EGGS AND LARVAE AND THEIR DEVELOPMENT 



93 



is composed of two layers, an inner and an outer. In the egg 

 of the smelt or sperling (Osmerus eperlanus), one of the salmon 

 family, the outer layer breaks at one part of the surface of the 

 egg and separates, turning inside out as it does so, and remain- 

 ing firmly attached only at one small circular patch. The 

 separated membrane is adhesive when the egg is first shed, and 

 attaches itself to objects in the water, e.g. the piers of bridges 

 or posts in the rivers where the fish 

 spawns, or the stones of the river 

 bed. The eggs are thus suspended 

 from their support by the flexible 

 outer membrane (Fig. 44). It has 

 been mentioned that in all the eggs 

 of bony fishes there is a small 

 opening in the egg-membrane 

 called the micropyle, through 

 which the sperm enters in the 

 process of fertilisation. In the 

 egg of the smelt this opening is 

 in the centre of the circular patch 

 over which the suspending mem- 

 brane remains attached. 



In other cases the outer mem- 

 brane is represented by threads or 

 cords which are attached to the 

 egg-membrane at two opposite 

 regions and become fastened to 

 foreign objects or entangled with 

 those of other eggs of the same 



fish by their free outer ends. This is the case in the eggs of 

 the gar-fish or long-nose (Belone vulgaris) and other fishes of 

 the same family, namely the skipper or saury pike (Scombresox) 

 and the flying-fishes (Exocoetus}. The conditions under which 

 the eggs of the gar-fish are naturally placed in British waters 

 Jiave not bee'n thoroughly ascertained, but Day mentions that 

 Mr. Dunn sent him in May 1881 a portion of a mackerel net to 

 which a number of the eggs of this fish were attached by means 

 of their filaments. These eggs had been shed by the fish when 

 captured, as frequently happens, in the mackerel net. Masses 

 of eggs held together by the tangling of such filaments have 



FIG. 44. Egg of the Smelt, with 

 its suspending membrane. 



