EMBRYOLOGY 381 



The albuminous envelope of the ovum is then cut open, and the ovum 

 extracted ixnder water. The ova are then brought into 0-5 per cent, 

 chromic acid for not more than twelve hours, or into alcohol of 70, 80, 

 and 90 per cent. Chromic acid makes ova brittle and attacks the pig- 

 ment, whilst alcohol preserves it, which is frequently important for 

 the study of the germinal layers. 



Morgan {Amer. Nat., xxv, 1891, p. 759, and Devel. of the Frog's Egg, 

 1897, p. 171) has the following. During the periods in which it is diffi- 

 cult or impossible to remove the inner jelly-membrane the eggs can 

 be freed as follows : Each egg is cut out with scissors from the general 

 jelly-mass, and put for from one to twelve hours into saturated solution 

 of picric acid in 70 per cent, alcohol containing 2 per cent, of sulphuric 

 acid. Wash in several changes of alcohol of 70 per cent. About the 

 second day in this the inner membrane begins to swell, and on the third 

 or fourth day may be pierced by a needle, and the egg removed and 

 placed in 80 per cent, alcohol. See also Whitman, Metfi. of Research, 

 p. 156. 



ScHUXTZE {Arch. niik. Anat., Iv, 1899, p. 174) removes with scissors 

 the outer layers of albumen, and puts the ova for five minutes in 2 per 

 cent, formol warmed to 75° or 80° C. The membrane left on the ova 

 then rises up sufficiently to allow the ova to be got out with needles. 



See also Born {ibid., xliii, 1894, p. 1). 



H. D. King {Journ. Morph., xvii, 1901, p. 295, and xix, 1908, p. 370) 

 fixes (for a few minutes) the spawn (of Biifo) in sublimate (saturated 

 with 5 per cent, of acetic acid), or in Flemming Zenker, or Hermann, 

 brings into alcohol, first of 50 and then 80 per cent., and removes the 

 jelly after a few days. 



Blks {Trans. Roy. Soc. Edinburgh, xli, 1905, p. 792) takes for ova, 

 formol of 10 per cent., but for embryos and larvae the mixture given 

 §114. 



Bouin takes for larvae of Rana the formol-sublimate mixture § 117. 



830. Sulphate of Copper Liquid (Fol, Lehrbuch, p. 106, after Remak 

 and Goette) ; for hardening ova of Amphibia : 



2 per cent, solution of sulphate of copper . 50 c.c. 



Alcohol of 25 per cent. . . . . . 50 ,, 



Rectified wood vinegar. . . . .35 drops. 



PISCES 



831. Teleostea in General. The ova of many of the bony 

 fishes can be studied by transmitted light in the living state ; 

 but those of the Salmonidae must be hardened and removed 

 from their envelopes for the study of the external forms of the 

 embryo. 



To this end they may be put for a few minutes into water 

 containing 1 to 2 per cent, of acetic acid, and thence into 1 per 

 cent, chromic acid. After three days the capsule of the ovum 

 may be opened at the side opposite to the embryo, and be removed 

 with fine forceps. The ovum is put for twenty-four hours into 

 distilled water, and then into successive alcohols. Embryos thus 

 prepared show no deformation, but the vitellus rapidly becomes 

 excessively hard and brittle, so as greatly to interfere with section- 

 cutting. 



