. OVA OP CEPHALOPODA. 271 



to remain in it for three to four hours, and then be brought, 

 into 70 per cent, alcohol. 



577a. Amaroecium (MAURICE and SCHULGIN, Ann. Sci. Nat. 

 ZooL, xvii, 1884). Stain in borax-carmine, wash out, and 

 stain for fifteen to twenty hours in very weak solution of bleu 

 de Lyon in 70 per cent, alcohol with a few drops of acetic 

 acid. In sections the epiblast and hypoblast appear chiefly 

 blue, the mesoblast cells on the contrary appearing almost 

 entirely red. 



Mollusca. 



578. Cephalopoda (Ussow, Arch, de BioL, ii, 1881, p. 582). 

 Segmenting ova are placed, without removal of the mem- 

 branes, in 2 per cent, solution of chromic acid for two minutes, 

 and then in distilled water to which a little acetic acid (one drop 

 to a watch-glassful) has been added, for two minutes. If an 

 incision be now made into the egg-membrane the yolk flows 

 away and the blastoderm remains ; if any yolk still cling to it, 

 it may be removed by pouring away the water and adding 

 more. The blastoderms thus prepared show, when appro- 

 priately stained, fine karyokinetic figures. 



579. Gasteropoda (HENNEGUY). Ova of Helix may be fixed 

 for from four to six hours in Mayer's picronitric solution 

 ( 56). The carbonate of lime that encrusts the external 

 membrane is thus dissolved, and the albuminous coat of the 

 egg is coagulated. The egg is opened with needles ; the 

 albumen comes away in bits, and the embryo can be removed. 

 Treat with successive alcohols, and imbed in paraffin. 



580. Limax (early stages) (MARK, Bull. MILS. Comp. Zool., Harvard 

 Coll., vi, 1881). The ova are treated with acetic acid of 1 to 2 per cent, for 

 four or more hours. The two external membranes are incised with fine 

 scissors, and the egg squeezed out in its albumen membrane. This is dis- 

 sected off on a slide, the egg is separated from the albumen, stained, and 

 mounted in glycerin. 



For later stages, or for making sections, osmic acid is used instead of 

 acetic acid, and the egg is hardened within its albuminous coats. 



