670 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 
example of the duration of a particular kind of ova in the bay, those of the gurnard may 
be taken ; for, appearing in April, they continue throughout May, June, and even part of 
July, being very abundant in June. It is clear, therefore, that with rapid growth, the 
differences in size between the post-larval forms produced from the ova at the extremities 
of the period must be considerable. 
Distribution.—Nothing was more striking, in the investigations in connection with 
H.M. Trawling Commission in 1884, than the abundance of the pelagic ova in the upper 
regions of the water, and indeed throughout it. They are not usually found quite at the 
surface, but as soon as the tow-net is sunk a fathom or two, they occur almost in every 
haul on suitable ground. Though on the banks frequented by the cod, haddock, and 
whiting, these pelagic ova are in greatest profusion at the breeding season, yet they are 
met with during many months from January till late in autumn, a continuous discharge 
of ova taking place from one or other group having this habit. Moreover, it is clear that 
the provision by which only a portion of the ovary in most fishes with pelagic eggs becomes 
ripe at a given time, greatly prolongs the spawning period, and tends to intensify the 
feature just mentioned. It is possible indeed to form an estimate of the number of 
spawning fishes in a given district by the abundance of pelagic ova, or the contrary. It 
is only necessary to illustrate this by reference to the surface of Smith Bank, off the coast 
of Caithness, where the ova and embryos were in vast numbers in the beginning of April, 
so much so that the area resembled a vast hatching-pond, even the sea-birds feeding in 
long lines on such as the currents swept to the surface. The same feature was shortly 
afterwards noticed, along with Lorp Datnovustn, off the Island of May, though both eggs 
and embryos were less numerous than in the former case. 
Again, recent investigations with the trawl-like tow-net on the bottom show that a 
vast number of pelagic ova, such as those of the cod, whiting, rockling, sole, flounder, 
gurnard, sprat, and other forms, are to be found there—when the large mid-water net and 
the surface-net are nearly devoid of them. Whether this aggregation of ova is due to 
cold at the surface or to the effect of currents has not yet been determined, but it is a 
feature of great interest. 
Sizes of Ova.*—As an example of the variety of pelagic ova common to the sea 
beyond the Firth of Forth in April, the following measurements from spirit-preparations 
are interesting. The ova were collected by the tow-net (sunk a fathom or thereabout) 
in the usual manner, and then placed in strong spirit, which caused considerable contraction, 
probably from ‘1 to °15. A very few measured 0216 of an inch, others had a diameter 
of 023, ‘03, 033 (probably Motella), -035, 0866, °04, ‘043, *045, ‘046, -05, the largest 
number ranging over the area covered by the last five, which probably included cod, 
haddock, ling, &e., 056, and a very few at ‘058 and ‘083 of an inch. A little variation 
appears to occur in each species. The average in fresh specimens of the haddock is 
‘056, the blastodise being °033; plaice, °0716; ling, ‘0916, and the oil-globule, -031; 
* A table of sizes of ova from RarraEte is given by one of us, in a Report on the Pelagic Fauna of St Andrews 
Bay, Seventh Annual Report, Fishery Board for Scotland, 1889. 
