146 FISHES OF SAN DIEGO—EIGENMANN. 
horizontal. The micropyle can, in these cases, most usually be found 
and the formation of the blastodisk observed. 
In pelagic eggs,in which the blastodisk invariably forms on the lower 
surface of the yolk, its formation might in part be explained by gravi- 
tation; not so in Atherinopsis and Ciupea, where it forms at the side of 
the yolk. In later stages it is seen that this is the normal position of 
the germ, for even if one succeeds in bringing it to the top of the egg, 
the egg slowly rotates, the oil globules changing position until the germ 
lies again at the margin. The process of the formation of the germ dif- 
fers considerably from that in Clupea. After the water space is formed 
a contraction begins near the ectodermal pole and travels to that pole. 
This is repeated rythmically. One series or one of the waves is repre- 
sented in Figs. 2-7. In Fig. 2 the contraction has begun; in Fig. 5 it 
has reached its culmination, and in Fig. 7 it has ended and another is 
about to begin. 
The first cleavage is completed after about three hours and twenty- 
five minutes from fertilization; twenty minutes afterward four cells are | 
formed. 
The sixteen-cell stage is reached in four hours forty-five minutes; 
thirty-two cells are formed five hours and ten minutes afterward. The 
first horizontal furrow was noticed after six hours, and the ‘ blastula” 
(Fig. 12) after twenty-eight hours. This figure is seen but a very short 
time; it is apparently formed by the sinking of the yolk below the cen- 
ter of the blastoderm. Cells from the overlying blastoderm frequently 
fall to the bottom of this segmentation cavity. The cavity is appar- 
ently obliterated by the settling down of the cells overlying it; imme- 
diately after its disappearance the blastoderm spreads and its margins 
begin to be infolded. At this stage periblast nuclei are very abundant 
and extend over half the yolk; twelve hours afterward they have ap- 
parently decreased in number and are much larger in size. 
The blastopore closes after about eighty hours. Kupffer’s vesicle ap- 
pears on the fourth day, the heart on the seventh; on the twelfth the em- 
bryos move vigorously; on the sixteenth day pigment spots appear on 
the top of the head and along the median line of the back. At this 
stage the embryos died, but the further developement may be gathered 
from the accompanying figures of larvee procured by skimming with the 
surtace net. 
Atherinops affinis (Ayres). Top smelt. 
J. & G. ’80, 29; R. Smith ’80; J. & G. ’80a, 456; id. ’81, 43; id., 82, 409. 
This species is found in San Diego Bay throughout the year. It is 
abundant through winter and spring, but is not esteemed as highly as 
the smelt. Large numbers are taken with seines. They spawn in May 
andJune. The larve are abundant in the bay, but its development has 
not been traced. From their habit of keeping near the surface, espe- 
cially surrounding offal, they are termed Top Smelt. 
ee 
