OVARIES OF FISHES. 159 
Another specimen, 15} inches long, exhibited a less advanced stage 
in the development of the yolk. The right ovary was 3} inches long, 
44 inches from the posterior end of the ventral fin. Inthe majority 
of the ova the yolk was present in an external zone of the egg. There 
was nothing in the germinal tissue to show that this ovary had spawned 
before, but a small free clump in the cavity seemed to consist of the 
membranes of old ova of last season. 
Another specimen, 144 inches long, was still less advanced, the yolk 
being present only as a thin layer of small granules in the most 
external region of the egg, and the quantity of it was so small that 
the transparency of the ova was scarcely diminished. Other speci- 
mens larger than this last showed no trace of yolk or indications that 
they had previously spawned, and were either immature, or in the 
resting condition. 
On August 8th I took a plaice from a tank in the aquarium, one 
of a number which were observed to spawn in the beginning of the 
year. The specimen was 18 inches long. The right ovary was 34 
inches long and 5} inches from the posterior end of the ventral fin. 
There were no indications that the fish had spawned before. The for- 
mation of yolk for next spawning was already somewhat advanced. 
The largest of the yolked eggs, which were quite opaque, was ‘5 mm., 
the smaller 2 to’3 mm. ; the largest of the yolkless eggs was ‘19 mm. 
Another large specimen from the same source, killed on August 17th, 
was found to be in a similar condition. 
It is quite certain from these observations that, so far as the 
microscopic appearance of the germinal tissue is concerned, the spent 
ovary of the plaice, when examined in the fresh state under the micro- 
scope, may be quite similar to an immature ovary. This is clearly 
exemplified by the specimen described above as examined on February 
24th. It is certain, I think, that all the yolked ova left in an ovary 
after spawning has taken place degenerate and disappear, and the for- 
mation of yolk in the succeeding crops of ova does not commence till 
some time after spawning is over. The earliest date at which I have 
observed the formation of yolk to have commenced is July 15th. It 
is also certain that the number of yolkless eggs left in the ovary after 
spawning is far less than the number of ripe eggs shed in the follow- 
ing season. Consequently the greater number of the eggs of 
one season’s crop are produced ab initio during the year. It is to be 
noted that after a comparison has been made between an ovary in 
which yolk is commencing to develop and a spent ovary containing 
superfluous yolked ova the latter cannot be mistaken for ova which 
are developing for the next season. They are few in number and 
scattered singly, while the developing ova are abundant everywhere, 
and they have a dull, unwholesome appearance. 
128 
