[27] SCIENTIFIC EXAMINATION OF THE GERMAN SEAS. 551 



arrival at the spawning places the eggs begin to loosen very rapidly 

 and the ovaries are emptied in a short time. Of many fish, snch as the 

 stickleback, the pike, and others, it is known that they lay their eggs 

 very rapidly, because they either, like the first-mentioned fish, deposit 

 them in a sort of nest, which could be observed, or because the spawn- 

 ing season, after it had once fairly commenced, was finished in about 

 two weeks. Other fish probably require a longer time for spawning. 

 The spawning season may be either delayed or accelerated by about a 

 month, according to difference in temperature. .After the eggs have 

 beep laid they must be immediately impregnated. The semen of the 

 male fish, inclosed in a white sexual organ (the spermary), matures at the 

 same time as the eggs, and when fully matured is ejected by the male 

 fish as a milky- white fluid. With the naked eye no compact particles 

 can be discovered in this fluid, but seen through the microscope, the 

 milky condition of the fluid is proved to be caused exclusively by the 

 so-called semen threads, spermatozoa composing about one-half of it. 

 These particles (see Fig. IX) consist of a head, which is generally round 

 and flat on the top, and a long thread termed the tail ; this thread moves 

 vehemently to and fro and propels these particles quite rapidly. As at 

 least one-half of the entire semen of a fish is composed of these small 

 particles, thousands of which do not weigh as much as a single fish-egg, 

 the male fish ejects millions of them. If an egg is to be impregnated, 

 it is necessary that at least one of these particles should enter it. This 

 process has frequently been observed. • 



When, for example, the codfish lays its eggs, they float on the surface 

 because they are a little lighter than the water. A somewhat marked 



Fig. XVI. — The lower part of a floating fish-egg, magnified about 100 times. 



7 



A, the yolk; B, the skin of the egg; C, a small orifice, the micropyle; D, semen threads (spermato- 

 zoa^, one of which is just entering the micropyle. 



portion of the egg is turned downward, because the egg is a little heavier 

 in that place than in others. Such an egg consists of a transparent mass, 

 containing in some fish a few drops of oil and of a skin, the egg mem- 

 brane (Fig. XVI, B) which protects the egg and keeps it together. 

 On the lower side of the floating codfish egg there is a very small 



