96 MARKETABLE BRITISH MARINE FISHES chap. 



the interior but at the surface of the yolk (Fig. 46). When the 

 egg is first shed the oil-globule is movable, and when the 

 egg is placed on a glass slip under the microscope, and rolled 

 over, the globule always floats to the uppermost point. When 

 the embryo develops, however, the globule becomes fixed by the 

 germinal membrane in a position beneath the tail end of the 

 embryo. Sometimes three or four globules are present in the 

 egg when shed, which afterwards run together into one. Eggs 

 of this kind occur in a large number of different families. Many 

 species of the cod family and flat-fish family have them, namely 

 the rocklings {MoU//a), the hake {^Merlucchis), the ling {Molva), 

 the turbot and brill {Rhombus), the topknots {Zeugopteriis)- 

 Among the spiny-finned fishes eggs of this kind have been dis- 

 covered to belong to the following : the mackerel, the gurnards, 

 the gray mullets, and the bass. The single oil-globule may be 

 present in conjunction with other conditions of the yolk. 



The next complication to be considered affects the yolk, 

 and consists in the division of the part of it beneath the germ 

 into a layer of separate segments, while the rest is undivided. 

 The egg remains transparent, the partitions between the seg- 

 ments, composed of germinal matter, being very delicate. This 

 condition is known to occur in the common sole and other 

 kinds of sole among the flat-fish family. In the common sole 

 (Fig. 47) there are also a number of very minute oil-globules 

 which are arranged in irregularly shaped patches on the 

 surface of the yolk and give the egg to some extent a chalk- 

 white appearance when it is seen floating in a bottle of water. 

 The same condition occurs in the red mullet (fam. Mullidce) 

 with a single large oil-globule in the yolk, and in the dragonet 

 (goby family) with no oil-globule. It occurs also in an egg 

 supposed to belong to the American blue-fish, one of the same 

 family as our scad or horse-mackerel, in the egg of which 

 also it is probably present. 



A still further complication in the structure of the yolk 

 consists in the complete division of it into segments, which like 

 the condition last described may occur together with the presence 

 of an oil-globule or without. Floating eggs having this peculiarity 

 are scarcely less transparent than those which have a simple 

 yolk. It is found in the floating eggs of the sprat and pilchard : 

 in that of the sprat there is no oil-globule, in that of the 



