278 EMBRYOLOGICAL METHODS. 



The eggs thus treated can often be cut 6/z on a rotary microtome 

 provided with a sharp knife. The method is indicated where large 

 numbers of stages of the early development of the frog are required 

 for junior class purposes. The main objection to the method is 

 that the chromic acid attacks pigment. Fertilisation and segmenta- 

 tion stages and general cytology are often extremely good ; stain in 

 any way (J. B. G.). 



0. HERTWIG (Jen. Zeit. Naturw., xvi, 1883, p. 249). The ova 

 are thrown into nearly boiling water (90 to 96 C.) for five or ten 

 minutes. The albuminous envelope of the ovum is then cut open, 

 and the ovum extracted under 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 pigment, whilst alcohol preserves it, which is 

 frequently important for the study of the germinal layers. 



MORGAN (Amer. Nat., xxv, 1891, p. 759, and Level, of the Frog's 

 Egg, 1897, p. 171) has the following. During the periods in which 

 it is difficult 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, Meth. of Research, p. 156. 



SCHULTZE (Arch. mik. 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). 



KING (Journ. Morph., xvii, 1901, p. 295, and xix, 1908, p. 370) 

 fixes (for a few minutes) the spawn (of Bufo) 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. 



BLES (Trans. Roy. Soc. Edinburgh, xli, 1905, p. 792) takes for 

 ova formol of 10 per cent., but for embryos and larvse the mixture 

 given 109. 



BOUIN takes for larvae of Rana the formol-sublimate mixture 

 112. 



