METHODS OF RECONSTRUCTION. 357 



convenient scale for the reconstruction of the anatomy of mammalian embryos 

 is a magnification of from i6 to 20 diameters. 



Let us suppose that a pig of 1 2 mm. has been drawn in a side view magnified 

 20 diameters ; that the embryo has been cut into 900 transverse sections and the 

 approximate plane of the sections is known. It may be more exactly deter- 

 mined by the study of the sections themselves; for instance, it may be deter- 

 mined what section is the last to pass through the surface of the head in the 

 region of the fore-brain and the last to pass through the border of the anterior 

 limb. Then it can be further ascertained through which dorsal segments these 

 two sections pass. By these data the plane of the two sections can be accu- 

 rately 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 complicated more details are needed, lines for the 

 additional sections can be interpolated. In our supposed case, our lines repre- 

 senting 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 

 formed from embryos imbedded in paraffin, because when an embryo is so im- 

 bedded 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 con- 

 stant, but varies from specimen to specimen, hence a camera drawing made from 

 the sections and magnified 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 measuring 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 given 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 recontructed from the point in the section corresponding to the out- 

 line of the embryo is measured off, and then marked upon the proper 

 line of the reconstruction diagran;.. A similar measurement is then taken 



