1 62 BRITISH FISHERIES 



spot at its summit : this spot is the " blastoderm " 

 or germinal disc, and it gives rise to the body of 

 the chick. 



There are, of course, many variations from this 

 type of structure. Fish eggs vary in size, the smallest 

 common sea-fish egg being that of the solenette 

 (Solea lutea), and the largest that of the halibut. 

 The former is, roughly speaking, about ^ o f an 

 inch in diameter, and the latter about -^ of an 

 inch, and within these limits the diameters of the 

 greater number of sea-fish eggs are placed. These 

 somewhat minute variations in size are, however, 

 of great importance, for they are fairly constant 

 for the eggs of each species in any one locality, and 

 it is possible to determine the species of an egg 

 found in the tow-net by measuring it with certain 

 microscopic apparatus. In such an identification we 

 are aided by other characters, such as the colour, 

 the structure of the yolk, the presence and size of 

 oil-globules within the latter, the markings and 

 appendages of the shell, and other things. 



There are two principal categories of fish eggs, 

 according to their specific gravities. All sea-fish 

 eggs are heavier than distilled water, and, if placed 

 in the latter, will at once sink to the bottom. 

 But the majority of sea-fish eggs are slightly 

 lighter than sea-water, and when shed into this 

 medium they rise to near the surface and float 

 there. Such are termed " pelagic " eggs, and to 

 this category belong the ova of the cod, whiting, 



