DE. W. H. EANSOM ON THE OVUM OF OSSEOUS FISHES. 
445 
causes ; and in eggs about in diameter it may be separated, but I failed to show 
any structure in it, probably from not succeeding in mounting it for examination 
with the highest powers. It is known by its characteristic foldings and refractive 
index. 
The best way to get out the yelk-sac is to cut up a fragment of the ovary with very line 
scissors in all directions, after having macerated it well in water ; then by removing the 
larger pieces several yelk-sacs are seen free from their ovisacs. 
The yelk-sacs of eggs of about xio" have certainly a fine dotted structure, and are 
furnished with buttons and a micropyle. The buttons can be seen on still smaller eggs, 
in which I found no dotted structure. As the eggs grow the yelk-sac gets thicker, and 
its markings more distinct : at first it is flaccid at all times ; but in eggs of the 2nd 
group, and larger, the segments, after imbibition of water, during which they seem to 
increase in thickness, become elastic, so that each segment springs into its shape again 
like a segment of an india-rubber ball. When the fine dottings can be observed, they 
have the same characters in all the stages of growth. 
The dots have a similar aspect on both inner and outer surfaces of the sac, are 
arranged in tolerably regular diagonal, curved lines, alternating, so that they enclose 
lozenge-shaped spaces. With powers up to X 500 they appear round, and even with 
the highest used, X 2600 and X 3000, they are but obscurely hexagonal. They are 
seen blackest when a plane rather deeper than the true surface is in focus, and then 
appear round. With very careful adjustments, and the true surface in focus, they have 
a polygonal or hexagonal, not very sharp outline, and seem like pits ; the elevated 
ridges between which look like a very fine, rather irregular reticulation. At a folded 
edge they produce an appearance of radial striation, the striae resolvable into dots, due 
to the laminated structure of the sac. This is best seen at a cut edge, especially after 
longer maceration. The cut edge reminds one of the edge of cut lace or perforated 
zinc, but whether this is due to an actual falling out of the matter which caused the 
appearance of dots, or is an illusion, I cannot say. The dots act, in focusing, like a 
substance of low refractive power, and I incline to the view that the appearance 
described is illusory. In the smallest-sized eggs of which I measured the dots, they 
were 2 4 0 o o" apart ; these eggs were about ibo" i n diameter. In nearly ripe ova, on the 
point of quitting the ovisacs, they were jtooo" apart, measured from centre to centre, 
each dot being about -3 0000 " in diameter; these results were the mean of several 
measurements with a power of x 1000. The dots are the same distance from each other 
on the inside as on the outside of the sac, and the radial lines are the same distance 
apart. All the measurements were made in the eggs of the 3-spined species. 
The outer surface of the sac suffers sometimes, after long maceration in water, a pecu- 
liar change in its consistence, so that on its rupture by very strong pressure, the surface- 
layer yields like a soft, almost viscid substance, seen as a colourless, structureless film, 
stretched across the rent, with a power of X 250 (Plate XVI. fig. 21). But with a power 
of X 3000 it has an exquisitely delicate structure, like net, very regular and perfect, and 
