GENERATION OF FISHES AN-D THEIR FECUNDITY 67 



from the ridges inside the roe are little round balls of yolk, of 

 course containing a germ, and they are covered with a trans- 

 parent membrane. These eggs are sent out into the water just 

 as they are, and no " white" or shell is added to them. In fact, 

 in all cases throughout the animal kingdom, the essential part 

 of an egg is the germ, from which as from a centre the forma- 

 tion of the new creature commences. A greater or smaller 

 quantity of yolk, which is the food of the germ, is connected 

 with it, and yolk and germ are contained in a membrane or 

 skin. If a number of hen's eggs were carefully broken, the 

 yolks separated and placed in water, they would correspond 

 exactly to the eggs of the herring or the salmon. The white 

 and the shell are not primary and essential parts of the egg, 

 though they are required under certain conditions. In the case 

 of the frog there is no shell, but each yolk is surrounded in the 

 egg-tube by a kind of " white " which swells up in water when 

 the spawn is laid, and causes a number of the eggs to stick 

 together in masses. 



An egg which is surrounded by a strong tough shell must 

 be fertilised before the shell is formed, and accordingly in dog- 

 fishes and skates as in birds the sperms are introduced into the 

 upper part of the egg-tube, where fertilisation takes place. But 

 the eggs of bony fishes are shed into the water unfertilised ; 

 there is, properly speaking, no egg-tube, and with the exception 

 of a few species no milt is introduced into the cavity of the roe. 

 These eggs come into contact with the milt when they are 

 expelled from the roe of the female into the water, and it is an 

 interesting fact that there is a minute aperture or pore in the 

 egg membrane for the admission of the sperm. This aperture, 

 technically known as the micropyle, is represented in the diagrams 

 Figs. 45-48, p. 95- 



In the case of the dog-fish or skate the eggs are laid singly 

 or in pairs at considerable intervals, as the formation of the shell 

 occupies a considerable time, and only two yolks come from the 

 ovary together.- The period of egg-laying too extends over a 

 considerable part of the year. The number of eggs laid in one 

 season could be ascertained, but I do not know of any special 

 inquiries on this point. On the other hand the period during 

 which eggs or spawn are produced by the bony fishes is limited 

 to a few months, and in the case of a single fish may last only a 



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