THE MARINE FAUNA AND THE OVA OF FISHES. 197 



eggs in tlie ovary are opaque, white grains, but by tlie time 

 they are shed they become as transparent as glass. These 

 ova as soon as tliey are shed are fertilized by milt in the 

 water supplied by male fish in the neighbourhood, and then 

 they rise towards the surface of the sea ; in calm weather 

 only do they actually reach the surface, because being but 

 slightly lighter than the water agitation causes them to be 

 uniformly distributed throughout the depth affected by the 

 wave-motion. A fine net made of muslin, or similar mate- 

 rial, drawn gently through the water at almost any season of 

 the year, collects numbers of these buoyant eggs, which can 

 be taken ashore and examined with the microscope. But as 

 these eggs are of many different kinds, and show constant 

 differences of structure, it is necessary to know what species 

 of fish each kind belongs to. One way of doing this is to 

 trace the development of the young fish after it is hatched, 

 until it reaches an age at which it can be recognised as a 

 whiting, sole, turbot, or other particular species. But this, 

 although easy enough to propose, is exceedingly difficult in 

 practice, and when followed usually leads to serious errors. 

 There is a more certain method, and that is to take the fish 

 when its ova are ripe, and by gentle pressure expel these 

 into a bottle of sea-water, then to add some milt from a 

 male of the same species, and keep the ova so obtained in 

 healthy conditions while they develop. This process of 

 obtaining ova is called artificial fertilization. 



A number of species have been subjected by various 

 observers in different places to this process, and the structure 

 of the eg'g and youug stages have been described in published 

 papers. Thus I myself published drawings and descriptions 

 of the ova of the cod, haddock, whiting, gurnard, smelt 

 [Osmerus eperlanus), plaice, common flounder, dab, and pole 

 flounder. But many species remained to be examined. 

 The first ova which I artificially fertilized after arriving at 

 Plymouth were those of Capros aper, a small fish with very 

 spiny fins, known sometimes as the boar-fish, but always 

 spoken of by Plymouth trawlers as the cuckoo. This fish 

 is taken in the trawl occasionally at all seasons of the year, 

 but in the latter part of summer, especially in August and 



