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in the autumn and winter. Leth remarlced that the yellow eels become silvery first 

 in spots and then later thoroughly and states as the result of his investigation : "I thus come 

 to the conclusion that the silver eel is not a special species but that it is 

 the breeding form of the yellow eel." A more definite determination of this is 

 given by C. G. JoH. Petersen ' who shows very clearly and distinctly, that the yellow 

 eels are eels in process of growth with well-developed digestive organs in full func- 

 tion but with less developed sexual organs, whereas the silver eels which migrate in 

 the autumn and winter are eels with a special breeding dress. In the latter the 

 digestive tract is shrunken, but the sexual organs on the other hand are more developed 

 than in the yellow eel. They take no nourishment or practically none and they migrate 

 evidently for the purpose of breeding. Petersen gives a large number of proofs for his 

 conclusions, amongst others, that he could not find silver female eels under a length of 

 ca. 42 cm., whereas yellow eels were found so small that the sex could not be determ- 

 ined. Similarly, he found no silver males less than ca. 29 cm., but yellow eels on the other 

 hand were to be got down to a length when the male organ could just be determined. 

 In addition to the changes in colour and in the appearance of the sexual and digestive 

 organs, several others occur both outer and inner during the development from "yellow" 

 to "silver" eel. One of the most important is that the eyes increase considerably in size. 



Petersen calls the dress of the migrating eel a breeding-dress % because there can 

 be no doubt that these eels migrate to the sea for the purpose of reproduction. All the 

 silver eels found both in fresh water and in shallow water at the coasts are however far 

 from being mature, and even if the sexual organs in the silver eels are considerably more 

 developed than in the yellow there is evidently still a very long way to go before the 

 breeding takes place. The opinion of many in past years, that the breeding of the eel 

 occurred in fresh water or in shallow water at the coasts (in the mouths of rivers etc.) 

 lacks any observation which might prove this. On the contrary we have every 

 justification for saying, that, if the breeding of the eel or spawning eels have not hitherto 

 been observed in fresh water or at the coasts in spite of the fact that so many have 

 been interested in this matter, then the reason very probably is, that spawning eels are 

 not to be found in fresh water nor at the coasts but at quite' other places where we have 

 not yet been able to follow them. 



That they have been followed some part of the way however may now be shown. 



3. Migration of the breeding eels to the sea. Whilst the fishing for the 

 silver eels when they are migrating in the autumn and winter to the sea is restricted at 

 most places in Europe to the rivers or estuaries, the conditions are different in the 



1 C. G. JOH. Petersen, The Common Eel {Angmlla vulgaris T.) gets a Particular Breeding-dress before 

 its Emigration to the Sea (Report Danish Biol. Stat., V, Copenhagen 1896). It should also be remarked that 

 the Italian engineer G. BuLLO (Previsioni sulla Laguna di Comacchio, Padova, 1894) staled in 1894, that the 

 eel assume a special dress when they migrate to the sea, and that the yellow eels were eels with sexual organs 

 in the embryonic condition, but he does not seem to have known Leth's views nor Petersen's first preli- 

 minary report, printed in the "Dansk Fiskeriforenings Medlemsblad, Nov. 1893". 



2 It would be more correct perhaps to call it a "migrating dress", if it is to have a special name, 

 because we do not yet know that the eel reproduces in this form, as no one has yet seen a fully ripe eel. It 

 is probable on the other hand that the eel's appearance and structure are changed even much more before 

 breeding. This is indicated both by Cunningham's observations on Conger and by Grassi and Calandruccio's 

 on the large-eyed Anguilla of the Messina Straits. 



