NORTH SEA INVESTIGATIONS. 365 



obvious difference in the ova of two plaice of five and ten Indies 

 respectively. 



The first approach to maturity is denoted by an enlargement of 

 some of the ova, and by various changes in their internal structure 

 (well known to students of Teleostean embryology), of which we 

 need notice only one. This is the assumption of an opaque condi- 

 tion ;* and since it is the character which is most readily apparent, I 

 have utilised it in separating ova which are approaching an individual 

 participation in the reproduction of the species from such as either 

 will or may be absorbed without ever ripening. The former, for 

 the purposes of the present Avork, may be termed " active," and the 

 latter '' inactive " ova, without implying the actual difference denoted 

 by the two words. Even before they are actually visible to the naked 

 eye, the presence of " active " ova may be detected in a germinal 

 epithelium by a change in the coloration and consistency of the latter.f 



Mr. Cunningham, in the last number of this journal, p. 227, deplores 

 the absence of any criterion by which one can find out whether a fish 

 has spawned or not ; if the fish be not ripe or ripening when caught 

 "it may be sexually immatui-e, or it may have spawned previously, 

 its sexual organs being merely in an inactive state at that particular 

 time of the year." 



To establish such a criterion has, of course^ been an essential 

 part of the present research ; as a result, I am satisfied that the 

 features upon which I myself, and I suppose most other observers, 

 have been accustomed to base a diagnosis, are adequate for the 

 purpose, provided a careful examination is possible. 



I have met with no single character, nor even combination of 

 several characters, which has a general applicability in this matter 

 for Teleosteans as a whole. The variation in structure and disposi- 

 tion of the reproductive organ is so remarkable that a different 

 treatment may be required even within the limits of a single genus. 



As we are here dealing only with the flat-fishes, there is no need 

 to advert to other forms, and I shall therefore confine myself to 

 discussing the conditions in so far as they affect the former alone. 



Undoubtedly the most important point is the distinction between 

 immature and spent (and resting) fish. 



The most important characters in distinguishing these stages may 

 be enumerated, in the order in which they become apparent, as follows : 



* The changes which give rise to the opaque condition are not the same in all species, 

 but they occur at much the same stage of development, and appear to possess the same 

 siguificance. 



t Specimens in which the ovaries exhibit the other characters of immaturity, but in 

 which the largest ova are just passing from the "inactive" to the "active" condition, are 

 included in the tables amongst the immature. 



