DE. W. H. EANSOM ON THE OVUM OF OSSEOUS FISHES. 
433 
have been recently impregnated. If eggs are examined in a similar manner about four 
or five minutes after having been fertilized, the funnel is seen very clearly, half with- 
drawn from a corresponding pit in the centre of the discus proligerus (Plate XV. fig. 6). 
The ordinary mode of examining the surface of these eggs under a lens is not convenient 
for observing the micropyle, on account of the strong reflection from the viscid layer, 
which cannot be got rid of by drying or wiping. 
The yelk-sac resists the action of water for a very long time, in so far as it is not 
decomposed or materially changed in its optical properties by it, and indeed it differs 
markedly from the contents of the egg by its greater stability. It is, however, 
changed by imbibition of water, from a soft easily lacerable membrane to a firm 
elastic one, each small section of which returns to its normal form very quickly after 
pressure. It is not much altered by spirit of wine ; it is rendered clearer, slightly swollen, 
and its markings are made less distinct by dilute acetic acid and by dilute solutions of 
potash. 
c. The yelk-ball . — A delicate, colourless, translucent, homogeneous membrane, which 
I call provisionally the inner sac, covers the whole surface of the yelk-ball within the 
yelk-sac. It is not so easily shown in the eggs which have not imbibed water, but with 
care may be seen to escape with its contents, by causing a sudden rupture with a large 
opening in the yelk-sac. By examining recently impregnated eggs, which, being elastic, 
allow the contents to escape with a jerk under suitable pressure, it may be better seen, 
as it escapes, thrown into distinct folds, contracting or collapsing as its contents pass out 
into the water around (Plate XV. fig. 7). I have reason to think that it is hardened by 
the action of water on its outer surface, its membranous characters being more distinct 
in eggs which have been some time impregnated. When free in water it shows a double 
contour at the folds or wrinkles only ; for where it still contains in its pouches yelk-sub- 
stance, the inner surface line is not well defined. Under pressure it seems capable of 
almost indefinite extension before it ruptures. It thus presents more the characters of 
a firmer layer or crust upon the surface of the thick fluid yelk-ball, than of a separable 
membrane. Perhaps the best view of it is got while it is yet within the yelk-sac, after 
partial escape of the yelk (Plate XV. fig. 8). I formerly spoke of- this membrane as 
elastic, but I have now some doubt of this, as its tendency to shrink and collapse after 
rupture may be due merely to the escape of its fluid contents. 
Dilute acetic acid and dilute solution of sal-ammoniac do not dissolve it ; dilute hydro- 
cyanic acid does. 
The formative yelk is that portion of the yelk-ball which is afterwards directly trans- 
formed into the germ. It exists in the unimpregnated egg as a superficial layer com- 
pletely surrounding the food-yelk, and is closely connected near the germinal pole with 
the soft, ill-defined internal surface of the inner sac, which, as it ultimately takes part 
in the cleavage, may to that extent be considered a part of the formative yelk. At the 
germinal pole it forms a thicker layer or disk, extending over about one-fourth to one- 
third of the surface of the yelk-ball, marked at its centre by a pit which receives the 
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