TELEOSTEA IN GENERAL. 313 



makes ova brittle ; they ought not to remain in it for more 

 than twelve hours. Chromic acid destroys or attacks the 

 pigment of the ova, whilst alcohol preserves it, which is fre- 

 quently important for the study of the germinal layers. 



MORGAN (Amer. Nat., xxv, 1891, p. 759 ; Journ. Roy. Mic. 

 Soc., 1892, p. 284) 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 35 per cent, alcohol containing " the same amount of sul- 

 phuric acid as in Kleiuenberg's solution/' Wash for several 

 hours in several changes of alcohol, beginning with 35 per 

 cent., and increasing the strength gradually up to 70 per cent. 

 About the second day in the 70 per cent, alcohol 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; and SCHULTZE, Zeit.f. wiss. ZooL, v, 1887, p. 177). 



Pisces. 



604. 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 Salmonidse must be hardened and removed 

 from their envelopes for the study of the external forms of 

 the embryo. 



To this end the ova 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 pnt for 

 twenty-four hours into distilled water, and then into successive 

 alcohols. Embryos thus prepared show no deformation, and 

 their histological elements are fairly well preserved. But the 

 vitellus rapidly becomes excessively hard and brittle, so as 

 greatly to interfere with section cutting. 



The following processes give good results as regards section 

 cutting. 



Put the ova for a few minutes into 1 per cent, osmic acid ; as 

 soon as they have taken on a light brown colour bring them 



