50 SALT-WATER FISHES 



more than another. In the eggs, moreover, of such outwardly 

 different, though anatomically allied, forms as the plaice 

 {JPleuronectes) and cod {Gadus) there is no oil-globule whatever. 

 In any case, therefore, since these last are both eggs of the 

 buoyant type, it is not the oil-globule which causes the floating 

 of the egg, though it may keep a particular part of it upper- 

 most in the water. Nor does the floating of eggs in sea water 

 appear to be a question, as Professor Huxley thought might 

 be the case, of temperature. 



The time occupied in hatching varies, as might be expected, 

 however, with this same condition of temperature, so that 

 figures are, unless all the attendant circumstances be carefully 

 noted, of little use. As some indication, however, and with 

 the reservation that such results are necessarily based on obser- 

 vations conducted in the artificial surroundings of the aquarium, 

 it may be mentioned that the eggs of the plaice developed at a 

 temperature of 53° F. in from ten to twelve days, while those 

 of the cod took, twelve days at 45° F.* It must further be 

 borne in mind that the eggs of many kinds of sea fish show 

 considerable range in size, and also that large eggs are found 

 to take longer in developing than small. It is also interesting 

 to remember that, providing the water has so large a percentage 

 of salt that it cannot freeze, the eggs of many northern fishes 

 are capable ot hatching out at a temperature below zero. 



The little larva, having at last, in a varying period, burst the 

 capsule of the egg, emerges in the sea a helpless being, without, 

 in the majority of cases, either red blood or an open mouth. 

 To change from this very rudimentary condition to the perfect 

 state, irrespective of size, may be the work of a few weeks or 

 of many months, according to the state of advancement when 

 hatched. Just as the naturalist in Australia is confronted 

 by a most remarkable contrast in this respect in the almost 

 embryonic new-born kangaroo and the young brush-turkey, 

 which does not escape from its native mound until practically 

 * Cunningham, op. cit., pp. 216, 284. 



