690 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



once ascend like minute crystalline globes of oil, and before fertilisation, as well as after, 

 they swim freely in the water (No. 11, p. 36, and No. 65, p. 450). Not only are these 

 pelagic ova found at and near the surface of the sea, but, in many areas, throughout the 

 greater part of its depth. Moreover, they occur in great numbers near the bottom. In 

 calm regions they congregate near the surface in scattered groups, and show no tendency 

 to adhere together, save in such exceptional instances as those before mentioned. The 

 slightest agitation scatters them, and they are carried to and fro by the currents in the 

 surrounding medium.* In very still water in tanks they often form layers extending over 

 a considerable area, the lower strata sustaining by their buoyancy the superimposed 

 layers, which are even and regular to a remarkable degree (PI. I. fig. 10). Their 

 buoyancy is readily affected by a variety of conditions, especially by adulteration of the 

 sea water in which they float. In impure sea water t and in fresh water they sink, as 

 they also do in alcohol, in which fluid they rapidly become opaque. Dead eggs never 

 float, and dying eggs, though remaining translucent, lose their buoyancy. Healthy eggs 

 are rapidly affected by unhealthy or putrid ova in their vicinity, a fact showing that the 

 zona radiata is pervious, and that endosmosis and exosmosis readily take place, as indeed the 

 absorption of water by the partially desiccated ova of the cod (vide p. 681) clearly shows. 



In demersal and pelagic ova unhealthy or dying eggs are readily recognised by the 

 opacity of their contents ; and an offensive odour, if the eggs are in masses, indicates that 

 they are dead. Small groups of demersal ova, such as those of Cottus and Cyclopterus, 

 when dead, may be kept for many weeks in still water in a flat vessel without undergoing 

 much change in outline, though of course opacity is complete. 



Fertilisation. — With very few exceptions {e.g., Gambusia patruelis^ Sebastes 

 norvegicus, and Zoarces viviparus) the ova of Teleosteans are fertilised after expulsion 

 by the shedding of the milt, on the part of the male, in their neighbourhood. The rapid 

 diffusion of the milt in water by the serpentine movements of the spermatozoa is very 

 striking — they spread through a large area, and in tanks used for artificial fertilisation and 

 rearing it is difficult to keep ova in an unfertilised condition if sperms can by any 

 possibility find access through the supply-tank. § 



In demersal ova deposited on the sea-bottom, on zoophytes or shells, or (in littoral 

 forms) beneath shelving rocks, in hidden nooks of tidal pools, and in some cases in 

 nests constructed by the male fish, fertilisation is usually ensured by the proximity of the 

 male, which may even carefully guard the ova during development, as is notably the case 

 in Cyclopterus lumpus (vide No. 107, pp. 81, 82); but even in this species masses of eggs 

 occasionally are found whose fertilisation has not been accomplished. This may some- 

 times happen in the case of pelagic ova, though experiments at the Laboratory have 

 shown that eggs of haddock may remain for a considerable time unfertilised, and yet be 



* See Hensen's observations proving that pelagic ova are widely scattered in the sea (No. 65, p. 449). 

 t Vide No. 104. J No. 141, p. 461. 



§ As occurred to Professor Ewart and Mr Brook at the Rothesay Aquarium, and also with Motella in the St 

 Andrews Laboratory. 



