OVARIES OF FISHES. 155 
.tinguishes the ova into three stages, the great, the small, and the 
minute. The great are those to be extruded at the next spawning 
season, and correspond to the “active” ova of Mr. Holt’s paper, 
while the small and the minute are the “inactive.’”’ Scharff in his 
paper in the Quart. Journ. Mic. Sci., 1888, on which Mr. Calderwood’s 
views are largely based, describes the smaller ova and the larger 
ova. ‘The latter correspond to the “active” ova of Holt’s paper, 
and Scharff describes the formation of yolk in them, but does not 
deal with the process in relation to the periodic changes which take 
place in the ovary. 
A comparison of these papers shows that the history of the 
formation of the yolk in the ova of fishes in connection with the 
periodic development of the crop of ova which are shed at each 
spawning season has not yet been thoroughly investigated. My 
own observations show that the opacity which distinguishes the 
active ova in Mr. Holt’s description is due entirely to the develop- 
ment of the yolk. I will describe what I have observed in the 
ovaries of various species, Commencing with the plaice. 
Of the plaice I examined in January some were ripe or nearly ripe 
and some immature. In the immature ovary the ova are all 
transparent, and when they are examined with the microscope in 
the fresh state their structure can be clearly seen. Leaving aside 
the tissue of the ovary, the stroma, which forms membranes round 
the eggs, the egg itself is seen to consist of structureless transparent 
protoplasm containing the nucleus, almost equally transparent, in the 
centre. The nucleus or germinal vesicle is enclosed by a membrane 
and contains the nucleoli, rounded bodies distributed at the periphery 
in contact with the inner surface of the membrane. The appearance 
of these young yolkless ova in the fresh state is shown in Fig. 1, a. 
In the other fish which are mature, and which are about to spawn, 
some of these yolkless eggs are present, but they are in small 
proportion to the opaque yolked eggs which make up the bulk of the 
enlarged ovary.. The yolked eggs are so opaque that it is impossible 
to see into their interior, but by examination of their surface the 
yolk can be seen to consist of separate globules or spherules of 
various sizes. In some specimens eggs which are very nearly ripe 
and ready for extrusion are seen. These are more transparent, and 
the transparency is seen to be due to a fusion of the yolk globules 
into a homogeneous mass. When this takes place, and the egg 
becomes ripe, the transparent mass of yolk occupies the whole central 
region of the egg, and the protoplasm forms a thin layer surrounding 
it. The germinal vesicle cannot be seen in the fresh ripe egg, but 
it is well known that it is represented by structures which can 
be demonstrated by appropriate methods in the external layer of 
NEW SERIES,—VOl,. III, NO. II. 12 
