Stroud, Mammalian Ccrcbclhnn, 77 



B. Microscopic 7iictJiods. 



1. Morphology . — Where the specimen is too small to be 

 readily handled, it can be examined much more easily if sec- 

 tioned than in any other way. However one should have both 

 a gross preparation and sections of the same stage of develop- 

 ment whenever possible. A perfect idea can be obtained in no 

 other way. Sections also should be cut in three different 

 planes : {a) Sagittal sections ; (^) Transections ; {c) Frontal 

 sections. 



It may not be possible to obtain four specimens of the 

 same stage, but transections and sagittal sections are almost in- 

 dispensable. Where only one specimen is available, the writer 

 has adopted the following method. First, cut sagittal sections 

 till the meson is just passed, then remove the tissue from the 

 block and fix it on again so as to cut the remainder of the tis- 

 sue in transections. Figure 32, PI. Ill is drawn from a section 

 which illustrates this procedure. 



The fixing, hardening, imbedding, etc., should be the 

 same, whether one wishes the sections for morphology or his- 

 tology. If for the former, sections may be cut much thicker, 

 from 40 microns to 120 microns, and should be stained only very 

 lightly, the thickness of sections and amount of tissue cut 

 noted so that one can make a wax model, if desired. Sections 

 are cleared and mounted in balsam in the usual manner. 



2. Histology proper. — Tissue may be fixed and hardened 

 after any of the approved methods. The writer has used alco- 

 hol and potassium dichromate; also 50 per cent, alcohol saturated 

 with common salt two to four days, then alcohols as in A, with 

 good results. Before fixing, the skull must be removed; but 

 great care is needed to avoid tearing the delicate telas and other 

 membranes. 



Imbedding. — This must be very carefully and thoroughly 

 done, or loss will be the inevitable result. After dehydrating, 

 the tissue is placed in a mixture of equal parts of strong ether 

 and 95 per cent, alcohol for two days; then into 2 percent, col- 

 lodion for 2 to 3 days ; then into 6 per cent collodion for 4 to 10 



