xv EGGS 411 



are perforated at the animal pole of the egg by a small aper- 

 ture or " micropyle," which is only large enough to admit of 

 the entrance of a single spermatozoon at a time (Fig. 235). 

 Generally, there is only a single micropyle, but, according to 

 Salensky, the Sturgeon (A. sturio) has from 3 to 9, and the 

 Sterlet (A. ruthenus) from 5 to 13. 



An important distinction may be made between the ova of 

 different Teleostomi as regards their location after extrusion from 

 the female. From this point of view two types of ova can be 

 distinguished, demersal and pelagic ova. Demersal eggs are 

 characterised by their larger size and greater weight, so that they 

 always sink after extrusion ; and by their opacity. They m;iy 

 either have an outer egg-membrane which is viscid and adhesive, 

 so that the eggs readily adhere to one another or to foreign objects, 

 or the membrane is smooth and non-adhesive. The Salmonidae, 

 for example, produce non-adhesive demersal eggs, which remain 

 separate after being deposited on the gravelly bed of a stream. 

 Most freshwater and many marine shore Fishes have adhesive 

 demersal eggs, which are deposited at the bottom of the water, 

 generally adhering to one another in larger or smaller clumps, 

 masses, or sheets, and attached to rocks, stones, or empty shells, 

 like the eggs of many shore Fishes, or to aquatic plants after the 

 fashion of the eggs of the Carp, Perch, and Pike, or even to 

 branching zoophytes, as is the case with the eggs of the Sea-snail 

 (Liparis). In some adhesive eggs the external egg-membrane 

 forms threads for their attachment. The eggs of the Gar-Fish 

 (Belone}, and those of the Saury Pike (Scombresox} and of the 

 Flying Fishes (Exocoetus), have viscid threads developed from 

 opposite points on the surface, which are either attached to 

 foreign objects or they become entangled with those of other 

 eggs of the same species. The oval eggs of some of the Gobies 

 have a bunch of fibres at one pole which serves to attach them. 

 In the Smelt (Osmerus eperlanus) a portion of the outer egg- 

 membrane breaks away from the rest and becomes turned back, 

 inside out, but remains attached to the egg at one point. By 

 means of this membrane the egg is attached to rocks or stones. 

 Pelagic eggs are distinguished by their lightness and buoyancy, 

 so that they always float near the surface of the water, and by 

 their smaller size and remarkable transparency (Fig. 235). A 

 conspicuous feature in many of them is the presence of a single 



