690 PROFESSOR W. C. M‘SINTOSH 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 (Pl. 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 (eg., Gambusia patruelis,t 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). 
+ Vide No. 104. t 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. 
