EMBRYOLOGY 369 



nitric, and Kleinenberg's picric acid. Flemming's fluid has also 

 been used. 



In later stages of development some workers open the uterus 

 under the 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. 



808. On Clearing Mammalian Material. This is an important 

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

 not properly dealcoholised, by injudicious methods. .1. P. Hill 

 always clears in two stages. Dehydrated embryos are brought 

 into cedar wood oil in which they are left overnight. See also 

 methyl benzoate method (§ 177). The cedar wood oil is sub- 

 sequently washed out in benzol for several hours according to 

 size of object. Paraffin parings are then added to the benzol, 

 contained preferably in a tube, and the latter is then left over- 

 night uncovered on the top of the bath, and subsequently put 

 into pure wax. This method ensures a gentle dealcoholisation, 

 and an efficient imbedding. Neither cedar wood oil nor benzol 

 cause the tissue to become brittle as happens often when one 

 uses xylol or chloroform (see §§ 135 et seq.). 



Imbedding. For embryological work of a critical character, 

 especially with post-blastoderm stages, double-imbedding in 

 celloidin and wax is generally indispensable (§ 190). It is only 

 necessary to contrast serial sections of chick blastoderms pre- 

 pared 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) ; Hartjl\.nn (Journ. 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 (blastodermic 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 haematoxylin), and Anat. Hefte, 1 Abth., viii, 1897, p. 476 (Rabbit ; 

 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 



