EMBRYOLOGY 379 



This may be done by putting them for two or three days into 

 1 per cent, sokition of chromic acid, and shaking well ; but ova 

 thus treated are very brittle, and do not afford good sections. 

 A better method is that described by Whitman {Amer. Natural., 

 xxii, 1888, p. 857), and by Blochmann {Zool. Anz., 1889, p. 269). 

 Whitman puts the fixed eggs into a 10 per cent, solution of 

 sodium hypochlorite diluted with 5 to 6 volumes of water, and 

 leaves them there till they can be shaken free, which happens 

 (for Necturus) in a few minutes. Blochmann takes eau de Javelle 

 (potassium hypochlorite), and dilutes it with 3 to 4 volumes of 

 water, and agitates the eggs, previously fixed with solution of 

 Flemming, for fifteen to thirty minutes in it. 



Lebrun {La Cellule, xix, 1902, p. 316) advises fixing ova of 

 Anura for not less than one and a half hours in liquid of Gilson, 

 § 74. The outer envelopes are then hard, and may be easily 

 incised and the ovum extracted by pressing on the pole opposite 

 to the incision. The operation should not be delayed until after 

 hardening in alcohol. Similarly {ibid., xix, 1902, p. 12) for 

 Urodela. 



GuYER {Amer. Nat., xli, 1907, p. 400) finds it suffice to roll the 

 ova (either fresh or fixed, but before bringing into alcohol) on 

 blotting paper. 



825. Imbedding. A great difficulty with the ova of Amphibia 

 lies in their becoming extremely brittle on imbedding in paraffin. 

 Carnoy and Lebrun {La Cellule, xii, 1897, p. 212) fix ovaries 

 or ovarian ova for fifteen minutes to three-quarters of an hour 

 (but see last §) in Gilson's mercuro-nitric fluid, § 74, and preserve 

 them in 80 per cent, alcohol. To imbed, they are brought for a 

 quarter of an hour into 95 per cent, alcohol, five minutes in 

 absolute alcohol, then into a mixture of alcohol and chloroform in 

 equal parts, and as soon as they sink in that they are put into 

 pure chloroform. Paraffin is added to the chloroform, enough to 

 about double the volume of the whole, and the whole is put for 

 about three hours into a stove at 35° C. Lastly, the ova are 

 put for not more than five minutes into a bath of pure paraffin 

 at 52° C. 



Lebrun {ibid., xix, 1902, p. 317) explains that it is important 

 not to dehydrate completely with absolute alcohol ; the ova 

 should be left in alcohol of 96 per cent, until chloroform can be 

 added without the mixture becoming turbid, and a second bath 

 of clean paraffin should be added. 



See also Morgan, Devel. of the Frog's Egg, New York, 1897, p. 171. 



826. Siredon. The ova are easier to prepai-e than those of the 

 Anura, because the yolk is separated from the albuminous layer 

 by a wide space filled with a liquid that is not coagulated by 

 reagents. Put the eggs for a few hours into picro-sulphuric acid, 



