294 ON THE OVUM OF OSMERUS EPERLANUS. [May 4, 
had not been fertilized and were all dead. But they all possessed 
a kind of membranous appendage, and there were two or three which 
were suspended from the surface of the stones by means of this 
membrane, the distal end of which had become attached at the 
moment of extrusion. In the free eggs no power of adhesion any 
longer existed. It was obvious enough that the membranous appen- 
dage was the so-called suspensory filament mentioned in the existing 
literature. But the word filament is a very inappropriate term. 
The membrane is flexible, and in the form of a hollow truncated 
cone, the sides of which are thrown into irregular folds; the narrow 
end of the cone is continuous with the envelope of the egg; the 
attachment between the enveloping and the suspensory membrane 
thus forms a ring on the surface of the former. Examination of the 
eggs in this condition does not afford evidence of the origin of the 
suspensory membrane. All that could be seen was that the suspen- 
sory membrane was dotted all over with pores of considerable size, 
and that the enveloping membrane was perforated everywhere by 
finer pores more closely crowded. The enveloping membrane is 
thus a zona radiata. 
Examination of the eggs freshly pressed from the female gave the 
complete explanation of all the facts. These eggs were nearly, and 
some of them quite, mature. They are enclosed in a thick zona 
radiata, which is differentiated into two layers, the outer of which 
is somewhat thinner than the internal. In the zona radiata 
externa the pores are larger and farther apart than in the interna. 
But the important fact, which I believe no one has previously 
observed, is that the external zona separates very readily from the 
internal, and, rupturing at one portion of the ovum, peels off, becom- 
ing turned inside out in the process, and, remaining attached over a 
small cireular area, forms the suspensory membrane which I have 
already described. Slight pressure and rolling of the eggs by means 
of a cover-glass was sufficient to cause the rupture of the external 
zona, and the two membranes were examined in all stages of separa- 
tion. Owsjannikow has described the presence of the two layers of 
the zona radiata in the ovum of Osmerus, and his description agrees 
with mine; but the eggs he examined were less mature than those 
I had to deal with, and it is this fact which prevented him discovering 
the curious function which the external zona performs. Owsjanni- 
kow has also described the micropyle in the unripe ova he studied. 
I was unable to detect the micropyle, but I am inclined to think it 
exists in the centre of the area over which the suspensory membrane 
is attached. 
A comparison between the adhesive ova of Osmerus and other 
adhesive ova can now easily be made. In all adhesive ova the 
exterior surface of the zona radiata is glutinous ; it adheres to solid 
objects, and, setting hard after attachment, securely fixes the eggs to 
one another and to surrounding solids. In some adhesive eggs the 
external layer of the zona is different in structure from the inner. 
In the Herring-ovum there is no distinct differentiation into two 
layers ; in Perca fluviatilis, as described by Owsjannikow, there aie 
