Unfortunately the shrinkage is not constant, but varies from 

 specimen to specimen, hence a camera drawing made from the sections and magni- 

 fied 20 diameters will not be of the right size to fit in the diagram, and these 

 drawings, must, therefore, be corrected. This may be done either, as is best, by 

 making the original camera lucida drawings of the right magnification for direct 

 use in reconstruction, or they may be made nearly the right magnification and 

 when they are measured off the necessary correction may be introduced by measur- 

 ing them with proportional dividers. 



From the camera lucida drawings of the single sections the measurements are 

 taken to fix the position of the parts in the reconstruction. 



For a given section the exact position in the reconstruction is fixed by the line 

 on the outline drawing of the embryo corresponding to the number of the section. 

 On the drawing of the section the distance of the organ to be reconstructed from 

 the point in the section corresponding to the outline of the embryo is measured off, 

 and then marked upon the proper line of the reconstruction diagram. A similar 

 measurement is then taken from the next section and transferred to the diagram in 

 the same manner, and so on . with successive sections until a series of dots is 

 obtained which mark the. outline of the organ. These dots are then connected by 

 a continuous line, .which will indicate the form and correct position of the organ. 

 Simple reconstructions may be easily made by these means. When, however, more 

 complicated reconstructions are attempted, much judgment and skill are necessary 

 in the selecting of parts which may be successfully represented in a single drawing, 

 bearing in mind always the point of view which is assumed for the reconstruction, 

 so that organs may be correctly represented in their relative positions, nearer or 

 farther from the observer as he looks at the drawing . After the outlines are 

 completed the shading of the parts must be added, and this often requires a special 

 degree of skill and a considerable faculty of plastic imagination. As examples of 



