400 
BULLETIN OF THE BUREAU OF FISHERIES 
rarely three oil globules. When more than one globule is present they are propor- 
tionately smaller than the single oil globule. The eggs float with the oil globule or 
globules uppermost and are never seen otherwise. 
The similarity of the eggs of the pigfish and those of the white perch ( Bairdiella 
chrysxira ) — the latter for the most part correctly described and figured by Kuntz 
(1914, p. 4, figs. 1 to 15) — is so great that they are confused easily. Confusion is 
especially liable to occur, because the eggs of both species are taken in the same 
areas and in the same towings during the greater part of the spawning season of 
the pigfish, as the spawning seasons of the two overlap. The separation of the eggs 
is especially difficult during the early stages, or until the embryo becomes well out- 
lined, and for that reason it seems advisable to state the differences noticed. In 
size the eggs are nearly identical. The range, as shown by the measurements of 
100 perch eggs spawned in a tank in which ripe fish were confined, is 0.66 to 0.72 
millimeter with an average of 0.686 millimeter. Although the eggs of the perch 
according to these measurements average a little smaller than those of the pigfish, 
so many are identical in size that they can not be separated on this basis. The eggs 
of both species commonly have a single oil globule. That of the perch egg averages 
slightly larger (0.18 millimeter) than that of the pigfish egg (0.16 millimeter), but 
the difference is so slight and so many are of equal size that the character is of little 
use in making identifications. However, the oil globule in the perch egg seldom is 
as clear as that in the pigfish egg. Furthermore, the oil globule in the perch acquires 
dark greenish specks in the advanced cleavage stage, which increase in number and 
become quite prominent when the embryo becomes well outlined. At this time the 
granular specks appear on the embryo also. These markings persist both on the 
oil globule and on the embryo until hatching and for a short time (24 hours or so) 
after hatching. In the pigfish, on the other hand, dark specks, if they appear at all, 
are present only during the early embryonic stages and they disappear when an 
advanced embryonic stage is reached. When present they are fewer and smaller 
and, therefore, less prominent than in the perch. 
The position of the, oil globules in relation to the embryos forms a ready and 
reliable recognition mark when an advanced embryonic stage is reached, and this 
character may be employed also in separating the newly hatched young of the two 
species. In the pigfish the oil globule occupies an anterior position with respect to 
the embryo. That is, it lies under or near the ventral surface of the head, whereas 
in the perch it occupies a posterior position, lying well behind the head. Similarly, 
in the newly hatched pigfish the oil globule lies near the anterior periphery of the 
yolk sac, under the head of the fish, whereas in the perch it occuppies a position near 
the posterior periphery of the yolk sac, on the ventral surface of the abdomen. 
Attention is called to the fact that Kuntz (1914, p. 4) gives the range in size of 
the perch egg as 0.7 to 0.8 millimeter, whereas the range obtained by us, as already 
stated, extends from 0.66 to 0.72 millimeter. It is evident from the range in size 
given that Doctor Kuntz included eggs of a larger size. It seems probable that he 
considered a certain larger egg, apparently always found in the tow with the perch 
egg herein described, as identical with the smaller egg which now is known definitely 
to be a perch egg. This larger egg appears to be identical, except for size, with the 
smaller one during the early developmental stages. In the advanced embryonic 
stage it, however, acquires a more profuse dotting with dark-green granules. The 
fish hatched from this egg is slightly larger, as would be expected, and it contains 
more pigment than the larva hatched from the smaller egg. This larger egg, as to 
