CEPHALOPODA. 315 



osmic acid treatment is necessary in the case of segmenting ova because the 

 Merkel's fluid does not kill rapidly enough, so that eggs placed in it may 

 even pass through one or two stages of cleavage before dying. This fluid 

 arrests the process of blackening by the osmium, or will even bleach the ob- 

 jects if blackening has set in. See also AGASSIZ and WHITMAN, in Proc. 

 Amer. Acad. Arts and Sciences, xx, 1884. For later stages the authors re- 

 commend the method of Perenyi. And see the experiments detailed by 

 COLLINGE, Ann. and Mag. Nat. Hist., x, 1892, p. 228 ; Journ. Boy. Mic~. 

 Soc., 1892, p. 883. 



Tunicata. 



611. Distaplia. DAVIDOJT (Mitth. Zool. Stat. Neapel, ix, 1, 

 1889, p. 118) lias some important observations on the fixation 

 of the ova of D. magnilarva. The best reagent is a mixture 

 of 3 parts of saturated solution of corrosive sublimate and 1 

 of glacial acetic acid. The ova to remain in it for from half 

 an hour to an hour, and be then washed for a few minutes in 

 water and brought through successive alcohols. Another 

 reagent, almost as good, consists of 3 parts of saturated solu- 

 tion of picric acid and one of glacial acetic acid, the objects 

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

 into 70 per cent, alcohol. 



612. 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. 



613. 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. 



WATASE (Journ. of Morphol., iv, 1891, p. 249; Journ. Roy. 

 Mic. Soc., 1892, p. 152) kills the ova in the macerating mix- 



