104 CHAPTER \ 7 III. 



A somewhat move complicated form of this process has been described by 

 WOODWORTH, Bull. Mus. Comp. Zool., xxxviii, vol. xxv, 1893, p. 45. 



A similar process has also been described by FIELD and MARTIN in Zeit. 

 f. wiss. Mile., xi, i, 1894, p. 11, small strips of gelatin being used instead of 

 paper. 



HOFFMANN (Zeit. f. wiss. Mik., xv, 3, 1899, p. 312) prefers to take, 

 instead of the ribbed paper, glass slides ruled with a diamond, and to com- 

 pletely imbed the objects in large drops of clove oil collodion (equal parts, 

 allowed to stand for twenty-four hours in an open vessel). The drops are 

 caused to set in xylol instead of turpentine. See also SAMTER, ibid., xiii, 

 4, 1897, p. 441, and PETER, Verh. Anat. Ges., xiii Vers., 1899. p. 134. 

 This subject is further treated under the head of " Reconstruction from 

 Sections " in the chapter on Enibryological Methods, which see. 



134. Knife-position and Shape of Mass to be Cut Even 

 with the most perfectly imbedded objects it is frequently 

 impossible to obtain sections that are good in every way- 

 thin, regular in thic-kness, not torn, not compressed, not 

 rolled or curled, and not folded or undulating in surface 

 Avithout careful attention to a host of minute details. These 

 details concern (a) the position of the knife, and (I) the 

 . consistency (and in a minor degree the shape and position) 

 of the mass to be cut. 



(a) The knife-position may be considered under two heads, 

 viz. its shmt and its tilt. 



By the slant of the knife is meant the angle that its edge 

 makes with the line of section : that is, with the line along 

 which it is drawn through the object (or along which the 

 object moves across it in the case of microtomes with fixed 

 knives). The position is fransverxe when the edge makes an 

 angle of 90 with the line of section, or the knife in that 

 case is said to beset square. It is oLlujni' or slanting \vheii 

 it makes a smaller angle with that line. 



It is a mistake to suppose that these two positions differ in that in the 

 transverse position the knife acts as a wedge or ploughshare, forcing its 

 way straight through the object, whilst in the oblique position it acts as a 

 saw, its edge being drawn along through the object, as in free-hand cut- 

 ting. On the contrary, in both cases the knife acts merely as a wedge, and 

 no microtome in general use at the present time* affords a drawing move- 

 ment such as can be given by the hand. In either position of the- knife 

 no point of the object is ever touched by more than one point of the 

 cutting-edge. The difference between the effect of the two positions is 



* A microtome with drawing motion to the knife is described by BECK 

 in Zeit.f. wiss. Mik., xiv, 3, 1S97, p. 324. 



