110 General Notes. [January | 
of ascertaining to within a very small fraction of a millimeter, the 
exact path of the knife. Such precise orientation can only be | 
arrived at in an indirect way; but the improved instruments and | 
methods of section-cutting make its attainment a no very diff- | 
cult task. To determine the /ocus of sections with accuracy, 
Blastoderm of the chick, 5™™ long. 
urface view magnified 20 diameters. ae 
Thickness of each section .o5™™, a | 
Plane of section at right angles to the long axis of the blastoderm. ce 
From these data we know that there should be just 100 S67 
tions, and that each section must correspond to 1™™ of the wg 
face view. Sy A 
Now if we draw a line at one side of the surface view, parallel i 
to, and of equal length with, its long axis, and divide this line into : 
100 equal parts, the number of the section will correspond to e- 
same number on the scale, and the exact position of the section i 
be recognized at a glance. = 
Of the conditions above named as essential to an exact kno 
edge of the locus of any given section, the only one likely tof 
sent any serious difficulties is that of obtaining sections of unif 
‘nee 
the gentlemen who are now associated in the management of 
Zoological Station of Naples. ` | 
Tue Reconstruction or Osjecrs rrom Secrions.—The _ 
portance of attending to all available means of orientati 
1 This microtome may be obtained fi Peas ba andl mechanic : 
Heidelberg. z rom Rudolph Jung, opticia 
