8 



Smear Preparations from Cut Surfaces 



General Principles 



The last chapter described the prepara- 

 tion of sniears from materials which were 

 fluid; the present chapter deals with the 

 method by which smears may be obtained 

 from the surface of materials which have 

 been cut into blocks. 



Preparation of the Smears 



There are only two methods of produc- 

 ing smears from the cut surface of sohd 

 bodies. Either the cut surface is rubbed on 

 a clean slide,- leaving behind a few cells 

 which have been detached from it, or the 

 cells are squeezed from a cut, and subse- 

 quently pressed out in the form of a smear. 

 The former method is used for animal 

 tissues and the latter for plant specimens. 



The difficulty in preparing smears from 

 the cut surface of animal tissue is not so 

 much to secure material as to avoid secur- 

 ing unwanted cells. The blood content of 

 the majority of organs is so high that, if a 

 freshly cut surface be smeared on a piece 

 of glass, the few cells which become de- 

 tached will be obscured by the red blood 

 corpuscles. The technique is, therefore, 

 usually applied to the central nervous 

 system, from the cut surfaces of which 

 cells not only detach themselves very 

 readily but which has also the advantage 

 of being only shghtly vascularized. The 

 only difficulty in preparing a smear from 

 a freshly cut surface of the central nervous 

 system lies in finding a pair of forceps 

 sufficiently wide and sufficiently blunt to 

 hold the material without it disintegrat- 

 ing. Apart from this the technique is essen- 

 tially that described in the last chapter. 

 A perfectly clean slide is taken and 

 rubbed with the cut surface of the mate- 

 rial. These smears cannot be dried satis- 

 factorily but must always be fixed, and it 

 is necessary to use the technique, dis- 

 cussed in the last chapter, of placing the 

 sUde face down across a pair of glass rods 

 lying in the bottom of the petri dish in 



such a manner that the lower surface, but 

 not the upper surface, is in contact with 

 the fixative. The fixative to be selected 

 naturally varies according to the material 

 to be studied, but fixatives containing 

 mercuric chloride are usually to be pre- 

 ferred. Smears may also be fixed in osmic 

 vapor, though they are not usually 

 as satisfactory w'hen prepared by this 

 method as are smears prepared from fluid 

 material. 



The smear technique in plant micro- 

 technique is largely confined to securing 

 sporogenous tissue from anthers at various 

 stages of their development. This tech- 

 nique, which was introduced by Taylor 

 1924 (3430, 78:236), has the advantage 

 that it permits the examination of chromo- 

 some material without the trouble of de- 

 hydrating, embedding, and sectioning. 

 This smear technique must, however, be 

 differentiated from the squash techniques, 

 described in the next chapter, in which 

 cells are dissociated. The smear technique 

 can only be used for materials which can 

 be squeezed out. There is some division of 

 opinion as to whether a small quantity of 

 adhesive (such as Mayer's albumen) 

 should first be smeared on the sUde, or 

 whether the material extruded from the 

 anther has enough protein to cause ad- 

 hesion. In either case the anther is cut 

 with a very sharp scalpel about one third 

 of the distance from its base and placed 

 on the sUde with the cut end in the region 

 where one wants the smear. The back of a 

 scalpel is then rolled from a position about 

 a milfimeter from the cut edge, toward the 

 cut edge. The material which is thus ex- 

 truded is rapidly smeared with the back 

 of the scalpel, the crushed anther removed, 

 and the slide inverted on glass rods in a 

 fixative. It has been found (Kauffman 

 1927, 20540b, 2:88) that these smears are 

 best stained with an iron hematoxyUn 

 technique (Chapter 20, DS 11.111). 



74 



