i6 
Fridtjof Nansen. 
[No. 7. 
and is, therefore, a Zona radiata, in the usual sense of the term) with 
the protective coverings of the ova of other Vertebrata, and he 
cornes to the conclusion that it is homologous with the single or 
double zona radiata in the ova of Teleosteans and Petromyzone. In 
Myxine, this vitelline membrane forms the sole protective cover ing 
of the deposited ovum ; there is no homology or comparison possible,. 
between this membrane and the horny capsule in which the eggs 
of oviparous Elasmobranchs are enclosed, as supposed by Dr. AlleN- 
THOMPSON; the Elasmobranch capsule being produced by a special 
gland in the oviduct. When the ova approach maturity, the thickness 
of the vitelline membrane increases rapidly, and at each pole of the 
ova. processes begin to be formed from its surface; these processes 
are the first stages of the well-known polar treads, which consequently 
consist of the same substance as the vitelline membrane, and are 
formed in the same way, probably by the follicular epithelium. On 
the examination of a polar thread of a deposited ovum in Prof. 
Turners Museum at Edinburgh, CunninGHAM found that these 
polar threads are not tubular as stated by Dr. Thompson. 
The deposited ovum which CunninGHAM examined, is one of 
the few deposited ova of Myxine existing in any Museum. It is 
to be regretted that nothing is known of where, nor when, this ovum 
was tåken. As already mentioned, I have quite recently detected a 
deposited ovum of Myxine in Bergens Museum, but neither in this 
case can it be said with certainty how, or where, the ovum was 
tåken. There is thus a void connected with the origin of all known 
deposited eggs of Myxine. 
Cunningham recognised several females that had recently 
discharged their ova. »In place of the 29 — 25 large ova which 
are usually present, there was a corresponding number of collap- 
sed follicles; each of these had a slit-like aperture at one end 
through which the ovum had been expelled.« He obtained 
similar recently » spent « specimens over a period of several 
months, and from what he has seen, he concludes that in Myxine y 
the deposition of ova occurs in the neighbourhood of the Firth of 
Forth during the months of December, January, February, and 
March; he believed that oviposition was limited to that season. 
Recently he has, however, stated (vide Zool. Anz. 1887, p. 391) 
that he has tåken similar spent females also in April, May, and 
in the first half of June. CUNNINGHAM now expects to find that 
oviposition is not limited to a particular season. As will be seen 
subsequently, this quite agrees with my results. 
