EGGS AND LARV^ AND THEIR DEVELOPMENT 97 



pilchard (Fig. 48) there is one. It occurs also in the egg of the 

 anchovy where there is no oil-globule, but the shape of this egg 

 is not round but sausage-like. The yolk is divided up in this 

 way also in certain floating eggs found at Naples in which there 

 were several oil-globules. These eggs may be' considered with 

 confidence to belong to some fishes of the eel family, but which 

 particular kinds cannot be decided. It is not known that our 

 common eel, or the conger, have floating eggs. 



It is interesting to notice that this last condition of the yolk 

 in floating eggs is the one which differs least from its condition 

 in attached heavy eggs, in which the yolk is always composed 

 of a number of separate globules. When the yolk has an outer 

 layer of separate segments it is in an intermediate condition, and 

 when it is not divided at all it is most altered from its condition 

 in fixed eggs, all the globules having run together into one large 

 globe. 



One other peculiarity among floating eggs remains to be 

 mentioned. In the majority the space between the membrane 

 and the yolk is not large, the two are in contact at more than 

 one point. In a few, however, there is a much greater separation 

 between the two, so that the yolk is only about half as wide 

 across as the envelope which contains it. This is the case in 

 the egg of the pilchard (Fig. 48), in that of the long rough dab 

 among the flat-fishes, and probably also in the halibut. 



There is scarcely any time of year or any part of the sea in 

 which floating eggs of some kind of fish cannot be found. As a 

 rule, however, they are not visible from a boat or vessel, it is 

 necessary to collect them from the sea-water by means of a tow- 

 net, a simple apparatus in the shape of a bag with a ring of iron 

 or cane to keep the mouth open, and made of some kind of 

 cloth which will allow the Avater to strain through. There are 

 many substances which grow in the sea at the surface and near it 

 and which may be mistaken for fish-spawn. One of these is very 

 common, and usually very abundant in May and June, when in 

 cilm weather it forms a visible scum on the surface. This 

 consists of little round grains, which under the microscope have 

 the appearance shown in Fig. 49. They belong to the lowest 

 and simplest classes of living things, and do not grow into any- 

 thing else. These grains have the power like many other marine 

 creatures of giving out light in the dark, and hence are called by 



H 



