o 



278 THE COD FAMILY. 



The egg of the ling (fig. 36, and Plate III, fig. 5), is of moderate 

 size, varj'ing in diameter between "042 and '043 of an inch (1"08 

 mm.). It belongs to the group of eggs 

 which is distinguished by the oily nutri- 

 ment in the yolk being collected together 

 into one large globule or sphere which 

 gives the egg a very distinctive apjiear- 

 ance. An oil-globule also occurs in its 

 fresh-water ally, the burbot. 



As has been already mentioned, to 

 this group of pelagic eggs also belong 

 those of the gurnard, rockling, brill, turbot and torsk, besides 

 numerous demersal species, so that the presence of an oil-globule 

 is not confined to any one group of fishes but may be present 

 or absent in closely allied species '- 



This sphere of oily fluid seems in all cases to lie freely in 

 the yolk below its protoplasmic coat (c£ Gurnard). The sphere 

 itself may also be seen by-and-by to have a coat of protoplasm 

 surrounding it. On moving the egg the oil-sphere may be 

 made to travel through the yolk and even under the developing 

 embryo, which in early stages rests like a cap upon one part of 

 the yolk-surface. We have no reason to believe that eggs with 

 oil-globules are more buoyant than those which have none : 

 one would not expect that the fact of the oily matter being 

 collected into one large sphere instead of being evenly distributed 

 throughout the yolk would have any direct effect upon the 

 buoyancy. The difference between an egg with an oil-globule 

 and one without is therefore merely a difference in degree of 

 segregation of the minute drops of oil scattered throvighout 

 the yolk. In the case of the ling, the oil-globule has usually 

 a diameter of about j^gths that of the egg, so that it is large 

 in comparison with the latter. 



The outer capsule of the egg is harder and more easily 

 ruptured than that of the cod's egg, and hence the egg bursts 

 rather than collapses under pressure. 



The hatching occupies about 9 days for eggs which are laid 



1 Prof. Prince ' On oleaginous spheres in the yolk of Teleostean ova, ' Ann. 

 Nat. Hist. Aug. 1886. 



