CHAPTER XXV. 269 



under fixative, or ligature one end of the organ and inject some 

 fixing medium. 



Corrosive formol mixtures have been much used for this purpose. 



Neutral formalin of from 3 to 10 per cent, strength is often 

 used for preserving later stages, after the uterus has been opened 

 out. The advantage of this procedure from the cytological point 

 of view is that any methods such as those of Regaud, Bensley- 

 Cowdry, Sjovall, or formol-silver nitrate neurological techniques 

 may subsequently be used. The chrome-picric or alcoholic acetic 

 formol mixtures are not so suitable if one has cytological study in 

 view. 



591. On Clearing Mammalian Material. This is an important 

 matter, because delicate embryos are easily shrunken up, or even 

 not properly dealcoholised, by injudicious methods. J. P. Hill clears 

 in two stages. Dehydrated embryos are brought into cedar wood 

 oil in which they are left overnight. The cedar wood oil is subse- 

 quently washed out in benzole for several hours according to size 

 of object. Paraffin parings are then added to the benzole, contained 

 preferably in a tube, and the latter is then left overnight uncovered 

 on the top of the bath, and subsequently put into pure wax. This 

 method insures a gentle dealcoholisation, and an efficient imbedding. 

 Neither cedar wood oil nor benzole cause the tissue to become 

 brittle as happens often when one uses xylol or chloroform (see 

 120-135). 



Imbedding. For embryological work of a critical character, 

 especially with post-blastoderm stages, double-imbedding in cel- 

 loidin and wax is generally indispensable. It is only necessary to 

 contrast serial sections of chick blastoderms prepared by this 

 method, with those obtained by wax imbedding alone to become 

 convinced of the inability of the latter method to do complete 

 justice to the details of the structure and relations of the embryonic 

 tissues (WILSON and HILL, Phil. Trans. Roy. Soc., 1907). 



See also J. P. HILL (Anat. Anz., Bd. xviii, 1900 ; Quart. Journ. Micr. 

 Science, Ivi, 1910) ; HARTMANN (Jour. Morph., xxvii, 1916). The latter 

 recommends punching a hole in the side of larger blastoderms to facili- 

 tate penetration of dehydrating and clearing fluids. WEYSSE, Proc. 

 Amer. Acad. Arts and Sci., 1894, p. 285 ( blast odermic vesicle of Sus 

 scrofa) ; SOBOTTA, Arch. mik. Anat., xlv, 1895, p. 15 (ovum of the Mouse ; 

 fixation in FLEMMING'S weak mixture, sections stained with BENDA'S 

 iron hsematoxylin), and Anat. Hefte, 1 Abth., viii, 1897, p. 476 (Kabbit ; 

 fixation with liquid of Flemming or picro-sublimate with 2 per cent, 

 acetic acid) ; BONNET, ibid., ix, 1897, p. 426 (Dog ; fixation in sub- 

 limate) ; SELENKA, Stud. Entw. d. Thiere, Wiesbaden, 1883, p. 5, and 



