334 



METHODS AND FORMULAS 



DS 12.30 



Preparation of a transverse section of 

 the head of a mouse using an acid- 

 alum hematoxylin stain (Masson 

 1934) followed by ponceau 

 2R-light green (Patay 1934) 



A large section of this type is required 

 not so much for the demonstration of his- 

 tological detail as for the demonstration to 

 classes of the morphological relationship 

 of the various parts of the head region. For 

 this reason the fixative to be employed 

 should be selected more on the basis of its 

 abihty to penetrate large structures than 

 in the hope that it will permit fine histo- 

 logical differentiation of detail. It is, more- 

 over, less necessary that the fine detail of 

 histological structures be preserved by im- 

 mediate fixation than that the mouse be 

 so arranged as to permit the penetration of 

 fixative to all parts. These considerations 

 must therefore dictate both the manner in 

 which the mouse is killed and that in 

 which it is fixed. If there are available to 

 the worker only the cruder methods of 

 killing, the mouse should be left until rigor 

 mortis has passed off before the different 

 parts are arranged. Or alternatively, and 

 far more satisfactorily, the mouse may 

 be killed by the injection of sodium bar- 

 bitol or some such reagent, in order that 

 it may die in a perfectly relaxed condi- 

 tion. The author has already expressed 

 in several places his conviction that a 

 mounter should experiment for himself 

 with fixatives, rather than follow already 

 estabhshed recommendations. But in the 

 present case his own preference is very 

 strongly for the mercuric-dichromate-ace- 

 tic mixture of Zenker 1894 (Chapter 18, 

 F 3700.0010). It must be remembered that 

 this specimen will have to be decalcified 

 and the use of the mercuric-dichromate is 

 therefore to render the tissues as hard as 

 possible and to prevent, as far as possible, 

 the hydrolysis of the softer parts by the 

 acids used for decalcification. 



If it is desired to secure the transverse 

 section through the region of the eyes, the 

 head should be severed from the body as 

 soon as the mouse is dead, and about 

 three-quarters of the lower jaw, the an- 

 terior end of the tongue, and the anterior 

 end of the snout should be removed with a 



pair of sharp scissors. A series of holes 

 should then be drilled, or driven with a 

 sharp point, into the skull in as many 

 places as possible, leaving free from holes 

 only a band approximately one-quarter of 

 an inch wide in the exact region of the eye 

 where the section is required. A band of 

 thread should then be tied tightly around 

 both the anterior and posterior cut sur- 

 faces so as to hold the skin in position. 

 Otherwise the differential contraction of 

 the latter while in the fixative will cause it 

 to become torn loose and to give a most 

 unsightly appearance to the section. The 

 more (and the more deeply) the holes are 

 drilled the better will the fixative pene- 

 trate and the subsequent section appear. 

 Previous to these operations one should 

 have prepared at least a liter of the re- 

 quired fixative and to have placed this in a 

 tall narrow jar. A liter-measuring cylinder, 

 with a cork to fit, is excellent, for the pur- 

 pose, although tall museum jars of approx- 

 imately the same shape are frequently 

 available. A staple or bent pin, should be 

 driven into the underside of the cork, to 

 which a thread or piece of string may be 

 attached, on the end of which is hung the 

 prepared head in a cheese cloth, or any 

 loosely-woven bag. Fixation with a di- 

 chromate solution is always best con- 

 ducted in the dark, so the jar with the sus- 

 pended head may now be placed in a dark 

 cupboard and forgotten for a period of two 

 or three weeks. The exact time of fixation 

 is unimportant; several months could be 

 allowed to elapse without any great risk 

 of damaging the piece. Nothing, however, 

 can be done with the piece if it is under- 

 fixed, and even if there are a relatively 

 large number of holes in it, two weeks is 

 not too long a period for the fixative to 

 penetrate. 



The head is now removed from the fixa- 

 tive and taken out of its bag, and then 

 washed in running water for at least 48 

 hours. If this can be done in the dark it 

 will prevent the possibiUty of the chrom- 

 ium being deposited as a dark-green layer 

 over the outside, an event which would 

 render after-staining difficult. 



One is now faced with the problem of 

 decalcification, for which purpose any of 

 the formulas given in Chapter 19 under 



